The use of motor-evoked potentials to monitor sciatic nerve status during revision total hip arthroplasty

Sciatic nerve palsy is an uncommon complication after total hip arthroplasty (THA) but can cause permanent functional impairment in the extremity. The goal of this study was to identify specific intraoperative maneuvers that may increase the risk of sciatic nerve injury during revision THA. Motor-evoked potentials (MEPs) were used in combination with electromyography (EMG) monitoring during revision THA in 27 consecutive patients to identify intraoperative events that cause conduction abnormalities through the sciatic and peroneal nerves. MEP monitoring required general anesthesia and a neurophysiologist in the operating room throughout the procedure. Significant electrical events occurred, most commonly during acetabular reconstruction. Hip flexion should be avoided during posterior acetabular retraction when using the posterior approach with posterior dislocation. The position of the sciatic nerve should be clearly identified when complex structural acetabular augmentation with allograft is performed during revision THA. MEP monitoring was used as a research tool for this study. However, we do not use MEP monitoring in our revision THA clinical practice.

Reversed latissimus dorsi muscle flap for repair of recurrent congenital diaphragmatic hernia

BACKGROUND/PURPOSE

Neonates with large congenital diaphragmatic hernias (CDH) require prosthetic patch closure of the defect because of the paucity of native diaphragmatic tissue. As the child grows, patch separation can occur necessitating reoperation. Use of vascularized autologous tissue may decrease the incidence of reherniation as tissue incorporation and growth may be improved. The authors report our early experience using a local muscle advancement flap with microneural anastomosis for those children in whom reherniation develops after prosthetic patch placement.

METHODS

Seven patients with CDH (6 left and 1 right) whose synthetic diaphragmatic patch separated from the chest wall resulting in a clinically significant recurrent hernia were followed up with prospectively. After dissecting the ipsilateral latissimus dorsi off the chest wall and dividing the thoracodorsal neurovascular bundle (based on its lumbar blood supply), the synthetic patch was removed via an eighth intercostal incision. The muscle flap was placed into the hemithorax through the bed of the tenth rib and sutured in place over a Vicryl mesh scaffold. The thoracodorsal nerve was anastomosed to the phrenic nerve. Functional analysis of the flap was performed in 4 patients.

RESULTS

Age at placement of the muscle graft ranged from 2 months to 48 months (median, 24 months). There has been no evidence of reherniation after placement of the muscle graft. Long-term outcome and functional analysis of the flap was available in 4 patients (mean, 19 months). Two infants had fluoroscopic and sonographic evidence of nonparadoxical neodiaphragmatic motion. In one of these, electromyographic evidence of function was documented with a phrenic nerve conduction velocity of 22 meters per second. The third infant showed no evidence of neodiaphragmatic motion, and the fourth infant had paradoxical motion.

CONCLUSIONS

This is the first direct documentation of phrenic nerve function in an infant with CDH. An innervated reversed latissimus dorsi (RLD) flap reconstruction for recurrent CDH provides an alternative to prosthetic patch repair. This technique offers the advantages of autologous vascularized tissue with potential phrenic nerve innervation and physiologic neodiaphragmatic motion.

Facial Nerve Monitoring in Middle Ear and Mastoid Surgery

HYPOTHESIS

Intraoperative electromyographic facial nerve monitoring, long accepted as the standard of care in surgery for acoustic neuroma and other cerebellopontine angle tumors, may be of aid in middle ear and mastoid surgery.

STUDY DESIGN

Retrospective series of 262 cases of middle ear/mastoid surgery in which monitoring was performed by a neurophysiologist.

METHODS

Neurophysiological monitoring events were classified as mechanical or electrical. The voltages producing facial nerve stimulation were compiled and compared with observed facial nerve dehiscence.

RESULTS

The most common use of monitoring was localization of the facial nerve by electrical stimulation (60%) or identification of mechanically evoked activity (39%). In 57 cases (36%), the first electrical stimulation event evoked a facial nerve response at less than 1 V threshold, indicating little or no bony covering. The minimum stimulation threshold throughout each of these cases was less than 1 V in 88 of the 159 cases (55%) in which stimulation was attempted. In contrast, the facial nerve was visibly dehiscent in only 35 cases (13%). Neurophysiological monitoring confirmed aberrant facial nerve course through the temporal bone in four cases resulting in cancellation of surgical treatment in two cases. Postoperative facial nerve function was preserved in all cases when present preoperatively.

CONCLUSIONS

An electrical stimulation threshold of less than 1 V is a more useful criterion of dehiscence than observation under the operating microscope. The absence of monitoring events allows safe dissection. Monitoring can help locate the facial nerve, guide the dissection and drilling, and confirm its integrity, thereby allowing more definitive surgical treatment while preserving neural function.

Identification of motor pathways during tumor surgery facilitated by multichannel electromyographic recording

OBJECT

The goal of this study was to determine the usefulness of electromyographic (EMG) recording in locating motor pathways near the central sulcus or internal capsule during surgery.

METHODS

Multichannel EMG recordings were compared with visual observation of contralateral body movement that was elicited by direct cortical or subcortical stimulation used to identify motor pathways before and during tumor resection. The EMG recordings were more sensitive than visual observation alone in identifying motor responses: in 30% of cases, responses were identified by EMG recording alone at some point during the operation and, in 9% of cases, EMG responses were the only responses observed. Additionally, EMG recordings often detected seizure activity resulting from electrical stimulation of the cortex that could not be appreciated on visual inspection. No new motor deficits were seen postoperatively in 88% of the patients in this series.

CONCLUSIONS

Using EMG recording in addition to motor pathway mapping results in greater sensitivity, allowing the use of lower stimulation levels and facilitating detection of stimulation-induced seizure activity.

Assessment of sensory function in neonatal sheep with somatosensory evoked potentials: methodology and normative data

Fetal sheep are increasingly used as animal models for fetal surgical interventions such as repair of myelomeningocele. Since behavioral observations cannot provide objective information about preservation of sensory function, we have developed a technique for reliably recording somatosensory evoked potentials in neonatal sheep. We determined anatomic criteria for placement of recording electrodes over the somatosensory cortex using external landmarks, and recorded normative data for both ulnar and posterior tibial nerve stimulation in a series of normal neonatal sheep. The methodology and normative data are presented in this report; a companion paper demonstrates the utilization of this technique in a variety of experimental fetal interventions.

Experimental fetal neurosurgery: effects of in-utero manipulations on somatosensory evoked potentials

Somatosensory evoked potentials (SEP) were used to objectively evaluate sensory function in neonatal sheep after experimental fetal surgery. Posterior tibial (PTN) and ulnar (UN) nerves were stimulated electrically and averaged SEP were recorded from scalp electrodes placed over the somatosensory cortex. Animals with experimentally-created myelomeningocele (MMC) showed no SEP to PTN stimulation, but normal SEP to UN stimulation. In-utero repair of the MMC resulted in preservation of neurologic function and normal PTN SEP. In-utero thoracic spinal-cord transection resulted in no regeneration, and no SEP to PTN stimulation. In-utero unilateral transection of the sciatic nerve, even with attempted repair, resulted in little or no regeneration and absent or grossly abnormal PTN SEP from the affected side. In summary, the SEP technique provides valuable information concerning preservation of sensory function in a variety of experimentally created neurologic abnormalities and can aid in functional evaluation of experimental therapeutic fetal interventions.

Very late pain-related activity identified with topographically mapped frequency domain analysis of evoked potentials

OBJECTIVE

To identify low-frequency activity in the pain-evoked potential at very late latencies, consistent with C-fiber transmission velocities.

METHODS

Brief (1 ms) painful (intracutaneous) and two levels of non-painful (mild and strong) electrical pulses were applied to the index and middle fingers of the left hand. Evoked potentials (EPs) were recorded from 30 electrodes covering the entire scalp. Data from the 3 stimulus conditions (approximately 60 trials per condition per subject) were compared using the frequency domain technique of complex demodulation applied to single trial data. Subjects were 14 normal right-handed male human volunteers, aged 19-36 years.

RESULTS

Using descriptive probability mapping, pain versus strong non-pain differences were found in grand average data as well as in 8 of 14 subjects, consisting of greater low-frequency power at latencies from 700 to 1100 ms at electrodes near the contralateral central sulcus and at the vertex.

CONCLUSIONS

There are topographically focal, pain versus non-pain differences in the 700-1100 ms latency range that can be seen using frequency-domain analytic techniques. These differences were not seen with traditional time domain analyses. They may be due to a C-fiber-related mechanism or to very late activity triggered by faster fibers.

Comparison of response amplitude versus stimulation threshold in predicting early postoperative facial nerve function after acoustic neuroma resection

OBJECTIVE

This study aimed to better predict the early postoperative facial nerve (FN) function after acoustic neuroma (AN) resection.

STUDY DESIGN

This study was a prospective series.

SETTING

The surgery was conducted in a tertiary referral center.

PATIENTS

A total of 44 patients undergoing AN resection with cranial nerve monitoring were observed for at least 1 year after surgery.

MAIN OUTCOME MEASURES

The predictive value of amplitude of the FN stimulus response on the early postoperative FN function was measured.

RESULTS

Cranial nerve monitoring in AN surgery was used to obtain the stimulation threshold and facial electromyograph response amplitudes to FN stimulation proximal and distal to the tumor at 0.2 V above threshold. Thirty-eight of forty-four patients studied had a low postresection threshold (< or = 0.1 V). Of these (10), 26% sustained a postoperative FN dysfunction of House-Brackmann (HB) grades 3-6. In an effort to improve the predictive value from cranial nerve monitoring, the response amplitude to suprathreshold stimulation was compared with the threshold and FN function. Eighty-nine percent of patients with an amplitude of > or =200 microV had a grade 1-2 early postoperative FN function, whereas only 41% of patients with < 200 microV had a grade 1-2 early postoperative FN function (p = 0.00035). Eighty-eight percent of patients with both a low threshold and high amplitude had a grade 1-2 early postoperative FN function, whereas the remaining 12% of patients had a grade 3-6 FN function (p = 0.0032). The false-positive rate of threshold alone in predicting a grade 1-2 FN function was 26% compared to 12% for low threshold and high amplitude combined.

CONCLUSIONS

The use of FN threshold and amplitude together is superior to threshold alone as a predictor of early postoperative FN function.

Midgestational sciatic nerve transection in fetal sheep results in absent nerve regeneration and neurogenic muscle atrophy

In order to test whether fetal nerve healing and regeneration result in complete functional recovery, we transected the sciatic nerve at trunk level in 13 midgestational sheep fetuses. In 10 fetuses immediate microsurgical nerve coaptation was performed. The neonatal lambs were evaluated clinically, electrophysiologically, and histologically. On the transected side, the 10 surviving lambs showed a sensorimotor sciatic nerve paralysis and atrophy of the muscles innervated by the sciatic nerve. Somatosensory evoked potentials were weakly present in 5 animals and absent in 5 animals. Histologically, minimal signs of axonal regeneration, massive degeneration of the entire nerve, and a marked neurogenic muscle atrophy were found. These unexpected results differ from the findings after peripheral nerve transections in late gestational sheep fetuses and also from the classic wallerian degeneration-regeneration pattern that follows adult nerve injury. We speculate that the almost absent regenerative potential at midgestation is related to axotomy-induced neurotrophic factor deprivation during a developmental phase where the neurons are critically dependent on growth factor for survival.

Auditory P3 responses to name stimuli

Auditory evoked potentials (AEPs) were recorded from 10 normal adults in response to their own first names and to other first names spoken on tape. The following experimental conditions were used: 30 repetitions of the subject's first name; 80 other assorted first names from the same gender; 30 repetitions of a first name other than the subject's name. A P3 component was recorded from all ten subjects in response to their own first name, but not to other first names. Utility of this procedure could include assessment of cognitive processing of nonresponsive populations such as comatose patients, stroke patients, demented patients, autistics, infants, and children.

Steady-state analysis of somatosensory evoked potentials

We report the development of a new method for frequency domain analysis of steady-state somatosensory evoked potentials (SEPs) to amplitude-modulated electrical stimulation, which can be recorded in significantly less time than traditional SEPs. Resampling techniques were used to compare the steady-state SEP to traditional SEP recordings, which are based on signal averaging in the time domain of cortical responses to repetitive transient stimulation and take 1-2 min or more to obtain a satisfactory signal/noise ratio. Median nerves of 3 subjects were stimulated continuously with electrical alternating current at several modulation frequencies from 7 to 41 Hz. Amplitude modulation was used to concentrate the power in higher frequencies, away from the modulation frequency, to reduce the amount of stimulus artifact recorded. Data were tested for signal detectability in the frequency domain using the T(circ)2 statistic. A reliable steady-state response can be recorded from scalp electrodes overlying somatosensory cortex in only a few seconds. In contrast, no signal was statistically discriminable from noise in the transient SEP from as much as 20 s of data. This dramatic time savings accompanying steady-state somatosensory stimulation may prove useful for monitoring in the operating room or intensive care unit.

The fetal spinal cord does not regenerate after in utero transection in a large mammalian model

OBJECTIVE

Regeneration and functional recovery after spinal cord transection do not occur in mammalian animals and humans postnatally. The goal of this study was to test whether in utero transection of the fetal spinal cord is succeeded by anatomic healing and functional recovery.

METHODS

In five sheep fetuses, at 60 days of gestation and 75 days of gestation (term = 150 d), the spinal cord was completely transected at T10. The animals were delivered near term by cesarean section for clinical evaluation, measurement of cortical somatosensory evoked potentials, and morphological assessment.

RESULTS

The newborn lambs demonstrated sensory-motor paraplegia, were incontinent of urine and stool, and exhibited a spinally generated, ambulatory pattern of the hindlimbs. No cortical somatosensory evoked potentials could be recorded in response to posterior tibial nerve stimulation, although potentials from the ulnar nerve, which enters the cord rostral to the lesion, were normal in all animals. Histologically, no neuronal connections across the transection site were identified. The cord proximal to the lesion was grossly normal, whereas distal to the transection, it appeared slightly smaller but with the cytoarchitecture preserved.

CONCLUSIONS

Unlike in lower vertebrate and avian species, the fetal ovine spinal cord has no detectable spontaneous regenerative capabilities when transected during midgestation. Gap formation after transection, secondary posttraumatic cell death, and missing guiding channels for sprouting axons may be factors involved in the absence of any regenerative response.

In utero repair of experimental myelomeningocele saves neurological function at birth

In a previous series of fetal sheep experiments, the authors demonstrated that midgestational exposure of the normal spinal cord to the amniotic space leads to a myelomeningocele (MMC) at birth that closely resembles human MMC phenotypes in terms of morphology and functional deficit. The present study tested whether delayed in utero repair of such evolving experimental MMC lesions spares neurological function. In 12 sheep fetuses, a spina bifida-type lesion with exposure of the lumbar spinal cord was created at 75 days' gestation (full term, 150 days). Four weeks later, the developing MMC lesions were repaired in utero for seven fetuses (five fetuses died before this time). Of those that had repair, three were delivered near term by cesarean section, and four died in utero or were aborted. All survivors had healed skin wounds and near-normal neurological function. Despite mild paraparesis, they were able to stand, walk, and perform demanding motor tests. Sensory function of the hindlimbs was present clinically and confirmed electrophysiologically. No signs of incontinence were detected. Histologically, the exposed and then covered spinal cord showed significant deformation, but the anatomic hallmarks as well as the cytoarchitecture of the spinal cord essentially were preserved. These findings show that timely in utero repair of developing experimental MMC stops the otherwise ongoing process of spinal cord destruction and "rescues" neurological function by the time of birth. Because there is evidence that a similar secondary damage to the exposed neural tissue also occurs in human MMC, we propose that in utero repair of selected human fetuses might reduce the neurological disaster commonly encountered after birth.

Delayed onset facial nerve dysfunction following acoustic neuroma surgery

Delayed onset facial nerve dysfunction following acoustic neuroma surgery is an under-appreciated phenomenon. The authors have recently reviewed long-term (> 1 year) facial nerve outcome in 129 patients who underwent acoustic neuroma removal with the aid of cranial nerve monitoring between 1986 and 1990. The facial nerve was anatomically preserved in 99.2% of the patients, and at one year, 90% of all the patients had House-Brackmann (H-B) grade I or II facial nerve function. Delayed onset worsening of facial nerve function was noted in 38 of 129 (29%) patients, most of which occurred in the first few postoperative days. The incidence increases to 41% (38 of 93) when corrected for those with immediate H-B grade VI weakness, and who therefore could not manifest further deterioration. The facial nerve function either deteriorated from normal to abnormal or increased in severity of weakness. Delayed facial palsy was not related to the size of tumor or the surgical approach. The most common occurrence was that of a patient with H-B grade I or II facial nerve function worsening to H-B grade VI in the postoperative period. The prognosis for recovery of facial nerve function following delayed palsy was excellent. In the majority of cases, the recovery was complete within the first 6 months without specific treatment. Comparable to the patients without delayed palsies, 89% (34 of 38) of the cases had H-B grade I or II and 97% (37 of 38) had H-B grade III or better facial nerve function at 1 year. This review suggests a surprisingly high incidence of delayed facial palsy following acoustic neuroma surgery, which fortunately has an excellent prognosis for spontaneous recovery.

Creation of myelomeningocele in utero: a model of functional damage from spinal cord exposure in fetal sheep

A recent study in human fetuses with myelomeningocele (MMC) suggested that the primary malformation is not neural but a failed closure of the posterior vertebral column and paraspinal soft tissue, which leads to exposure and secondary destruction of the spinal cord. The goal of this study was to test whether chronic exposure of the normal spinal cord to the amniotic space produces a lesion similar to human MMC. In fetal sheep at 75 days' gestation (group A) and 60 days' gestation (group B) (term = 150 days), the lumbar spinal cord was exposed to the amniotic cavity by excising skin and paraspinal soft tissues, and by performing a laminectomy. Some animals from both groups were fetectomized and assessed morphologically at 100 days' gestation. The remainder were delivered near term and assessed clinically, electrophysiologically, and morphologically. In group A, all animals showed MMC-type pathology. The exposed spinal cord was herniated out of the spinal canal and rested on the dorsal membranes of a cystic sac. The neural tissue was stretched and flattened out. Histologically, the hallmarks of the spinal cord were not discernable and the cytoarchitecture was lost. These changes were less severe at 100 days than at term. The three survivors in group A were paraplegic. In group B, the two survivors and two fetuses harvested at 100 days had healed skin wounds and near normal spinal cord histology. The other animal harvested at 100 days had a MMC-type lesion with less severe histological changes. The two survivors had a mild paraparesis.(ABSTRACT TRUNCATED AT 250 WORDS)

In utero surgery rescues neurological function at birth in sheep with spina bifida

We hypothesize that the neurologic deficit associated with open spina bifida is not directly caused by the primary defect but rather is due to chronic mechanical and chemical trauma since the unprotected neural tissue is exposed to the intrauterine environment. We report here that exposure of the normal spinal cord to the amniotic cavity in midgestational sheep fetuses leads to a human-like open spina bifida with paraplegia at birth, indicating that the exposed neural tissue is progressively destroyed during pregnancy. When open spina bifida was repaired in utero at an intermediate stage, the animals had near-normal neurologic function. The spinal cord was deformed but largely preserved. These findings suggest that secondary neural tissue destruction during pregnancy is primarily responsible for the functional loss and that timely in utero repair of open spina bifida might rescue neurologic function.

A multichannel, model-free method for estimation of event-related potential amplitudes and its comparison with dipole source localization

We present a multichannel, model-free method for estimation of event-related potential (ERP) amplitude ratios and amplitudes using singular value decomposition (SVD), and compare with the Dipole Components Model (DCM). When the ERPs are generated by a single or multiple dipoles with equal amplitude ratios, the SVD method is superior to DCM in terms of reliable estimation of amplitude and is comparable with DCM for reliable and unbiased estimation of amplitude ratios. We show that dipole model misspecification leads to unbiased amplitude ratios and biased amplitudes when the ERP data sets are (1) generated and fit with a single dipole, or (2) generated by N dipoles with equal amplitude ratios and fit with M < or = N dipoles, because the effect of model misspecification 'cancels' for a ratio. Similarly proof that DCM estimates amplitude ratios more reliably than amplitudes for these cases is given.

Facial nerve outcome after acoustic neuroma surgery: a study from the era of cranial nerve monitoring

The introduction of intraoperative cranial nerve monitoring in posterior fossa surgery has greatly aided the surgeon in identification and anatomic preservation of cranial nerves. As a result, the long-term function of the facial nerve continues to improve after removal of acoustic neuroma. Herein, we report our long-term (1 year or greater) facial nerve outcome in 129 patients who underwent surgical removal of their acoustic neuromas with the aid of intraoperative neurophysiologic monitoring between 1986 and 1990. The facial nerve was anatomically preserved in 99.2% of the patients, and 90% of all the patients had grade 1 or 2 facial nerve function 1 year after surgery. Long-term facial function was inversely correlated with the size of tumor (chi-squared, p < 0.02) and was not related to the side of tumor, the age and sex of the patient, or the surgical approach. In a comparison among tumor groups matched for size, no statistically significant difference in facial nerve outcome between the translabyrinthine and retrosigmoid approaches was detected. The proximal facial nerve stimulation threshold at the end of surgical removal was predictive of long-term facial nerve function (analysis of variance, p < 0.02). At 1 year, 98% (87 of 89) of the patients with electrical thresholds of 0.2 V or less had grade 1 or 2 facial nerve function compared with only 50% (8 of 16) of those with thresholds between 0.21 and 0.6 V. In the era of cranial nerve monitoring, patients can be better advised about long-term facial nerve outcome after surgical intervention. Preoperatively, the size of the tumor is the most critical factor in predicting long-term facial function. Postoperatively, the proximal seventh nerve stimulation threshold at the end of the surgical procedure can be used as one prognostic measure of long-term facial nerve function.

Effects of prefrontal cortex lesions on visual evoked potential augmenting/reducing

Visual evoked potentials to 4 stimulus intensities were recorded from Fz, Cz, and Oz, all referred to A2, in 23 neurologically normal subjects and 21 neurosurgical patients with prefrontal lesions. The response amplitude as a function of intensity was evaluated for P1/N1 and N1/P2 components. At the Oz lead, the EP amplitude consistently increased with brighter stimuli more rapidly in the prefrontal group than in the controls. Similar, though less consistent, effects were seen at Fz and Cz. These findings suggest that the phenomenon of augmenting/reducing is at least partially influenced by prefrontal-mediated inhibition of sensory processes.

Identification of pain, intensity and P300 components in the pain evoked potential

This study examined the relationships among 3 components of the somatosensory evoked potential (SEP) to painful stimuli. Painful stimuli were produced using intracutaneous electrical stimulation of a fingertip and two levels of non-painful stimuli were produced by superficial electrical stimulation of a neighboring fingertip. SEPs were recorded from Cz-A1 and Pz-A1, and difference waves were computed for 3 components: (1) a pain component (the difference between SEPs to painful vs. strong but non-painful stimuli); (2) an intensity component that is not related to pain (the difference between SEPs to strong non-painful vs. mild non-painful stimuli); and (3) a P300 component (the difference between SEPs to the same stimuli under Target instructions vs. Standard instructions). The positive peaks in the 3 types of difference waves differed in both latency and topography, although with latency and topography overlap. The intensity component had an earlier positive peak than the pain component, and the pain component had an earlier positive peak than the P300 component. The pain and intensity components were larger at Cz than Pz, whereas the P300 component was larger at Pz than Cz. Under certain conditions, the pain evoked SEP consists of a weighted combination of the 3 components, complicating interpretation of the positive peaks in the recorded wave forms.

Immune function and neuropsychological performance in HIV-1-infected homosexual men

This study explores the relationship of immune dysfunction to the neuropsychological performance of individuals infected with HIV-1. Fifty-five HIV-positive homosexual men and 37 negative homosexual controls were evaluated using neuropsychological measures, physical exams, and measures of immune functioning. There were no significant differences favoring HIV-negative subjects over HIV-positive subjects. HIV-positive subjects, in fact, performed slightly better on attention and memory procedures. The HIV-positive subjects were then stratified according to the Centers for Disease Control symptom groupings (Group II, asymptomatic, n = 19; Group III, lymphadenopathy, n = 17; and Group IVA or C-2, symptomatic, non-AIDS, (n = 19). There were no significant neuropsychological differences among the three CDC groups. The HIV-positive subjects were also stratified on two measures of immune functioning: absolute CD4 counts (< 200, 201-400, > 400) and beta 2-microglobulin (beta 2M) (> or = 5.0, 3.0-5.0, < 3.0). Individuals with greater immune compromise, as measured by beta 2M, were more impaired on measures of attention and memory and had greater overall neuropsychological impairment (p < 0.05). Furthermore, 57% of the subjects who were abnormal on beta 2M were also impaired on measures of attention and memory, whereas only 14% of those with normal beta 2M were impaired on these same measures (p < 0.05). These results suggest that HIV-positive asymptomatics without evidence of immune compromise do not appear to be at greater risk of cognitive impairment than HIV-negative controls. However, for those HIV-positive individuals who are immune-compromised (even while asymptomatic), there is increased risk of neuropsychological impairment. These results also suggest that knowledge of serostatus and the use of the CDC classification system alone are insufficient in exploring the development of neuropsychiatric changes in HIV-1 infection.

Relationship of CD4 counts to neurophysiological function in HIV-1--infected homosexual men

OBJECTIVE

To explore the relationship of immune dysfunction to neurophysiological measures of brain-stem conduction time.

DESIGN

Three-year longitudinal prospective cohort study; results of time 1 analyses reported.

SETTING

San Francisco (California) General Hospital, Departments of Psychiatry and Epidemiology.

PATIENTS

Volunteer sample of 55 human immunodeficiency virus (HIV)-positive and 37 HIV-negative homosexual men recruited from a larger cohort of homosexual men followed up since 1983 at San Francisco General Hospital as part of an ongoing study of the natural history and course of HIV type 1 infection.

INTERVENTION

None.

MAIN OUTCOME MEASURES

Auditory brain-stem responses and somatosensory evoked potentials for subjects stratified separately on HIV serostatus, Centers for Disease Control and Prevention symptom groupings, and absolute CD4 counts.

RESULTS

The HIV-positive subjects had an increased wave III-V interpeak latency of the right ear auditory brain-stem response compared with the HIV-negative subjects (t test, P < .05). There were no significant differences among the three Centers for Disease Control and Prevention groupings on any evoked potential measure. When HIV-positive subjects were stratified on a measure of immune functioning, ie, CD4 counts, individuals with greater immune suppression were more impaired on speed of auditory brain-stem conduction time (Mann-Whitney U test, P < .05). Furthermore, 85% of subjects impaired on this evoked potential measure had CD4 counts of less than 0.40 x 10(9)/L (400/microL), whereas only 15% of those impaired on this measure had CD4 counts of greater than 0.40 x 10(9)/L.

CONCLUSIONS

Asymptomatic HIV-positive subjects who do not have evidence of immune suppression do not appear to be at greater risk for neurophysiological impairment than HIV-negative subjects. The HIV-positive individuals who are immune suppressed (even while asymptomatic) appear to have an increased likelihood of central conduction time slowing as measured by evoked potential procedures.

Modality specificity of evoked potential augmenting/reducing

Visual and auditory evoked potentials to 4 stimulus intensities in each modality were recorded from Fz, Cz, and Oz (visual) and Fz and Cz (auditory). Response amplitudes for P1-N1 and N1-P2 components were analyzed. Visual responses at Oz showed no increase in amplitude with brighter stimuli for either component. At both Fz and Cz, response amplitude increased with stimulus intensity at each lead for both modalities. For P1-N1 the slope of the stimulus/response function was similar for both modalities. However, for N1-P2 the slope was significantly higher for auditory than for visual EPs. These results are inconsistent with a view of augmenting/reducing as resulting from sensory modulation by a non-specific mechanism, such as the mesencephalic reticular formation, and instead suggest that a more selective mechanism such as regulation of sensory transmission by the prefrontal cortex may underlie this phenomenon.

Intraoperative monitoring of facial and cochlear nerves during acoustic neuroma surgery

The likelihood of successful preservation of facial and cochlear nerve function during acoustic neuroma surgery has been improved by the advent of intraoperative monitoring techniques. The facial nerve is monitored by recording EMG from facial muscles, with no muscle relaxants used; mechanical irritation of the nerve during surgery causes increased EMG activity, which can be detected in real time using a loudspeaker. Brief episodes of activity associated with specific surgical maneuvers aid the surgeon in avoiding damage to the nerve, whereas prolonged tonic EMG activity may reflect significant neural injury. Electrical stimulation with a hand-held probe elicits evoked EMG responses, which can be used to locate and map the nerve in relation to the tumor. The threshold for eliciting evoked EMG responses provides a rough indicator of the functional status of the nerve. Different nerves in the posterior fossa (trigeminal, facial, spinal accessory) can be identified in multichannel recordings by the spatial distribution and latency of responses to electrical stimulation. The ability to elicit EMG responses from low amplitude stimulation of the facial nerve at the brain stem after tumor removal is a reasonable predictor of postoperative facial function. Cochlear nerve function is assessed by recording the ABR from ear canal and scalp electrodes or the CNAP with an electrode placed directly on the nerve at the brain stem root entry zone. The ABR is a well-known, noninvasive technique that can be adapted to intraoperative use relatively easily but is of limited utility owing to the delay inherent in signal averaging. Direct CNAP recordings require placement of an intracranial electrode in such a way as to contact the cochlear nerve without interfering with surgical access but have the distinct advantage of rapid feedback on changes in cochlear nerve status.

Narrative speech deficits in dyslexics

Narrative speech of 31 dyslexics and 33 good readers was studied. Subjects were 10- to 12-year-old right-handed boys of normal intelligence, and with normal neurological status, vision, and hearing. The dyslexics used shorter communication units (independent clauses with all their modifiers), and a higher percentage of their words were noncommunications (words which are extraneous to the speaker's intended meaning). Rate of speech was the same in both groups. These findings were replicated in an independent second cohort of 21 dyslexics and 21 controls, aged 9 to 13. Our findings support theories of a general language deficit in dyslexia, and demonstrate the involvement of the previously unexamined area of narrative speech.

EEG asymmetry in schizophrenic patients before and during neuroleptic treatment

Lateral asymmetry of electroencephalographic (EEG) spectra was assessed in schizophrenic patients compared to normal controls. Ten predominantly unmedicated schizophrenic inpatients and nine normal controls performed monitored cognitive tasks during bilateral recording of EEG from parietal and temporal sites. Lateralization of EEG power in five frequency bands was compared between the groups; separate analyses were performed for linked ears and vertex references. A subsample of schizophrenic patients was restudied after a period of neuroleptic treatment. All significant group differences were obtained with the linked ears reference only. Pretreatment schizophrenics manifested relatively less alpha power over the right hemisphere during all conditions than controls, particularly in the parietal leads. After treatment, there was a significant shift in alpha lateralization toward the control values. These latter effects were also present in the theta frequency band to a lesser extent.

Neurometrics does not detect 'pure' dyslexics

Thirty-eight severely dyslexic boys and 38 good readers were evaluated with neurometrics, a diagnostic procedure based on the application of numerical taxonomy to EEG spectra obtained during resting conditions, supplemented by selected evoked potential features. This procedure generates deviance scores for the EEG spectra by comparing each individual's values to those obtained from a normative population and has been reported to discriminate learning disabled children from normal controls (Ahn et al. 1980). In the present study, all subjects, dyslexic and control, passed stringent screening to assure normal intellectual, neurological, sensory and emotional status. The false positive rate obtained in our control group was comparable to that reported earlier. However, none of the deviance scores significantly discriminated dyslexics from controls; most subjects from both groups were classified as normal. Severe dyslexia per se is thus not associated with the specific neurometric abnormalities reported previously in more heterogeneous learning disabled populations.

EEG spectra in dyslexic and control boys during resting conditions

We studied the resting eyes open and eyes closed EEG in carefully screened samples of 9-13-year-old dyslexic and control boys within a 2-cohort cross-validation design with repeat testing 1-3 years later. We found: no difference between groups in delta or theta activity in either cohort or in repeat testing 1-3 years later; in the dyslexics, decreased beta (19-24 c/sec) activity at bilateral central, parietal and mid-temporal leads referenced to vertex; inconsistent group differences in the alpha band: the first cohort dyslexics had lower alpha power than did the controls, but there was no group difference in the second cohort; and significant relative delta differences between groups in the first cohort, which were shown to be a statistical artifact of the use of relative power. We conclude that: dyslexia per se is not associated with increased absolute power in the delta and theta bands; lower power in the high beta band is reliably found in these samples of dyslexics without other disorders; and alpha power levels are not consistently lower in the dyslexic group.

Use of antidromic evoked potentials in placement of dorsal cord disc electrodes

Intraoperative recordings of somatosensory evoked potentials were made in 16 patients undergoing implantation of a dorsal cord stimulation system. Antidromic recordings, obtained by stimulating through the dorsal cord electrode placed in the epidural space and recording over peripheral nerves in the painful region of the body, and much higher signal-to-noise ratios and could be obtained with greater reliability than standard orthodromic recordings. When the placement of the electrode was adjusted to obtain evoked responses in the painful region, paresthesias referred to that region were obtained in virtually every case. Use of this procedure allows implantation and internalization of the electrodes in a single procedure under general anesthesia, and reduces the necessity of subsequent revisions.

EEG spectra in 9-13-year-old boys are stable over 1-3 years

Good reliability was found in EEG spectra recorded during resting conditions in two studies 1-3 years apart in groups of adolescent dyslexic and control boys who were well screened for neurological, sensory and emotional disabilities. The studies used different equipment and analysis (FFT in study 1 vs. iterative digital bandpass filters in study 2). The central leads (C3 and C4) referenced to vertex were recorded in both studies. Pearson correlation coefficients were computed as reliability estimates for each spectral band separately for each group, for absolute and relative power and for eyes open and eyes closed recording conditions. For both groups and for all conditions, the absolute power reliabilities were above 0.71 for all bands except delta for which they were above 0.54. Reliabilities were more variable for relative power measures.

A subcortical correlate of P300 in man

Event-related potentials in visual and auditory target detection tasks were recorded simultaneously from the scalp, somatosensory thalamus and periaqueductal gray in a chronic pain patient with electrodes implanted subcortically for therapeutic purposes. Short latency tactile responses confirmed the location of the thalamic electrodes. Rare auditory stimuli which were detected by the subject were accompanied by a prominent P300 component at the scalp, and by negative activity at the subcortical sites with the same latency as the scalp positivity. This activity was not seen in responses to frequent non-target stimuli and was not dependent on an overt motor response. Similarly, rare visual stimuli generated a scalp P300 and negative activity subcortically; both scalp and subcortical waves had a longer latency than in the auditory experiment. The reaction time was similarly longer to visual targets. These data are inconsistent with a hippocampal generator for P300, but are consistent with a generator in the thalamus or more dorsally located structures.

Evoked responses to frequency shifted tones: tonotopic and contextual determinants

Event-related potentials were recorded from 13 normal adults in response to a wide range of frequency shifts of a constant amplitude tone, in four separate experiments. Based on the tonotopic organization of the auditory system, we predicted a logarithmic relationship between amount of shift (delta f) and response amplitude. As predicted, the amplitudes of the exogenous N100 and P200 components increased with progressively larger shifts. Log (delta f) predicted 38.6% of the within-subjects variance for N100-P200 amplitude. In addition, an unexpected contextual effect was observed: the largest shift in each experiment tended to produce similar amplitude responses, despite large differences in absolute magnitude of delta f. A two-factor model including a contextual measure of delta f accounted for 48.7% of the variance, indicating that both physical and contextual stimulus parameters determine the amplitude of exogenous components. N100 latency was also influenced by both physical and contextual parameters, whereas P200 latency was almost constant over a wide range of delta f.

Dyslexic children have normal vestibular responses to rotation

We examined the rotational vestibular responses of carefully screened dyslexic and control populations (34 dyslexics and 33 controls). The subject groups had equivalent performance IQs but differed significantly on verbal IQ and on silent and oral reading. Children with significant neurologic, visual, or hearing deficits were excluded. We measured eye movements provoked by sinusoidal rotation of the subjects (in total darkness) at low frequencies (0.01 to 0.16 Hz). Gain, phase, and preponderance (asymmetry) of the responses were calculated from the eye velocity and stimulus velocity waveforms. There were no differences between the groups in any of these measures. We conclude that there are no clinically measurable differences in this aspect of vestibular function in our carefully selected populations of dyslexic and control children.

Tracking eye movements are normal in dyslexic children

We measured saccadic and smooth eye tracking movements in 34 dyslexic and 35 control subjects and were unable to show differences in any of the tasks or parameters measured. Subjects were carefully screened for neurological, optometric, audiological, and behavioral problems before admission to the study. We suggest that differences between our results and those of previous investigators are based on sampling differences. We conclude that while there may be children with reading disorders secondary to visual or oculomotor deficits, dyslexia can and does exist independent of such deficits. Because, in this and other studies, we have ruled out peripheral involvement, we feel that dyslexia is primarily a deficit of central information processing of written or spoken material.

Predictive eye movements do not discriminate between dyslexic and control children

Pavlidis has suggested that predictive saccadic tracking eye movements distinguish between reading disabled and normal children. We have examined saccadic eye movement tracking in 34 dyslexic and 33 control subjects, between 10 and 12 yr old. The subjects were equivalent in performance IQ (WISC-R), with differences between verbal and performance IQ of less than 30 points. Children with neurological, visual or auditory deficits or histories of emotional problems and hyperactivity were excluded. There were no differences between the groups on the eye movement tracking task when scored objectively by computerized analysis of the eye movement records, or by ranking of the records by an experienced observer. Our results (and those of other recent studies) fail to support Pavlidis' contention that eye movements hold the key to dyslexia.

EEG patterns during 'cognitive' tasks. I. Methodology and analysis of complex behaviors

This paper presents a methodology which uses nonlinear pattern recognition to study the spatial distribution of EEG patterns accompanying higher cortical functions. The multivariate decision rules reveal the essential EEG patterns which differentiate performance of two tasks. Cross-validation classification accuracy measures the generality of the findings. Using this method, EEG patterns were derived from a group of 23 adults during performance of several complex tasks, including Koh's block design, writing sentences, mental paper folding, and reading silently. These patterns discriminate between the tasks, are consistent with, and extend the results of, visual EEG interpretations and univariate analysis of spectral intensities. Since writing sentences could not be distinguished from mere scribbling, it is unclear whether the EEG patterns found to distinguish complex behaviors were related to the cognitive components of tasks, or to sensory-motor and performance-related factors.

Electroencephalogram correlates of higher cortical functions

By means of two-stage, nonlinear multivariate pattern recognition, electroencephalograms (EEG's) were analyzed during performance of verbal and spatial tasks. Complex scalp distributions of theta-, beta-, and, to a lesser extent, alpha-band spectral intensities discriminated between the two members of a pair of tasks, such as writing sentences and Koh's block design. Small EEG asymmetries were probably attributable to limb movements and other uncontrolled noncognitive aspects of tasks. Significant EEG differences beteeen cognitive tasks were eliminated when controls for inter-task differences in efferent activity, stimulus characteristics, and performance-related factors were introduced. Each controlled task was associated with an approximately 10 percent reduction, as compared with visual fixation, in the magnitude of alpha- and beta-band spectral intensity. This effect occurred bilaterally and was approximately the same over occipital, parietal, and central regions, with some minor difference over the frontal region in the beta band. With these controls, no evidence for lateralization of different cognitive functions was found in the EEG.

Selective regulation of thalamic sensory relay nuclei by nucleus reticularis thalami

Stimulation in the segment of nucleus reticularis thalami adjacent to the lateral geniculate body (RLG), abolished visual evoked potentials for up to 150 msec. Both photic stimulation in the contralateral visual field and electric stimulation in the ipsilateral optic tract elicited primary cortical responses that were markedly reduced or abolished by prior conditioning stimulation in RLG. Stimulation of the segments of nucleus reticularis thalami adjacent to the medial geniculate (RMG) or the ventrobasal complex (RVBC) had the effect of markedly reducing or abolishing unilaterally projected primary evoked responses in the auditory and cutaneous systems, respectively. Only the sensory evoked potentials mediated by the relay nucleus adjacent to the region of R stimulated were affected. The reduction of the cortical evoked potentials was not due to the processes underlying the cortical recovery cycle, because conditioning stimulation on either side of RLG stimulated the primary geniculocortical fibers, but had a minimal or no effect on the visual test evoked response. These results suggest that R functions as a topographically organized inhibitory gate which can regulate the patterns of sensory input from the thalamus to the cortex. The regulatory effects on R by the mesencephalic reticular formation and the mediothalamic-frontocortical system may mediate both generalized and selective control of cortical sensory evoked potentials.

Regulation of slow potential shifts in nucleus reticularis thalami by the mesencephalic reticular formation and the frontal granular cortex

Novel stimuli or electric stimulation of the mesencephalic reticular formation (MRF) produced large positive slow potentials (SPs) in rostral nucleus reticularis thalami (RVA) that accompanied the negative SPs known to occur in frontal cortex. SP durations (20-30 sec) were similar to the periods of unit inhibition that occur in RVA following MRF stimulation. Trains of 8 c/sec medial thalamic stimuli produced phasic negative SPs in RVA similar in duration to the intervals of unit excitation that follow each stimulus pulse. These results suggest that the polarity and duration of the SPs in RVA reflect changes in excitation of the underlying neurons. Direct activation of a specific region of RVA produced complete inhibition of visual cortex responses evoked by optic tract stimuli, a finding which suggests that RVA has an inhibitory action on the thalamus. A tone reinforced by electric shock also elicited SPs in frontal cortex (negative) and RVA (positive). In contrast to the long duration of the MRF- or novelty-elicited SPs, the durations of the conditioned SPs were phasic and were regulated by the tone--shock interval. Bilateral cryogenic blockade of the interconnections between the frontal cortex and medial thalamus abolished SPs of all origins in the frontal cortex. The blockade also abolished conditioned SPs in RVA, but did not affect the MRF-elicited ones. Thus, the subcortical SPs that accompany orienting to novel stimuli are distinct from those which occur during the higher cognitive process of conditioned expectancy and require the integrity of the mediothalamic-frontocortical system.

Regulation of unit activity in nucleus reticularis thalami by the mesencephalic reticular formation and the frontal granular cortex

Recruiting responses and related synchronous activities appear to be mediated by thalamic inhibition originating in nucleus reticularis thalami, a structure jointly regulated by an ascending projection from mesencephalic reticular formation and a descending influence from the frontal cortex. Extracellular unit activity was recorded in the anterior nucleus reticularis thalami (RVA) during recruiting responses, augmenting responses, stimulation of the mesencephalic reticular formation (MRF), and cryogenic blockade of the inferior thalamic peduncle (ITP). During recruiting responses, RVA units responded to medial thalamic (MT) stimulation with prolonged high frequency bursts. Analysis of the post-stimulus time histograms of these responses showed tham to have the same latency, duration, incrementing character, and envelope shape as the phasic thalamic inhibitory postsynaptic potentials (IPSPs) which appear to mediate recruiting responses. Brief stimulation of the MRF, which abolishes recruiting responses and thalamic IPSPs, prevented the response of RVA units to MT stimuli, and inhibited the spontaneous discharge of these units for 20 sec or more. We propose that the desynchronizing effect of MRF activation results from the abolition of thalamic inhibition originating in RVA. Cryogenic blockade of the ITP, which abolishes recruiting responses in the thalamus and cortex, also prevented R units from responding to MT stimuli. This result suggests that the MT activates R units via a thalamo-frontocortico-R pathway and explains the long latency of R bursts and thalamic IPSPs following MT stimulation. R cells that fired prolonged bursts during recruiting responses did not respond during augmenting responses. This result suggests that separate thalamic inhibitory mechanisms are involved in these two types of synchronization.