Vascular pathology of the brain stem due to experimentally increased intracranial pressure: changes noted in the micro-and macrocirculation

Brain stem blood flow, pupillary response, and outcome in patients with severe head injuries

OBJECTIVE

Acute pupillary dilation in a head-injured patient is a neurological emergency. Pupil dilation is thought to be the result of uncal herniation causing mechanical compression of the IIIrd cranial nerve and subsequent brain stem compromise. However, not all patients with herniation have fixed and dilated pupils, and not all patients with nonreactive, enlarged pupils have uncal herniation. Therefore, we have tested an alternative hypothesis that a decrease in brain stem blood flow (BBF) is a more frequent cause of mydriasis and brain stem symptomatology after severe head injury. We determined the relation of BBF to outcome and pupillary response in patients with severe head injuries.

METHODS

One hundred sixty-two patients with a Glasgow Coma Scale score of 8 or less underwent stable xenon computed tomographic blood flow determination at the level of the superior colliculus, and this blood flow was correlated with pupillary features, intracranial pressure, computed tomographic scan pathology, and outcome.

RESULTS

A BBF of less than 40 ml/100 g/min was significantly associated with poor outcome (P < 0.009). In patients with bilaterally nonreactive pupils, the BBF was 30.5+/-16.8 ml/100 g/min, and in those with normally reactive pupils, the BBF was 43.8+/-18.7 ml/100 g/min (P < 0.001). Intracranial pressure and the presence of a brain stem lesion observed on the computed tomographic scan did not correlate with BBF, pupillary size, or reactivity. Unfavorable outcome at 12 months was directly related to age (P = 0.062) and inversely related to pupillary responsiveness (P = 0.0006), pupil size (P = 0.005), and BBF of less than 40 ml/100 g/min (P = 0.009).

CONCLUSION

These findings suggest that pupillary dilation is associated with decreased BBF and that ischemia, rather than mechanical compression of the IIIrd cranial nerve, is an important causal factor. More important, pupil dilation may be an indicator of ischemia of the brain stem. If cerebral blood flow and cerebral perfusion pressure can be rapidly restored in the patient with severe head injury who has dilated pupils, the prognosis may be good.

Outcome of brain stem auditory electrophysiology in children who survive purulent meningitis

Brain stem auditory electrophysiology was assessed in children long after purulent meningitis by examining the central components of brain stem auditory evoked response (BAER) at different presentation rates of clicks. Children with peripheral hearing loss were excluded from this study to avoid any possible influence of the loss on the measurements of BAER central components. Of the 70 children who had recovered from meningitis 2 to 8 years earlier, 9 (12.9%) showed abnormalities in BAER central components--mainly a slight reduction of wave V amplitude, at the click presentation rate of 10/s. Of the remaining 61 children who did not show any obvious abnormalities in the BAER at 10/s, an abnormally prolonged I-V interval and a reduced amplitude of wave V were seen in 5 children (8.2%) at the click rate of 90/s. The total BAER abnormality rate was therefore 20.0% (14/70 cases). The results suggest that most children who survive purulent meningitis have a favorable outcome for the brain stem auditory pathway, with about 1 in 5 having mild dysfunction or a suboptimal outcome in the pathway.

Brainstem auditory evoked potential in Japanese encephalitis

Japanese encephalitis (JE) is associated with varying degrees of coma and brainstem involvement is frequent which can be evaluated and monitored by brainstem auditory evoked potential (BAEP). The present study has been undertaken to evaluate the BAEP changes and their role in predicting the outcome. Twelve adult patients with JE were subjected to CT scan, MRI and BAEP studies after detailed neurological evaluation. The severity of coma was assessed by Glasgow coma scale and outcome was defined at the end of 3 months into good and poor recovery on the basis of Barthel Index score (BI). The mean age of the patients was 28.3 years (range 14-50), and four of them were females. Most of the patients were comatose. The mean Glasgow coma scale (GCS) score was 7 (range 4-11). There were no brainstem signs or cranial nerve palsy. Cranial CT scan revealed thalamic hypodensity in four, whitematter oedema in three and left putaminal hypodensity in one patient. Cranial MRI was carried out in eight patients which revealed bilateral thalamic lesions in all, basal ganglia and midbrain lesions in three each and pontine and cerebellar lesions in one patient each. Brainstem auditory evoked potentials were recordable bilaterally. The absolute latency of wave I, II, III, IV and V and interpeak latencies (IPL) of I-V, III-V, and I-III were normal. The V/I amplitude ratio were significantly reduced in five patients. The BAEP abnormalities correlated with brainstem lesions on CT or MRI but not with severity of coma or outcome. The reduced amplitude ratio of wave V/I may be due to raised intracranial tension or brainstem involvement in JE.

Serial somatosensory and brainstem auditory evoked potentials in monitoring of acute supratentorial mass lesions

OBJECTIVE

To determine the relevance of serial evoked potentials (brainstem auditory evoked potentials and somatosensory evoked potentials) and clinical parameters (pupillary response and intracranial pressure) in patients with acute supratentorial mass lesions.

DESIGN

Prospective case series of comatose patients with acute supratentorial mass lesions.

SETTING

Neurocritical care unit of a tertiary care center.

PATIENTS

Thirty consecutive patients with the following study inclusion criteria: a) clinical and computed tomography evidence of an acute supratentorial mass lesion; b) implantation of an intracranial pressure monitoring device; and c) a persistent comatose state during the observation period.

INTERVENTIONS

Brainstem auditory evoked potentials, somatosensory evoked potentials, intracranial pressure, and pupillary responses were recorded at the time of three particular events: a) immediately after implantation of an epidural intracranial pressure monitoring device; b) during intracranial pressure therapy; and c) at termination of intracranial pressure therapy. Evoked potential results were ranked into three categories: a) normal on both sides; b) abnormal or absent on one side; and c) evoked potentials on both sides abnormal or absent. Spearman's rank correlation was performed to analyze serial recordings. Cross tables were generated to determine the prognostic value of evoked potentials and clinical parameters. Fisher's exact test was applied to calculate statistical significance.

MEASUREMENTS AND MAIN RESULTS

Intracranial pressure values correlated with pupillary responses and brainstem auditory evoked potentials during and at the termination of intracranial pressure therapy. Pupillary findings correlated with brainstem auditory evoked potentials only at the time of termination of intracranial pressure therapy. There was no correlation between somatosensory evoked potentials and clinical parameters. Pupillary responses indicated a good or poor recovery during and at the termination of intracranial pressure therapy. Brainstem auditory evoked potentials and intracranial pressure values distinguished between good and poor outcome only at termination of intracranial pressure therapy. Somatosensory evoked potential results did not predict outcome.

CONCLUSIONS

Shortly after manifestation of supratentorial mass lesions, the results of evoked potentials and clinical parameters indicate increased intracranial pressure and incipient transtentorial herniation but do not predict sequelae. Our results indicate that after institution of effective therapy, pupillary abnormalities and brainstem auditory evoked potentials serve as valuable prognostic predictors. In contrast, somatosensory evoked potentials reflect neither therapeutic efficacy nor outcome in our patient population.

The effects of an extradural expanding lesion on regional intracranial pressure, blood flow, somatosensory conduction and brain herniation: an experimental study in baboons

Intracranial pressure (ICP) differences, change of local blood flow (CBF) using the hydrogen clearance technique, change in the somatosensory evoked potential (SEP) to median nerve stimulation and pupillary size were investigated during progressive elevation of the ICP (using an extradural balloon) in 6 anaesthetized baboons. CBF was measured in the frontal cortex, somatosensory cortex, thalamus (nucleus ventralis posterior lateralis-VPL), medial lemniscus (ML), lateral lemniscus (LL) and caudate nucleus (CN). Conduction along the somatosensory pathway between C2 at the neck and VPL was compared with conduction between VPL and primary somatosensory cortex. The amplitude of the cortical SEP was also studied. ICP gradients between hemispheres developed as the pressure was increased to in excess of 50 mm Hg. CBF was significantly reduced from control in the cortex and VPL on the side ipsilateral to the balloon at 50 mm Hg ICP. A significant decrease in ML flow occurred bilaterally at 70 mm Hg ICP. Conduction time was increased significantly between the right VPL and cortex at a pressure of 50 mm Hg. The amplitude of the cortical response was significantly reduced at 30 mm Hg on the right side and 50 mm Hg on the left. Aniscoria occurred at 50 mm Hg ICP and the pupils became dilated at 70 mm Hg. The SEP was possibly more sensitive than the pupillary reactions as an indication of tentorial herniation in these experiments.

Sequential changes of auditory brain stem responses in relation to intracranial and cerebral perfusion pressure and initiation of secondary brain stem damage

The relationship of supratentorial intracranial pressure (ICP) and cerebral perfusion pressure (CCP) with serial changes in auditory evoked brain stem responses was investigated. Eighty-one patients without primary brain stem damage admitted to our emergency unit were studied. When ICP over 50 mm Hg persisted for 4 hours, the I-V interpeak latency was significantly prolonged. The threshold of this prolongation was 8 hours for the ICP over 45 mm Hg and 24 hours for that of over 40 mm Hg. The ICP of 35-40 mm Hg for 24 hours was the border zone. CCP did not show a significant relation with I-V interpeak latency changes. The loss of wave V was observed in a wide range of the ICP (30-147 mm Hg) and CPP (0-60 mm Hg). Wave III disappeared when the ICP exceeded 50 mm Hg. Wave I became undetectable with an ICP above 50 mm Hg or a CPP below 40 mm Hg. These results indicate that an increase of ICP over 40 mm Hg definitely initiates secondary brainstem dysfunction if it lasts for more than 24 hours and that the ICP should be reduced below this level, preferably below 35 mm Hg, to maintain brain function. The fact that both low CPP and high ICP were involved in the loss of wave I clearly shows that both ischaemia and displacement of the brain stem are the important pathophysiological factors for the disappearance of wave I.

Auditory brain stem responses in infants with posthaemorrhagic ventricular dilatation

Nineteen infants with posthaemorrhagic ventricular dilatation had auditory brain stem responses measured during the period of maximal ventricular dilatation. These showed various patterns ranging from normal, through various abnormalities, to complete absence of responses. When serial auditory brain stem responses were studied in parallel with the evolution of posthaemorrhagic ventricular dilatation it was seen that the abnormalities of auditory brain stem response usually resolved irrespective of the persistence or progression of ventricular dilatation. No correlation was found between cerebrospinal fluid pressure and prolonged interpeak intervals on the auditory brain stem response. In three patients with posthaemorrhagic ventricular dilatation improvement in the auditory brain stem response occurred when cerebrospinal fluid was withdrawn. Intermittent withdrawal of cerebrospinal fluid (by ventricular tap or lumbar puncture) in two of these infants was followed by improvement in the auditory brain stem response after a period of 24 hours (but not sooner). In one infant born at full term improvement in the auditory brain stem response was noted one week after shunting.

Quantitative analysis of reversible dysfunction of brain-stem midline structures caused by disturbance of basilar artery blood flow with the auditory brain-stem responses

To clarify the effects of disturbances in basilar artery blood flow, basilar artery angiospasm was induced in 2 cats and 4 guinea pigs and auditory brain-stem responses (ABRs) were continuously recorded preceding, during and following the angiospasm. The angiospasm caused specific ABR changes in that waves II (P2-N2) and III (P3-N3) were attenuated without any corresponding amplitude reduction of P4. Those changes were equivalent following stimulation of either ear. Moreover, the ABR changes gradually recovered within 5 h. On the basis of the animal experiments, 52 patients with subarachnoid hemorrhage, supratentorial tumor showing increased intracranial pressure or hydrocephalus were selected for repeated ABR examinations. ABR abnormalities similar to those observed in the animal experiment were obtained especially from the patients exhibiting grade 3 or 4 symptomatology with subarachnoid hemorrhage. In these cases, the wave III to wave IV-V amplitude ratio was significantly decreased. In some cases the ABR abnormalities and the wave III to wave IV-V amplitude ratio recovered as the clinical course improved. These results support the conclusion that specific ABR changes (wave III to wave IV-V amplitude ratio) reflect transient ischemic dysfunction of the midline portion of the brain-stem caused by disturbances of basilar artery blood flow.

Effects of an expanding supratentorial mass on the auditory brain-stem responses in baboons

This study was carried out to investigate the effects of an expanding supratentorial mass on auditory brain-stem responses (ABRs). A balloon was inserted into the supratentorial epidural space of seven baboons (in two cases, in the right occipital area; in five cases, in the right temporal area). The balloons were inflated at a rate of 0.2 ml/minute to increase intracranial pressure (ICP). ICP (right frontal epidural pressure) and blood pressure (BP) were continuously recorded. Recordings of ABRs (vertex to mastoid on both sides) were made serially. Pupillary changes were also recorded. At 30 mmHg ICP, the amplitude of wave V on the right side was observed to be significantly attenuated (p less than 0.02). At 50 mm Hg ICP, the latency of wave V on the right side was significantly prolonged compared with that at 30 mm Hg ICP (p less than 0.02). At 70 mmHg ICP, significantly decreased amplitude of wave V on the left side was also observed (p less than 0.02, from the control), associated with significant increased latency of wave IV on the right side (p less than 0.01, from the control; p less than 0.05, from 50 mm Hg ICP). Finally, waves IV and V on both sides substantially disappeared at 100 mm Hg ICP. Anisocoria appeared in four animals at 30-50 mm Hg (mean +/- SD; 45 +/- 8.7) ICP. The amplitude of wave V was significantly decreased in these circumstances (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Regional cerebral blood flow and CSF pressures during the Cushing response induced by an infratentorial expanding mass

An experimental study was carried out in eight dogs to investigate whether the Cushing response (CR) during intracranial hypertension is due to pressure per se, tissue distortion, or ischemia in the brain stem. To minimize the effects of rostrocaudal displacement, intracranial pressure was raised by an expanding mass lesion located in the posterior fossa. Regional cerebral blood flow (rCBF) was measured with radioactive microspheres and compartmental cerebrospinal fluid (CSF) pressures were recorded during the CR which was induced by the continuous inflation at a constant rate of an infratentorial epidural rubber balloon in two groups of four dogs. In one group (A) rCBF was measured at the onset of the CR and in the other group (B) at the peak of the systemic blood pressure rise. In the animals of group A blood flow in the mesencephalon, pons and upper medulla oblongata was reduced from control values by 32%, 57% and 85% respectively. In group B blood flow in the same areas did not differ significantly from pre-inflation values. In contrast, the recorded balloon volume, which was assumed to be an index of mechanical distortion of the brain stem, varied considerably at the beginning of the blood pressure rise (from 2.5 to 4.7% of the calculated intracranial space). Similarly, CSF pressure in the posterior fossa at the onset of the CR also varied considerably (from 52 to 117 mmHg). Thus, the large quantitative variations meant that both absolute pressure and tissue distortion were poor predictors of the onset of the CR. The findings suggest that ischemia, rather than brain stem distortion per se or pressure by itself, is responsible for the initiation of the CR. The rise in blood pressure elicited during the CR seems capable of restoring blood flow in the brain stem back to control values.

Auditory brain-stem responses in hydrocephalic patients

Auditory brain-stem response (ABR) was measured in 40 patients (80 ears) with confirmed hydrocephalus. Eighty-eight percent of these patients showed some form of ABR abnormality. Responses indicative of brain-stem dysfunction consisted of prolonged I-V interwave latency (38%), reduced V/I amplitude ratio (33%), and abnormalities in wave-shape of components III (27%) and V (53%). In addition, 70% of the patients had elevated ABR thresholds; 45% had responses in excess of 20 dB HL and the remaining 25% had no ABR activity. The etiology of the hydrocephalus, head circumference and brain-stem symptoms were not associated with particular ABR abnormalities. Communicating hydrocephalus correlated significantly with both prolonged I-V conduction time and absence of ABR activity, compared with non-communicating hydrocephalus. Four of the 9 patients retested showed ABR improvement on follow-up; one patient showed deterioration. The results were compared to our prior studies of ABR in 60 post-meningitic patients and in 100 severely neurologically impaired institutionalized children in whom the incidence of intrinsic brainstem abnormalities was one-third and two-thirds that of the hydrocephalic group, respectively. The results of this study suggest that ABR can be used to document clinically unsuspected brain-stem pathology that may accompany hydrocephalus. Auditory brain-stem dysfunction is likely to complicate the assessment of hearing sensitivity in hydrocephalic patients.

Acute intracranial hypertension and brain-stem blood flow. An experimental study

This study has been carried out to evaluate the effect of supratentorial mass lesions on the local cerebral blood flow (CBF) of the brain stem. Local CBF of the thalamus, inferior colliculus, and medulla oblongata, and supra- and infratentorial pressure were serially measured in 52 cats with intracranial hypertension produced by supratentorial balloon expansion. The mean control local CBF's in the thalamus, inferior colliculus, and medulla oblongata were 37.5, 42.1, and 30.7 ml/100 gm/min, respectively. At 20 to 30 mm Hg of supratentorial pressure, the local CBF of the thalamus started to decrease, and at 20 mm Hg of infratentorial pressure, the local CBF of the inferior colliculus began to decrease. Finally, at 40 to 60 mm Hg of infratentorial pressure, the local CBF of the medulla oblongata was affected. At the beginning of uncal herniation, indicated by anisocoria, the mean local CBF of the inferior colliculus abruptly decreased from 33.7 to 19.6 ml/100 gm/min in 16 cats. The Cushing response was evoked at a mean supratentorial pressure of 93.4 mm Hg and infratentorial pressure of 49.9 mm Hg in 16 cats. When the systemic arterial pressure was increased to the highest level in 13 cats, the mean local CBF of the medulla oblongata did not show significant change (a decrease from 22.8 to 20.9 ml/100 gm/min). The results suggest that at the beginning of uncal herniation, the local CBF of the upper brain stem markedly decreased. During the Cushing response, the local CBF of the medulla oblongata did not change significantly.

Acute intracranial hypertension and auditory brain-stem responses. Part 2: The effects of brain-stem movement on the auditory brain-stem responses due to transtentorial herniation

Movement of the upper brain stem (inferior colliculus) was correlated with the alterations in the amplitude of wave V of the auditory brain-stem responses (BER's) during supratentorial brain compression in cats. In vivo observation of the brain stem and postmortem inspection show that suppression of the amplitude of BER wave V reflects the extent of caudal displacement of the inferior colliculus. Marked suppression of the amplitude of BER wave V (approximately 30% of control) correlates with the beginning of transtentorial herniation, and complete suppression of the wave V indicates complete transtentorial herniation of the brain-stem and supratentorial structures. The BER wave V is thought to be a sensitive index of caudal movement of the upper brain stem due to transtentorial herniation.

Acute intracranial hypertension and auditory brain-stem responses. Part 1: Changes in the aduitory brain-stem and somatosensory evoked responses in intracranial hypertension in cats

Changes in auditory brain-stem responses (BER's) and somatosensory evoked responses (SER's) were investigated to correlate mass volume, intracranial pressure, and neurological dysfunction in mass-induced intracranial hypertension in cats. As the intracranial pressure was raised by expansion of a supratentorial balloon, the late components of the SER's were suppressed first, followed by the early components of the SER's, then Wave V and Wave IV of the BER's, in that order. This suggests that the nonspecific reticular projections are most vulnerable to compression ischemia, and the specific somatosensory pathways are the next most vulnerable. Neural activity of the auditory pathways in the upper brain stem was also gradually suppressed, but less so than that of the somatosensory pathways. After complete transtentorial herniation, in spite of immediate mass evacuation, the function of the somatosensory pathways was greatly impaired, often irreversibly. The neural activity of the auditory pathways in the upper brain stem revealed progressive recovery during a 3-hour period. The measurements of BER Wave V is thought to be useful in predicting transtentorial herniation.

Release of antidiuretic hormone during mass-induced elevation of intracranial pressure

There are complex osmotic and non-osmotic factors regulating release of antidiuretic hormone (ADH). A wide variety of intracranial pathological processes may trigger ADH release sufficient to produce clinically recognizable hyponatremia, or the "inappropriate ADH syndrome." We systematically studied one non-osmotic trigger, namely mass-induced elevated intracranial pressure (ICP). Initial experiments established baseline data in normal rhesus monkeys: anesthetized animals displayed appropriate rises and falls in immunoreactive urinary ADH in response to intravenously administered hypertonic and hypotonic infusions. Next, ballon catherters were implanted subdurally over temporal lobes and the animals were allowed to recover. The final experiment consisted of anethetizing the animals, monitoring arterial blood pressure and blood gases, and retrieving timed urinary specimens while continuously recording ICP during infusion-pump expansion of the subdural ballon. A nonlethal and a lethal series of ballon-expansion experiments were done. Control values of urinary ADH were 783 +/- 125 muU/15 min, and ICP was less than 10 mm Hg. During nonlethal mass expansion ADH output rose of 3433 +/- 269 millimicronU/15 min while ICP averaged 65 mm Hg (measured at completion of mass expansion). While the mass was maintained, hypotonic infusion produced unchanged urinary ADH output of 3452 +/- 277 muU/15 min. During lethal experiments, urinary ADH rose still higher to 4339 +/- 1887 muU/15 min associated with ICP averaging 100 mm Hg. We concluded that there is a direct relationship between the magnitude of ICP and the amount of ADH release, and that during elevated ICP the ADH release is not suppressed by hypotonic infusion.

Dynamic changes in regional CBF, intraventricular pressure, CSF pH and lactate levels during the acute phase of head injury

The authors measured regional cerebral 133xenon (133Xe) blood flow (rCBF), intraventricular pressure (IVP), cerebrospinal fluid (CSF) pH and lactate, systemic arterial blood pressure (SAP), and arterial blood gases during the acute phase in 23 comatose patients with severe head injuries. The IVP was kept below 45 mm Hg. The rCBF was measured repeatedly, and the response to induced hypertension and hyperventilation was tested. Most patients had reduced rCBF. No correlation was found between average CBF and clinical condition, and neither global nor regional ischemia contributed significantly to the reduced brain function. No correlation was found between CBF and IVP or CBF and cerebral perfusion pressure (CPP). The CSF lactate was elevated significantly in patients with brain-stem lesions, but not in patients with "pure" cortical lesiosn. The 133Xe clearance curves from areas of severe cortical lesions had very fast initial components called tissue peaks. The tissue peak areas correlated with areas of early veins in the angiograms, indicating a state of relative hyperemia, referred to as tissue-peak hyperemia. Tissue-peak hyperemia was found in all patients with cortical laceration or severe contusion but not in patients with brain-stem lesions without such cortical lesions. The peaks increased in number during clinical deterioration and disappeared during improvement. They could be provoked by induced hypertension and disappeared during hyperventilation. The changes in the tissue-peak areas appeared to be related to the clinical course of the cortical lesion.

Effect of systemic arterial blood pressure on cerebral blood flow in intracranial hypertension

In five baboons and 11 cats cerebral ischaemia was produced either by inflating an epidural balloon and or by ligating major arteries supplying the brain. Fifteen of the animals developed intracranial hypertension after cerebral ischaemia. If ICP were high, but still significantly lower than MABP, elevation of MABP by noradrenaline infusions was accompanied by a proportional increase of ICP. However, the increase of ICP was lower than that of MABP so that CPP was raised. CBF measured by the 133Xenon clearance technique was significantly increased by arterial hypertension in eight cases. The proportional increase of CPP and CBF by elevation of arterial blood pressure was substantially greater, the lower ICP was immediately after ischaemia. There was no effect of MABP in cases in which ICP equalled MABP.