Blink reflexes in severe traumatic coma

Brainstem dysfunction in critically ill patients

The brainstem conveys sensory and motor inputs between the spinal cord and the brain, and contains nuclei of the cranial nerves. It controls the sleep-wake cycle and vital functions via the ascending reticular activating system and the autonomic nuclei, respectively. Brainstem dysfunction may lead to sensory and motor deficits, cranial nerve palsies, impairment of consciousness, dysautonomia, and respiratory failure. The brainstem is prone to various primary and secondary insults, resulting in acute or chronic dysfunction. Of particular importance for characterizing brainstem dysfunction and identifying the underlying etiology are a detailed clinical examination, MRI, neurophysiologic tests such as brainstem auditory evoked potentials, and an analysis of the cerebrospinal fluid. Detection of brainstem dysfunction is challenging but of utmost importance in comatose and deeply sedated patients both to guide therapy and to support outcome prediction. In the present review, we summarize the neuroanatomy, clinical syndromes, and diagnostic techniques of critical illness-associated brainstem dysfunction for the critical care setting.

Electromyographic evaluation of blink reflex as a tool for early diagnosis of neurological dysfunction in patients of hypothyroidism

BACKGROUND

Neurological dysfunction is an important consequence of hypothyroidism. Some of the neurologic manifestations of hypothyroidism include somnolence, lethargy, impaired memory and concentration, depression and entrapment neuropathy. Rarer but reversible neurological manifestations include cerebellar ataxia, psychosis, dementia and myxedema coma.

PURPOSE

The aim of the present study was to evaluate the usefulness of the blink reflex as a method for obtaining an early diagnosis of central nervous system dysfunction in hypothyroid patients who do not have signs or symptoms of nervous system dysfunction.

METHODS

Forty-eight patients with mean age 40 (± 11), with newly diagnosed primary hypothyroidism and 20 healthy control subjects were included in the study. The patients were divided into subclinical and overt hypothyroidism. Patients with normal T3/T4 with elevated TSH were considered subclinical hypothyroids. For blink reflex testing, subjects lay supine on a bed in a warm room with eyes gently closed. Recordings were performed with an EMG machine (Nihon Kohdeni'Neuropack), with a filter setting of 20 Hz to 10 kHz, using an analysis time of 50 ms. Recordings were performed with surface recording electrodes (Dantec 13K60, Copenhagen, Denmark).

RESULTS

Second ipsilateral response (R2I) and second contralateral response (R2C) latencies in hypothyroidism were prolonged relative to controls, and the differences were statistically significant (P<0.001 and P<0.001, respectively). Latency of Rl, R2I, and R2C did not correlate linearly free T3, free T4, or TSH values in the hypothyroid group.

CONCLUSION

The finding of abnormal blink reflex responses in hypothyroid individuals raises the notion that they may be useful in detecting early changes and in the follow-up of the patients with the disorder.

Methodological considerations in the neuropsychological study of central nervous system underarousal with a specific emphasis on coma

Several obstacles exist that impede the scientific study of pathological underarousal (stupor and coma). For instance, there is a lack of a clear, rational, standardized taxonomy with which to describe the phenomenon. Moreover, there is considerable confusion of the construct of arousal with other neurobehavioral constructs. The field also suffers from a general lack of acceptably reliable and valid instruments, especially measures of long-term outcome. Additionally, current treatments for chronic pathological underarousal are frequently presumptive and applied haphazardly, and thus do little to elucidate the process of recovery. It is suggested that biomedical and neuropsychological approaches to the study and treatment of pathological underarousal are complementary and that it is useful to view pathological underarousal as a behavioral, as well as a medical, phenomenon.

Effect of the inhibitory phenomenon following magnetic stimulation of cortex on brainstem motor neuron excitability and on the cortical control of brainstem reflexes

We studied the effect of the inhibitory phenomenon following transcranial magnetic stimulation (TMS) on brainstem motor neuron excitability and on the cortical control of brainstem reflexes in 5 normal subjects. The R1 component of the blink reflex assessed excitability of the facial nucleus during the period of post-TMS inhibition (experiment 1). TMS effect on the cortical control of brainstem reflexes was assessed by delivery of a stimulus that was less than or equal to the threshold for the R1 with the subject relaxed, yet elicited a potentiated reflex during maneuvers requiring cortical input (i.e., voluntary eye-closure and anticipation, experiment 2). In experiment 1, a suprathreshold electric stimulus applied during post-TMS inhibition elicited an R1 response that was preserved in amplitude (mean 476 +/- 44 microV; 123%) compared to control responses not preceded by TMS (mean 386 +/- 31 microV). Conversely, in experiment 2, when the R1 was dependent on cortical drive, the mean R1 amplitude during post-TMS inhibition was only 15% (P < 0.01) and 8% (P < 0.01) of control values obtained during eye-closure and anticipation, respectively. These findings imply that the inhibition following TMS originates rostral to the facial nucleus, at a cortical level.

Effects of general anesthesia on the human blink reflex

The blink reflex was studied in 10 patients undergoing elective procedures under general anesthesia. Anesthetic agents were isoflurane, halothane, nitrous oxide, methohexital, and thiopental in various combinations. At induction, blink reflexes were diminished by low-dose thiopental (1-1.5 mg/kg) and abolished by high-dose thiopental (4-8 mg/kg) and methohexital (1.5 mg/kg). Blink reflexes were absent during halogenated volitile inhalational anesthesia and did not return until patients were in the recovery room, well after end-tidal anesthetic levels were zero by mass spectroscopy. Recovery of consciousness and the ability to blink on command often preceded return of any blink reflex activity, indicating that the blink reflex is not useful as a postoperative test of facial nerve function in the operating room after anesthesia. In six patients, blink reflexes were still diminished 2-3 hours after cessation of anesthesia, at a time when patients were fully oriented and corneal and eyelid reflexes were clinically normal. This finding suggests that the blink reflex might be a sensitive test of subtle CNS dysfunction after inhalational anesthesia and potentially could serve as a useful objective indicator of recovery from anesthesia for outpatient surgery.

Electrically elicited blink reflex and early acoustic evoked potentials in circumscribed and diffuse brain stem lesions

In the present paper, the function of the brain stem in patients with brain stem lesions of various aetiology is investigated with electrophysiological methods. The clinical observations are supplemented by experimental investigations on cats, in which the blink reflex and the early acoustic evoked potentials were registered during the acute elevation of intracranial pressure. The findings in patients with circumscribed space-occupying lesions in the posterior fossa document that the registration of the BR and the BAEP have a functional diagnostic significance above and beyond the neurological and radiological investigation. In the case of the cerebellar space occupations, specific alterations could not be observed. On the contrary, the alterations of BR and BAEP indicate a general disturbance of brain stem function, possibly as a result of a general increase of intracranial pressure. In cerebellopontine angle tumours, both BR and BAEP showed specific alterations which were usually asymmetrical. The BR changes ipsilateral to the tumour are of major topodiagnostic significance, whereas the alterations of the contralateral potential are especially informative in the registration of BAEP. The alterations of BR and BAEP also allow an appraisal of the localization and extent of the lesion in primary space occupations in the brain stem: A pathological R1 indicates a pontine lesion, whereas pathological R2 responses are found in medullary and in oral pontine and mesencephalic lesions. In contrast to cerebellopontine angle tumours, the BAEP tends to show symmetrical alterations in primary brain stem lesions. The prolongations of interpeak latencies correspond to the brain stem segment concerned, and the same also applies to pathological amplitude reduction and deformations of individual potentials. In patients with localized brain stem damage, the reflex pathway of R2 is discussed on the basis of the BR findings. In contrast to the view held up to now that only structures situated caudal of the facial nucleus area are responsible for the genesis of the R2 response, it is assumed on the basis of our own observations that pontomesencephalic structures rostral to the facial nuclei are also important for the genesis of R2. Registration of BR and BAEP in patients with acute diffuse brain stem damage shows that both methods have a high diagnostic and prognostic value. Isolated damage and combined brain stem lesion can be demonstrated and the course can be followed up. Normalization of pathological findings reflects clinical recovery, and conversely a secondary deterioration indicates the presence of complications.(ABSTRACT TRUNCATED AT 400 WORDS)

Evoked potentials and brain stem reflexes

The recording of evoked potentials (EPs) has become one of the most useful functional diagnostic techniques in the neurosciences during recent years. In combination with the neurophysiological investigation of brain stem reflexes (BSRs) EPs provide good information concerning circumscribed and diffuse brain and brain stem lesions. In this article the role of EPs and BSRs will be pointed out with special regard to their use in neurosurgery concerning awake and comatose patients as well. Pathological findings caused by extracerebral factors or due to neurological (systemic) and otological diseases will be discussed from the aspect of differential diagnosis only. Evoked potentials described in this paper are short latency potentials which are related to more or less defined generators in the peripheral and central nervous system. EPs of long latency seem to play a role in cognitive, affective and integrative functions of the central nervous system and they will not be dealt with in this article. In recent years an increasing number of review articles and monographs dealing with several aspects of evoked potentials has appeared [16, 17, 18, 25, 69, 80, 84, 122, 123, 124, 127, 130].

Effect of increased intracranial pressure on the blink reflex in cats

During an acute increase in ICP produced by balloon inflation three different phases could be observed. In the first phase (ICP 40-50 mm Hg) the latencies of R 1 and R 2 showed an initial decrease followed by increase in latency. In the second phase (ICP 50-70 mm Hg) R 2 disappeared, whereas R 1 showed marked alterations, prolongation of the latency and duration, and a decreasing amplitude. In the third phase no response could be evoked. The pathophysiological observations correlated with the morphological alterations. Two ischaemic zones with BBB damage which "transsected" the mesodiencephalic and pontomesencephalic border were found. The results suggest that the disappearance of R 2 depends on rostral damage, whereas alteration of R 1 is caused by a pontomesencephalic lesion.

Some observations on the blink reflex in posttraumatic coma

Blink reflex studies were carried out on 51 comatose patients with signs of brain stem impairment due to head injury. Twenty-five patients were studied in acute coma on day 1 or 2 after trauma. Twenty-six patients were studied in prolonged coma during days 3-12 after brain injury. Brain stem involvement was divided clinically and by CT scan into secondary lesions due to supratentorial mass displacement and primary lesions due to direct violence to the brain stem. Further, the comatose states were separated by behavioural and EEG signs of sleep into "sleep' and more "alert' states. The blink reflexes were recorded after mechanical and electrical stimulation. Mechanical stimulation appeared to be more effective in evoking late responses than the electrical stimulus. The elicitability of the late responses was also dependent on the time of stimulation. During "sleep', usually accompanied by a spindle-EEG, the late responses were totally absent after both mechanical and electrical stimulation in acute coma. In more "alert' states, usually accompanied by high voltage delta waves in the EEG, the late responses, especially direct R2, could be frequently elicited by mechanical stimulation. These differences decreased during prolonged coma. All patients with signs of secondary brain stem involvement who had at least one late component in the acute stage of coma recovered well. So did all patients, with one exception, with primary brain stem injuries. Early recovery of the blink reflexes in cases of primary brain stem injury was a further favourable sign. Consistent absence of R2 (and R1) in prolonged coma indicated a bad outcome. The presence of all components in prolonged coma was of less prognostic significance than in the acute stage.

The acoustic blink reflex: stimulus dependence, excitability and localizing value

The acoustic blink reflex was examined in 26 subjects with open eyes and 20 subjects with closed eyes by means of an electrooculogram. Amplitude and excitability are highly dependent on the loudness of the stimulus used and on the opening or closure of the eyelids. Particularly if the eyes are closed, it may be necessary to use a stimulus of high (105-110 dB SPL) intensity (hand-clapping is often not loud enough). If an acoustic blink reflex can be elicited, it can be assumed that certain pontine and mesencephalic structures are intact.