Brain stem function

The brain stem reflexes are particularly important in helping to identify those lesions that may affect the ARAS, explain the reason for coma, and potentially help in assessing the prognosis. The following reflexes are predominantly related to the eyes and the pattern of respiration.

1. Pupillary reactions. The size, equality, and reaction of the pupils to light is recorded. Unilateral dilatation of the pupil with loss of the light response suggests uncal herniation or a posterior communicating artery aneurysm. Midbrain lesions typically cause loss of the light reflex with mid-position pupils, whereas pontine lesions cause miosis but a retained light response. Fixed dilatation of the pupils is an indication of central diencephalic herniation and may be differentiated from the fixed dilatation due to atropine-like agents by the use of pilocarpine eye drops which cause miosis if the dilatation is as a result of loss of parasympathetic innervation but are ineffective if it is pharmacological. A Horner's syndrome may be seen ipsilateral to a lesion in the hypothalamus, thalamus, or brain stem, when it will be associated with anhydrosis of the ipsilateral side of the body. It can also be due to disease affecting the wall of the carotid artery when anhydrosis will only affect the face.15 Hepatic or renal failure and other forms of metabolic coma may make the light reflexes appear unduly brisk and the pupils therefore relatively small. Most drug intoxications tend to cause small and sluggishly reactive pupils, and pontine haemorrhage will cause pinpoint pupils due to parasympathetic stimulation (Figure 1.3).16

2. Corneal responses. The corneal reflexes are usually retained until coma is very deep. If these responses are absent in the patient who is otherwise in a light coma then the possibility of a drug induced coma or of local causes of anaesthesia to the cornea should be considered. The loss of the corneal response when drug overdose is excluded is a poor prognostic sign.

3. Eye movement. The resting position of the eyes and the presence of spontaneous eye movements should be noted. Conjugate deviation of the eyes raises the question of an ipsilateral hemisphere or contralateral brain stem lesion. Abnormalities of vertical gaze are less common with patients in coma but depression of the eyes below the meridian may be seen with damage at the level of the midbrain tectum and in states of metabolic coma. The resting position of the eyes is normally conjugate and central but it may be dysconjugate when there is damage to the oculomotor or abducens nerves within the brain stem or along their paths.

Roving eye movements seen in light coma are similar to those of sleep. They cannot be mimicked and their

/ Diencephalic Small reactive

Tectal Large fixed

Tectal Large fixed

Pontine Pinpoint

/ Diencephalic Small reactive

Figure 1.3 Pupillary changes

Pontine Pinpoint

\ Third nerve Fixed dilated ptosed

Figure 1.3 Pupillary changes presence excludes psychogenic unresponsiveness.14 Periodic alternating gaze or "ping-pong" gaze is a repetitive conjugate horizontal ocular deviation of uncertain aetiology.17 Spontaneous nystagmus is rare in coma as it reflects interaction between the oculovestibular system and the cerebral cortex. Retractory nystagmus in which the eyes jerk irregularly back into the orbits and convergent nystagmus may be seen with midbrain lesions.18 Ocular bobbing, an intermittent jerking of downward eye movement, is seen with destructive lesions in the low pons and with cerebellar haematoma or hydrocephalus.19 4. Reflex eye movements. These are tested by the oculocephalic and oculovestibular responses (Figure 1.4). The oculocephalic, or "doll's head", response is tested by rotating the patient's head from side to side and observing the eyes. In coma with an intact brain stem the eyes will move conjugately and in a direction opposite to the head movement. In a conscious patient such a response can be imitated by deliberate fixation of the eyes but is not common. In patients with pontine depression the

Figure 1.4 Reflex eye movements. (a) Normal doll's eye; (b) tonic oculovestibular response; (c) dysconjugate oculovestibular response; (d) absent oculovestibular response

oculocephalic response is lost and the eyes remain in the mid-position of the head when turned. The oculovestibular response is more accurate and useful. It is elicited by instilling between 50 and 200 ml of ice cold water into the external auditory meatus. The normal response in the conscious patient is the development of nystagmus with a quick phase away from the side of stimulation. A tonic response with conjugate movements of the eyes towards the stimulated side indicates an intact pons and suggests a supratentorial cause for the coma. A dysconjugate response or no response at all indicates brain stem damage or depression. Both ears should be stimulated separately. If unilateral irrigation causes vertical eye movements the possibility of drug overdose arises because many drugs affect lateral eye movement.

The value of oculovestibular testing in patients without lateralising eye signs is considerable because it identifies not only the intactness of the brain stem and corticopontine connections but also may reveal the presence of an intrinsic brain stem lesion by causing dysconjugate eye posturing. In addition, it is the definitive way of identifying patients in psychogenic coma who will show normal nystagmus and frequently be distressed by the manoeuvre. 5. Respiration. Modern techniques of assisted respiration and the need to examine patients in intensive care units where respiration is controlled complicates the assessment of normal respiratory function. If the patient is seen before respiration is controlled, the presence of long cycle periodic respiration suggests a relatively high brain stem lesion; central neurogenic hyperventilation implies a lesion at the level of the upper pons; and short cycle periodic respiration, which carries a poor prognosis, is seen with lesions lower in the brain stem. In general the presence of regular, rapid breathing correlates with pulmonary complications and a poor prognosis rather than with site of neurological disease in patients in coma.20

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