Prognosis in Coma Related to Severe Head Injuries

NEUROLOGIC EXAMINATION

It is important to carry out careful physical and neurologic examinations. Evaluation of the patient’s spontaneous limb and bulbar movements, pupillary reactions, eye movements, and response to painful stimuli usually indicates the level of brain lesion causing coma. If the patient is able to blink, yawn, lick, and swallow, which are complex brainstem reflexes, lower brainstem function is preserved.

Pupillary size depends on the balance between sympathetic function (descending sympathetic fibers course in the lateral brainstem tegmentum) and parasympathetic function (parasympathetic fibers exit with the oculomotor [cranial nerve III] in the midbrain).

Pupillary reaction depends on the afferent light stimulus reaching the superior colliculus, as well as efferent transmission through the oculomotor nerve. The light reflex arc is located in the diencephalon and midbrain.

Eye movements are observed by retracting the upper eyelids and watching spontaneous activity. When the head is rotated to one side—a maneuver to be performed only when it is clear that the cervical spine is not injured—the eyes should move fully and conjugately in the opposite direction if the appropriate brainstem oculomotor and vestibular centers are preserved (doll’s eye phenomenon, or oculocephalogyric reflex). When the head is moved to the right, the eyes move conjugately to the left; when the head is moved downward, the eyes should roll upward. Ice water introduced into one ear canal with the patient’s head-of-bed elevated to 30 degrees should evoke conjugate eye movements toward the side of the stimulation (vestibulo-ocular reflex). In this position, the horizontal semicircular canal is in a vertical position, and the endolymph falls within the canal, thereby decreasing the rate of vestibular afferent firing. The eyes turn toward the ipsilateral ear, with horizontal nystagmus to the contralateral ear. Horizontal reflex eye movements are controlled by the oculomotor, trochlear (cranial nerve IV), and abducens (cranial nerve VI) nerves and their nuclei; the medial longitudinal fasciculus and parapontine reticular formation (pontine lateral gaze center); and the vestibular nuclei and nerves (cranial nerve VIII). All these structures are located within the pontine tegmentum. Vertical movements are controlled by centers in the rostral midbrain and caudal diencephalon.

Last, spontaneous limb movements should be observed. If absent, then testing for a response to a noxious stimulus is appropriate.

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