Special Neurologic Tests and Procedures

Background

In a comatose individual with normal brain stem and cranial nerve function, stimulation of the vestibular labyrinth results in well-described and reproducible extraocular movements through the vestibulo-ocular reflex (VOR). Pathologic conditions involving the labyrinth, vestibulocochlear nerve, or the oculomotor reflex pathways in the brain stem will alter or abolish the VOR.

Physiology and Functional Anatomy

The anatomic pathways underlying the VOR begin in the lateral semicircular canal of the inner ear. Movement of endolymphatic fluid within the canal results in changes in polarization in the underlying hair cells, which in turn are relayed via cranial nerve VIII (vestibular) to the brain stem ( Fig. 61.1 ).

There are two main pathways between the vestibular and oculomotor nuclei in the brain stem. The direct pathway travels from the vestibular complex to the nuclei of the ipsilateral third cranial nerve (oculomotor) and contralateral sixth cranial nerve (abducens), and ultimately to the ipsilateral medial rectus and contralateral lateral rectus muscles. The indirect pathway occurs over multisynaptic circuits in the tegmental reticular formation.

Rotation of the head generates flow of endolymphatic fluid within the semicircular canals; the direction of flow increases or decreases stimulation of the vestibular neuron, which results in conjugate deviation of the eyes away from or towards the affected side, respectively. This principle forms the physiologic basis of caloric testing. When the lateral canal is in the vertical position (head elevated 30 degrees above horizontal) and ice water is infused into the ear, the endolymph nearest the canal cools and sinks, which results in decreased stimulation of the vestibular neuron, and the eyes deviate conjugately toward the side of irrigation. When warm water is used in the same position or when the canal is inverted 180 degrees, the opposite occurs.

Indications and Contraindications

Caloric testing is infrequently used in emergency department (ED) settings in the United States where advanced neuroimaging is readily available. However, in resource-limited settings, caloric testing at the bedside may assist in the evaluation of comatose patients by differentiating structural, metabolic, and psychogenic causes of unresponsiveness.

Few contraindications exist to caloric testing of an unresponsive patient. An absolute contraindication is the presence of a basilar skull fracture, either documented radiologically or suspected by clinical signs, because of the risk of introducing infection into the central nervous system (CNS) through an associated dural tear. If active bleeding or cerebrospinal fluid otorrhea or rhinorrhea is present in a trauma patient, defer caloric testing and evaluate the patient for a potential basilar skull fracture. Tympanic rupture, hemotympanum, or step-off deformities of the auditory canal may indicate fracture of the temporal bone, which is a contraindication to caloric testing. Signs of active ear infection and perforation of the tympanic membrane are relative contraindications to caloric testing.

Equipment

A 30- or 50-mL plastic syringe is ideal for irrigation. The syringe may be used as is, or a short length of soft plastic tubing (e.g., a butterfly catheter with the needle cut off) may be attached. At least 100 mL of ice water should be available, although larger quantities of cool tap water (<25°C) can be used with similar results if ice is unavailable. Saline or tap water may be used. A small, curved plastic emesis basin is useful to collect water as it drains from the ear canal. Other equipment required includes an otoscope, ear specula, and equipment for removal of cerumen. Towels and a thermometer may be helpful.

Procedure

Inspect the ears before inserting the otoscope. Remove excess cerumen and foreign material. Clearly visualize the tympanic membrane. Leave the ear speculum in the canal as a guide for irrigation. Perform the test with the patient supine and the head and upper part of the body raised to 30 degrees, if possible. Drape the patient with a towel and position a small emesis basin below the ear to collect the outflow of water from the ear. Fill a container with ice water and place it near the bedside.

Fill a syringe and catheter system (without the needle) with 10 mL of ice water and direct the irrigation stream toward the tympanic membrane. Because the goal of qualitative caloric testing is to induce a maximum response, the amount and rate of infusion are not critical. As a general guide, infuse 5 to 10 mL of ice water initially over a period of 5 to 10 seconds. Amounts less than 5 mL may be advisable in suspected cases of light coma or psychogenic unresponsiveness. If no response is noted at first, infuse at least 100 mL before declaring that there is no response. Ask an assistant to hold the patient's eyelids open, which makes it easier to observe for eye deviation. Movement usually occurs after a latency of 10 to 40 seconds, with the response persisting for as long as 4 to 5 minutes. Focusing on a scleral vessel makes small deviations easier to detect. Wait at least 5 to 10 minutes after the eyes have returned to their original position before testing the contralateral ear.

Complications

Carefully selecting equipment can avoid the few complications that are possible with caloric testing. Using plastic syringes and soft catheter tubing instead of needles to irrigate the ear will reduce the risk of perforation of the tympanic membrane or canal wall injury if the patient moves unexpectedly.

As mentioned in the contraindications section, caloric testing should not be performed in patients with possible basilar skull fractures, due to the theoretical risk of meningitis. Other potential complications include otitis media and induction of vomiting with subsequent aspiration. Although ice water irrigation might produce nausea and emesis in awake patients, vomiting with aspiration has not been reported as a complication of caloric testing in comatose patients. Nevertheless, some may prefer to delay testing until the patient's airway is protected.

Interpretation

After irrigation, ocular movements should be observed for any response to the stimulus. Typically, there is a latency of response of 10 to 40 seconds. Reactions to ice water irrigation may be divided into four categories: (1) caloric nystagmus, (2) conjugate deviation, (3) dysconjugate deviation, and (4) absent responses ( Fig. 61.2 ).

The first reaction, caloric nystagmus, is seen in normal, alert individuals. Ice water infusion induces a rhythmic jerking of the eyes that includes a slow deviation toward the irrigated side followed by a quick compensatory saccade toward the midline. In an apparently comatose individual, caloric nystagmus usually signifies psychogenic unresponsiveness due to catatonia, conversion reactions, schizophrenia, or feigned coma, but can also be present in patients with very mild organic disturbances in consciousness. The response is present in more than 90% of children by 6 months of age and declines in magnitude only after the seventh decade of life.

In the second type of response to cold caloric stimulation, conjugate deviation, the eyes deviate conjugately toward the side of ice water stimulation (they “look” toward the source of irritation). When present, this reaction indicates intact brain stem function, as well as intact afferent and efferent limbs of the VOR. This is seen during general anesthesia, in supratentorial lesions without brain stem compression, and with many metabolic and drug-induced comas.

Dysconjugate reactions constitute the third type of caloric response to ice water stimuli. The most common dysconjugate reaction is internuclear ophthalmoplegia, in which a lesion of the medial longitudinal fasciculus causes weakness or paralysis of the adducting eye after caloric irrigation. Internuclear ophthalmoplegia can be due to acute damage to the rostral pons or as a manifestation of multiple sclerosis or stroke. With acute supratentorial lesions, the development of dysconjugate caloric responses is a significant sign that may indicate compression of the brain stem and impending herniation. Caloric responses of this type are less common with metabolic and drug-induced coma. Reversible internuclear ophthalmoplegia has been reported in patients with hepatic coma and may occur during toxic responses to phenytoin, barbiturates, or amitriptyline.

Absent caloric response is the fourth category of reactions to ice water stimuli. Loss of caloric responses in comatose patients with structural lesions is usually a sign of brain stem damage. The VOR may also be transiently absent or decreased on the side opposite massive supratentorial damage during the first hours after injury. Whereas the initial absence of the VOR is generally a poor prognostic indicator, it does not uniformly predict a poor outcome when performed in the first 24 hours after presentation. This has particular relevance in cases of traumatic coma, where testing in the first 24 hours after injury may generate variable responses and is of less prognostic value. Absent caloric responses may occur with any subtentorial lesion that affects the vestibular reflex pathways, including pontine hemorrhage, basilar artery occlusion, cerebellar hemorrhage, or infarction with encroachment on the brain stem, and with any expanding mass lesion within the posterior fossa. Caloric responses may disappear in patients with deep coma resulting from subarachnoid hemorrhage, perhaps because of pressure on the brain stem.

The VOR is usually retained until the late stages of metabolic coma. Nevertheless, caloric responses may be transiently absent in certain types of drug-induced coma, with eventual complete recovery of the patient. The VOR seems to be particularly sensitive to the effects of sedative-hypnotic drugs, antidepressants (e.g., amitriptyline, doxepin), and anticonvulsants (e.g., phenytoin, carbamazepine). As one would expect, neuromuscular blocking agents (e.g., succinylcholine) will abolish caloric-induced ocular movements.

Finally, the caloric response may be absent for reasons other than the neurologic causes responsible for the coma. Inadequate irrigation because of excessive cerumen or poor technique and unilateral or bilateral dysfunction of the peripheral vestibular apparatus must be considered. Bilateral loss of the caloric response (areflexia vestibularis) is uncommon in conscious patients, constituting 1.7% and 0.2% in two large series of patients.

Summary

Caloric testing is a simple, easily performed bedside procedure that may enhance the neurologic assessment of comatose patients. In the ED this test should be reserved for stable patients undergoing secondary assessment. The examination requires minimal equipment and can be particularly useful in settings where access to advanced neuroimaging is limited or delayed. Complications are few if patients are properly selected and correct technique is used.

Background

Brain death is defined as irreversible and complete loss of cerebral and brain stem function with preserved cardiac function. Prior to declaring brain death, the cause of the brain injury should be definitively identified, as some toxicologic syndromes may mimic brain death. The most common scenario where emergency clinicians may be asked to perform a brain death examination is to identify potential organ and tissue donors. It should also be noted that a determination of brain death is not necessary for withdrawal of life-supporting measures. If a cause of coma or severe neurologic injury is identified and a poor prognosis is shared with the family, withholding or withdrawing life-sustaining treatment may be appropriate and formal determination of brain death need not be performed.

Indications and Contraindications

Evaluation for brain death implies that severe CNS dysfunction has been identified, that the cause of the CNS dysfunction is known, and that reversible causes of coma have been confidently excluded. Complex medical issues that may confound the assessment should be considered and ruled out, including severe electrolyte disturbances, hypothermia (defined as a core temperature <32°C), hypotension, drug intoxication or poisoning, and pharmacologic neuromuscular blockade. Neuroimaging studies should be carefully reviewed.

Procedure

The clinical neurologic examination remains the standard for determination of brain death. It typically involves assessment for cerebral function, brain stem reflexes, and respiratory drive. The emergency clinician should be familiar with local practices and policies, which may also require an electroencephalogram, documentation of absent cerebral blood flow by angiography or radioisotope brain scan, or other techniques. The following components of the clinical examination are generally utilized to establish brain death.