Special Neurologic Tests and Procedures

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Chapter 61

Special Neurologic Tests and Procedures

J. Stephen Huff

Neuro-otologic tests and procedures are used in a variety of clinical scenarios ranging from evaluation of a dizzy patient to diagnosing brain death. Some of these procedures have become less common with the advent of advanced neuroimaging and electrophysiologic testing; nonetheless, their simplicity and effectiveness give them continued relevance, especially in practice environments with limited resources.¹

Caloric Testing

Accurate assessment of a comatose patient requires a thorough neurologic examination, including careful evaluation of the patient’s responses to a variety of external stimuli. In a comatose individual with normal brainstem and cranial nerve function, stimulation of the vestibular labyrinth results in well-described and reproducible extraocular movements. This response is known as the vestibulo-ocular reflex (VOR) and forms the physiologic basis for caloric testing. Caloric testing (calor from Latin, meaning heat) involves delivering a thermal stimulus to the external auditory canal to activate the labyrinth. Pathologic conditions involving either the vestibular or the oculomotor reflex pathways will alter or abolish the usual response to caloric stimulation.

Caloric tests may be performed in either conscious or unconscious patients, depending on the diagnostic circumstances. Quantitative caloric examination is conducted in awake ambulatory patients for evaluation of possible vestibular dysfunction. This type of testing requires precisely controlled irrigation temperatures and specialized recording devices and is best undertaken in a properly equipped laboratory under the supervision of an experienced neuro-otologist. However, a neurologist, neurosurgeon, or emergency clinician may perform qualitative caloric testing in a comatose patient to detect gross disruption of the VOR pathways that may indicate structural lesions or metabolic abnormalities affecting the labyrinth, vestibulocochlear nerve, or brainstem. In this clinical setting, ice water is used to provide stimulation of the vestibular system. Such testing needs no special expertise and can be done at the bedside with equipment readily available in the emergency department (ED). This simple procedure can provide valuable diagnostic and prognostic information necessary for the management of a comatose patient.

Brown-Séquard ² first described the effects of introducing cold water into the ear canal in the mid-19th century. The clinical importance of the phenomenon was first realized in 1906 by Bárány,³ who developed a caloric procedure involving an ice water stimulus. He postulated, correctly, that caloric stimulation of the auditory canal induces the formation of convection currents within the semicircular canals of the vestibular labyrinth.

Standardization of the procedure awaited introduction of the Fitzgerald-Hallpike technique in 1942. This technique used both warm and cool water stimuli under rigidly specified conditions, which permitted quantification of normal and abnormal caloric responses. Today, most formal caloric testing of conscious patients is based on variations of the original Fitzgerald-Hallpike procedure.⁴

The value of caloric testing in the assessment of comatose patients was emphasized by the work of Klingon ⁵ and Bender and associates.⁶ Electronystagmography (ENG) can provide a graphic record of reflex eye movements and permit precise determination of the intensity of the caloric response.

Physiology and Functional Anatomy

Proper performance and interpretation of a caloric test requires a basic understanding of the structure and function of both the vestibular and oculomotor systems. The anatomic pathways underlying the VOR begin in the posterior portion of the labyrinth of the inner ear. The peripheral vestibular apparatus is located within the temporal bone and consists of the utricle, the saccule, and the lateral, anterior, and posterior semicircular canals. Because of its proximity to the external ear canal, the lateral or horizontal canal is of principal interest in caloric testing. The lateral canal is oriented at a 30-degree angle to the horizontal plane. Deflections of the cupola because of movement of endolymphatic fluid within the canal result in changes in polarization in the underlying hair cells, which in turn are relayed to the afferent limb of the primary vestibular neuron.

Impulses of the primary neuron travel via Scarpa’s ganglion and cranial nerve VIII to the brainstem and synapse with secondary vestibular neurons in the superior and medial vestibular nuclei of the upper medulla and lower pons (Fig. 61-1). Although the connections between the vestibular and oculomotor nuclei in the brainstem are quite complex, two main pathways exist. The direct projection runs from the vestibular complex to the nuclei of cranial nerves III and VI via the medial longitudinal fasciculus (MLF) and involves only three neurons: (1) primary vestibular, (2) secondary vestibular, and (3) oculomotor. The indirect projection between these nuclei occurs over multisynaptic circuits in the tegmental reticular formation. Another brainstem structure that contributes to the vestibulo-ocular interaction is the paramedian pontine reticular formation (PPRF), a poorly characterized group of pontine neurons that coordinate both voluntary and involuntary lateral gaze. The PPRF receives multiple inputs. Including projections from the vestibular system and the contralateral frontal cortex, and sends output to oculomotor neurons through both direct and indirect pathways. Excitatory impulses originating in the lateral canal finally travel via the oculomotor and abducens nerves to the ipsilateral medial rectus and contralateral lateral rectus muscles.⁷

Figure 61-1 Vestibulo-ocular reflex and its contribution to horizontal eye movements. The semicircular canals respond to rotational acceleration of the head by driving the vestibulo-ocular reflex to maintain the eyes in the same direction in space during head movement. Fibers from the horizontal semicircular canal travel first to the vestibular nuclei and then to each paramedian pontine reticular formation. Excitatory projections that travel to the contralateral sixth cranial nerve nucleus and, via the medial longitudinal fasciculus, to the ipsilateral medial rectus subnucleus cause gaze to the left. In a similar manner, inhibitory projections are sent to the antagonist ipsilateral lateral rectus and contralateral medial rectus. (Adapted from Lavin PJ, Donahue SP. Disorders of supranuclear control of ocular motility. In: Yanoff M, Duker JS, eds. Ophthalmology. 3rd ed. St. Louis: Mosby; 2008.)

Rotation of the head generates flow of endolymphatic fluid within the semicircular canals. The firing rate of the primary vestibular neuron is dependent on the direction of flow. For example, in the lateral canal, flow toward the ampulla (ampullopetal) increases the firing rate, whereas flow away from the ampulla (ampullofugal) decreases the firing rate. Increased firing on one side results in conjugate deviation of the eyes toward the opposite side, whereas decreased firing causes deviation to the same side. This principle forms the physiologic basis of caloric testing. When the lateral canal is in the vertical position (patient placed supine) and ice water is infused into the ear, the endolymph nearest the canal cools and sinks, which results in ampullofugal flow (Fig. 61-2). As the firing rate decreases, 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.

Figure 61-2 Convective flow mechanism of the caloric response. Irrigation with warm or cold water (or air) results in a temperature gradient across the horizontal semicircular canal. With the horizontal canal oriented in the earth-vertical plane, gravity induces convective flow of endolymph from the cooler area of the canal, in which endolymph is more dense, into the warmer area of the canal, in which endolymph is less dense. For the warm caloric irrigation shown in this diagram, an ampullopetal deflection of the cupula results from this flow of endolymph. Vestibular nerve afferents innervating the horizontal semicircular canal are excited, and a horizontal nystagmus with slow-phase components directed toward the opposite ear is produced. A cold caloric stimulus results in an oppositely directed response, with ampullofugal deflection of the cupula, inhibition of horizontal canal afferents, and nystagmus with slow-phase components directed toward the ear to which the cold caloric stimulus is applied. (Adapted from Baloh RW, Honorubia V, eds. Clinical Neurophysiology of the Vestibular System. 3rd ed. Philadelphia: Davis; 2001.)

The eye movements induced by caloric stimulation in conscious, neurologically normal individuals are more complex. 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. This is known as caloric nystagmus. By convention, caloric nystagmus is named for the fast component, thus the popular mnemonic “fast COWS” (Cold irrigation—Opposite beating nystagmus; Warm irrigation—Same-sided beating nystagmus). Most sources attribute the slow phase of nystagmus to vestibular activity transmitted over the direct pathway, whereas the fast phase is believed to be generated by the PPRF in conjunction with cortical activity and carried over indirect pathways within the reticular formation. Numerous factors, including physiologic, pharmacologic, and pathologic, can alter caloric-induced eye movements.

Second Phase of Interpretation

After irrigation, ocular movements should be observed for any response to the stimulus. Again, 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. 61-3). The first reaction, caloric nystagmus with the fast component beating away from the side of ice water irrigation, is seen in normal, alert individuals, in cases of psychogenic unresponsiveness, and in those who have very mild organic disturbances in consciousness. The intensity of nystagmus is highly variable in conscious subjects and depends on the degree of visual fixation and the level of mental alertness. 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.¹³

Figure 61-3 A, The four types of caloric responses seen with unilateral and bilateral irrigation. 1, Normal nystagmus. 2, Conjugate deviation. 3, Disconjugate deviation. The most common type, internuclear ophthalmoplegia, is shown here. Vertical eye movements usually remain intact in this lesion. 4, Absent caloric responses. MLF, medial longitudinal fasciculus. B, Oculocephalic and oculovestibular testing in patients with selected clinical conditions. (A, Modified from Plum F, Posner JB, eds. The Diagnosis of Stupor and Coma. 3rd ed. Philadelphia: Davis; 1980:55; B, from Smith M, Bleck T. Techniques for evaluating the cause of coma. J Crit Illness. 1987;2:51.)

Caloric-induced nystagmus after cold water irritation in an apparently comatose individual usually occurs when testing individuals with psychogenic unresponsiveness as a result of catatonia, conversion reactions, schizophrenia, or feigned coma. Hyperactive caloric responses may result from testing in the presence of tympanic perforation or mastoid disease. Hypoactive caloric responses are recorded in patients with a wide variety of vestibular and neurologic disorders.

Hypoactive or abnormal caloric responses may be further evaluated with quantitative caloric testing, ENG, or other techniques such as auditory evoked responses. Caloric-induced nystagmus may be inverted (beating to the wrong side) or perverted (beating in the wrong plane); both responses may be seen with brainstem lesions. Pseudocaloric nystagmus is a preexisting latent nystagmus that is brought out by the general arousal of ice water irrigation; it can be distinguished from true nystagmus by its failure to reverse direction with warm water irrigation.

As the level of coma deepens, the fast phase of nystagmus becomes intermittent and then disappears, probably because of decreased activity in the cerebral cortex.

In the second type of response to cold caloric stimulation, the eyes deviate conjugately toward the side of ice water stimulation (they “look” toward the source of irritation). When present, this reaction indicates intact brainstem function, as well as intact afferent and efferent limbs of the reflex. This is seen during general anesthesia, in supratentorial lesions without brainstem compression, and with many metabolic and drug-induced comas. In such situations, bilateral simultaneous irrigation with ice water might result in conjugate downward deviation, thus implying that the brainstem centers for vertical gaze are functional.

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 MLF causes weakness or paralysis of the adducting eye after caloric irrigation. Internuclear ophthalmoplegia might be due to acute damage to the rostral pons or could be seen 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 brainstem and impending herniation. Caloric responses of this type are less common with metabolic and drug-induced coma; when present in metabolic coma, their significance is less ominous. Reversible internuclear ophthalmoplegia has been reported in patients with hepatic coma and may occur during toxic responses to phenytoin, barbiturates, or amitriptyline. Forced downward deviation of the eyes, either conjugate or dysconjugate, may be seen in sedative-hypnotic drug–induced coma when unilateral caloric testing is performed.¹⁴

Palsies of the oculomotor nerves are another cause of dysconjugate reactions, although most should be apparent before irrigation. Causes include diabetic neuropathy, increased intracranial pressure, and Wernicke’s encephalopathy. Finally, Plum and Posner ¹⁰ reported that unusual and poorly characterized caloric responses may be obtained when testing comatose patients with long-standing severe brain injury.

Absent caloric response is the fourth category of reactions to ice water stimuli. As a general rule, the VOR is preserved more than other brainstem reflexes; however, the oculocephalic, or doll’s eye, response may persist in the absence of caloric responses. Loss of caloric responses in comatose patients with structural lesions is usually a sign of brainstem damage. With supratentorial lesions, progressive loss of caloric responses may be seen in the final stages of transtentorial herniation. The VOR may also be transiently absent or decreased on the side opposite massive supratentorial damage during the first hours after injury.¹⁵ 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 brainstem, 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 brainstem.

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).16,17 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% of the ENG clinical population in two large series of patients.18,19

The VOR has prognostic as well as diagnostic significance in comatose patients. In a study of 100 patients who were comatose as a result of head trauma, absence of caloric responses 1 to 3 days after injury was associated with extremely high mortality.²⁰ Testing in the immediate posttraumatic period may yield inconsistent responses and is of considerably less prognostic value. Levy and coworkers²¹ studied 500 cases of nontraumatic, non–drug-induced coma in a large multicenter study. Absence of the VOR correlated with less than a 5% chance of achieving functional recovery within 1 year when tested within 6 to 24 hours of the onset of coma. In one study of comatose patients, the combination of absent VOR and absent pupillary light reflex at 24 hours was associated with 100% mortality.²² Complete loss of caloric responses is part of the criteria for the diagnosis of brain death and correlates with irreversible cessation of cerebral function at least as well as an isoelectric electroencephalogram (EEG) does.²³ Excessive reliance on a single clinical sign must be avoided during consideration of brain death and in decisions regarding neurologic prognosis, and therapy should be based on complete evaluation of all the evidence available (see the later section “Brain Death Testing”).

Summary

Caloric testing is a simple, easily performed bedside procedure that may enhance the neurologic assessment of comatose patients. When reliably interpreted, caloric testing may furnish valuable diagnostic and prognostic information. Even if the cause of the coma is known, the test may provide a baseline for the evaluation of changes in the patient’s status. In the ED this test should be reserved for stable patients undergoing secondary assessment. The examination requires minimal equipment and can be conducted in a few minutes while awaiting the results of laboratory testing or neuroimaging. Complications are few if patients are properly selected and correct technique is used.

DIX-Hallpike Test For The Diagnosis of Positional Vertigo

Vertigo occurring only and repeatedly with change in position is probably benign paroxysmal positional vertigo (BPPV); the head-hanging positioning maneuver (Dix-Hallpike test, sometimes referred to as the Nylen-Báràny maneuver) is useful in confirming clinical suspicion of BPPV because the abnormal nystagmus provoked is characteristic of the disorder.24,25 In BPPV, it is thought that calcium crystal material displaced from the vestibule floats within the endolymph of the posterior semicircular canal. Head movement induces bidirectional forces in the fluid acting on the cupula that trigger the attack of BPPV.^26–29 The posterior semicircular canal is most commonly affected,^25,28 but at times the horizontal canal is thought to be involved, thereby leading to variants of typical BPPV.^30–32

Background

BPPV is a common mechanical disorder of the inner ear in which vertigo is precipitated by certain head movements; nystagmus and autonomic symptoms such as nausea and vomiting commonly accompany the vertigo. Although patients may have quite dramatic findings with severe autonomic symptoms, the actual episodes of vertigo are extremely brief and typically last less than 1 minute. Syndromes of positional vertigo and provocative maneuvers have been described by clinicians for more than 100 years; the description of BPPV has been attributed to Adler, Báràny, Nylèn, Bruns, Borries, Dix, and Hallpike.³³ Dix and Hallpike most fully defined the disorder, and the provocative technique that they described is superior; thus it most accurately should bear the eponym Dix-Hallpike positioning test or the Dix-Hallpike test.^25–33 Rarely, paroxysmal positional vertigo from a central cause has also been described in patients with small cerebellar hemorrhages.³⁴

Indications and Contraindications

The Dix-Hallpike test may be a useful diagnostic test in confirming BPPV by provoking a specific type of nystagmus and in selecting patients suitable for positional therapy (discussed later). The maneuver should not be performed on patients with severe cervical spine disease, unstable spinal injury, high-grade carotid stenosis, or unstable heart disease.²⁵ Patient discomfort and physical infirmity are relative contraindications; some elderly patients or those with ongoing nausea or vertigo may not tolerate the changes in body position necessary for performance of the procedure. If nystagmus is present at rest without any provocation or if associated neurologic signs or symptoms exist, the diagnosis of BPPV is probably excluded and the maneuver is not clinically indicated.

Procedure

The Dix-Hallpike maneuver is illustrated in Figure 61-4. Place the patient initially in the seated position on the stretcher with the head turned 45 degrees to one side. With the patient instructed to keep the eyes open and focused on the examiner, quickly lay the patient down flat with the head hanging over the edge of the bed and observe the eyes for induced nystagmus. Repeat the entire maneuver with the head turned 45 degrees toward the opposite side.²⁵