Directed Neurological Examination Sensory Function Of Cranial Nerve V

When assessing the trigeminal system, it is important to perform a directed and assiduous neurological examination because subtle alterations in function may provide clues to detect potentially serious neurological disease. Sensory and motor components should be tested separately, comparing right and left sides, and light touch, pin, and temperature sensation should be tested individually in V1 to V3. The cornea and sclera should be inspected for evidence of keratitis, which may be suggestive of diminished corneal (trigeminal) sensation or lacrimation, as well as the facial skin, oropharynx, teeth, gums, and nares for evidence of inflammatory, infectious, or neoplastic disorders. When considering trigeminal neuralgia or, less often, post-herpetic neuralgia, trigger points that elicit pain should be identified (see later). Localizing signs from involvement of the neuraxis in close proximity to the affected trigeminal region may be helpful. For example, other cranial neuropathies such as ocular motor dysfunction or facial weakness and Horner's syndrome may provide clues as to the lesion locus in combination with the trigeminal symptoms. In general, lesions that affect branches of V1 to V3 distal to the gasserian ganglion result in highly focal, circumscribed sensory loss within one division or subdivision of the trigeminal nerve. In contrast, pathological processes affecting the ganglion itself typically cause ipsilateral hemifacial sensory dysfunction. Lesions within the brain stem that affect the individual trigeminal nuclei may give a distinct picture of dissociated sensory loss in which pin, temperature, and light touch sensory modalities are differentially affected.

Sensory examination should assess pain, temperature, light touch, and vibration as well as checking the corneal and jaw jerk reflexes. Painful stimuli can reliably be delivered with a non-reusable pin applied evenly and gently to the skin (.Fig 10-2 A through 10-2C). Two-point discrimination testing, especially on the lips, can be useful. Although temperature discrimination can best be assessed with glass tubes of hot or cold water, in clinical practice, touching the face with a tuning fork and asking if it feels cold is effective. Light touch can be tested with a cotton wisp or tissue paper stroked gently against the skin. Vibration may be assessed with a 125-Hz tuning fork held against the frontal bones, maxilla, and mandible.

It is important when performing a sensory examination to avoid suggesting or directing the patient into desired

Figure 10-2 Sensory examination of the trigeminal nerve. Testing pain sensation in the cutaneous distribution) V1, (B) V2, and C) V3 by lightly touching the skin with a pin. Light touch and temperature modalities should also be assessed in the same distributions.

responses and to be vigilant for the malingering or hysterical patient. Thus, a conservative way to test sensation is to ask the patient to respond yes if present or no if not and then to determine a more subjective difference between the two responses. Comparisons should be made with the contralateral face. It is especially important to pay attention to upper cervical dermatomal distributions when assessing sensation along the angle of the jaw because there may be close overlap between sensory loss from a lesion affecting V3 and cervical spinal nerves of C2 to C3. Similarly, the distribution of the sensory fibers to the upper face and into the scalp merges near the vertex with fibers of the greater occipital nerve, which innervate the posterior scalp. The overlap in the midline of V1 to V3 cutaneous sensory fields is helpful in determining a malingering or hysterical patient. Thus, in patients complaining of hemifacial sensory loss, it is important to determine whether the pattern of loss abruptly stops at the midline or if there is a subtle gradation of loss proceeding from the affected to the normal side. In the former case, nonorganic causes should be suspected. By testing vibratory sensation on the frontal bone, malingering or hysterical patients can often be identified as those who report a difference in sensation despite the physiological impossibility of differences in vibratory sensation when essentially applying the tuning fork to the same albeit "right" or "left" bone. Similarly, loss of sensation on the forehead, which ends abruptly at the hairline, may suggests a nonorganic etiology, although facial sensory loss from leprosy affects facial cutaneous regions, which have a cooler temperature, but spares fibers at the hairline where the skin is warmer (personal communication, MJ Brown).

MOTOR FUNCTION OF CRANIAL NERVE V

Evidence of trigeminal motor involvement such as muscle atrophy, spasm, or fasciculations; jaw deviation; difficulty chewing; and hyperacusis may be present. Atrophy of the temporalis is often easy to observe. To assess muscle bulk, the masseter may be pinched between the fingertips and the temporalis muscle should be palpated as the patient opens and closes the mouth ( Fig 10-3 A). The strength of jaw opening ( Fig 10-3 B), closure, and lateral deviation should be assessed because jaw closure is very strong and difficult to evaluate clinically, and subtle changes may not be easily detected.

If it is difficult for the patient to open the mouth against resistance, some degree of facial motor weakness is likely

Figure 10-3 Motor examination of the trigeminal nerve. A, palpation of the masseter muscles to assess muscle bulk, which may be diminished in lesions affecting the trigeminal motor nucleus or trigeminal nB, Assessing strength of jaw opening is clinically easier than jaw closure.

to be present. Weakness of the pterygoids resulting from nuclear or nerve lesions may produce ipsilateral jaw deviation ( .Fig, 10-3 ). Lower motor neuron processes involving the trigeminal motor nucleus such as motor neuron disease may produce fasciculations in association with muscle weakness and atrophy. One useful sign is the presence of masseteric spasm and contracture in association with ipsilateral hearing loss, facial numbness, diminished corneal reflex, and paretic facial muscles in patients with tumors infiltrating the dorsal pontine tegmentum. y , y

TRIGEMINAL REFLEXES

The corneal reflex is a reliable measure of afferent trigeminal V1 and efferent facial seventh nerve fibers (a V-VII reflex) and is present at infancy. Lightly touching the cornea with a tissue or cotton swab induces a rapid bilateral blink reflex (.Fig, 10-4 A). Touching the sclera or eyelashes, tapping the glabellar regions (glabellar blink reflex), presenting a light flash, or stimulating the supraorbital nerve induces a less rapid but still reliable response. Anatomically, afferent corneal V1 fibers may synapse within the spinal trigeminal nucleus as well as the main sensory nucleus. Short projection neurons project bilaterally to the facial nuclear neurons, which in turn project to the orbicularis oculi muscles. The latency of this response is between 25 and 72 msec and increases with advancing age. A brief latency difference exists between the stimulated and contralateral side of less than 6 msec. Extensive electrophysiological studies have characterized this reflex in great detail y in a number of neurological disorders. The corneal reflex may be slowed in various disorders affecting the trigeminal nerve, ganglion, or brain stem nuclei including posterior fossa and cerebellopontine angle tumors such as acoustic

Figure 10-4 Trigeminal reflexesA, The corneal reflex is elicited by lightly touching the cornea with a tissue or cotton swa, The jaw jerk reflex may be elicited by gently tapping a finger placed on the jaw. Both of these reflexes may be brisk in supranuclear processes but diminished in nuclear or peripheral lesions.

neuromas, multiple sclerosis, brain stem strokes (especially in Wallenberg's syndrome), and Parkinson's disease. Rarely, a delay in the corneal reflex has been reported ipsilateral to a hemispheric lesion. y

The jaw jerk reflex may indicate dysfunction of afferent sensory or efferent motor V3 fibers. Afferent axons for the jaw jerk reflex are from stretch receptors (muscle spindles) within the masseter, temporalis, and medial pterygoid muscles, which project centrally to the mesencephalic nucleus and induce a rapid, single jaw closure (jerk) mediated via motor trigeminal neurons that project back to these muscles. The jaw jerk is elicited by tapping the chin when the mouth is closed and the jaw relaxed (...Fig 10-4 B). Other variations include tapping the thumb placed on the chin or a tongue blade placed on the lower teeth. Peripheral or brain stem nuclear processes that affect V3 neurons attenuate the jaw jerk reflex, whereas in lesions involving supranuclear trigeminal motor projections, exaggeration of the jaw jerk may be identified. Other trigeminal reflexes include the oculocardiac, corneomandibular, snout, and trigemino-abducens reflexes. y

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