Download principles and techniques of the examination of the trigeminal nerve

TRIGEMINAL NERVE AND ITS CENTRAL CONNECTIONS
charge (an internal efference copy of the motor command
[106]), and eye muscle proprioception (107,108). The reader
is referred to Steinbach’s review (107), in which he summarizes the possible role of eye muscle proprioception in (a)
maintence of stability in conjugacy and fixation; (b) specification of visual direction; (c) development of some aspects
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of normal visual function; and (d) depth perception, vergence, and other binocular functions. In the final analysis, it
is most likely that the muscle afferents in human extraocular
muscles provide no conscious perception of eye position but
are certainly involved in subconscious control and monitoring of muscular contractions (109,110).
PRINCIPLES AND TECHNIQUES OF THE EXAMINATION OF THE TRIGEMINAL
NERVE SYSTEM
Because the trigeminal nerve is mainly a sensory afferent,
sensory testing remains the mainstay of the clinical examination. However, a thorough understanding of the efferent
motor pathways and trigeminal reflexes allows the examiner
to evaluate the various functions of the trigeminal system
more effectively, leading to more accurate neuroanatomic
localization. In addition, trigeminal nerve function should
be examined within the context of its three divisions.
TESTS OF CUTANEOUS SENSATION
The sense of touch may be easily tested with a small wisp
of cotton or the edge of a tissue. One may also use a light
brush of the fingertips against the skin of the face. If reliability is in doubt, the patient should be asked to close the eyes
and then indicate each touch. Although the most sensitive
test is to compare the sense of light touch on one side with
the other, this method frequently leads to spurious information. The nasal mucosa is highly sensitive to the lightest
touch, and the comparison of the ‘‘nasal tickle’’ response
may also be useful. Because the gingiva is also innervated by
the trigeminal nerve, testing inside the mouth may produce
useful information. In particular, patients with nonorganic
sensory loss often do not note sensory changes inside the
mouth. One may use the sharp end of a broken tongue blade
to check gingival sensation, again comparing one side with
the other. It is imperative to check all three branches of the
trigeminal nerve bilaterally, as they may be differentially
affected, depending on the location of the lesion.
Pain sensation is estimated by response to pinpricks. If
an area of decreased response or subjectively blunted sensation is found, it can be outlined by proceeding from the
region of blunted sensation outward, noting the borders of
normal sensation. As is observed in plotting relative defects
in the visual field, there is often a graduated cutaneous sensory loss with margins that vary with the degree of stimulation. Consequently, heavier pinpricks are likely to show a
smaller field of sensory loss than are light pinpricks. When
an area of suspected decreased sensation is found, it should
be compared directly with the equivalent area of skin on the
other side of the body. The use of the Wartenberg wheel, a
pinwheel that applies a constant stimulus by rolling over the
skin, can no longer be recommended because of universal
precautions that need to be followed to avoid the transmission of blood-borne illness, such as hepatitis C or the acquired immune deficiency syndrome (AIDS). Many clinicians employ a single-use straight pin or safety pin to test
pain sensation, but there are disposable plastic-tipped sharp
devices designed specifically for this task.
Sensations of heat and cold may be tested with glass tubes
containing comfortably hot and cold water. With the patient’s eyes closed, the skin is lightly touched by tubes alternately while the patient indicates if the sensation is hot or
cold. Another convenient method is to use the coolness of
a metal examination tool, such as a tuning fork or ophthalmoscope handle, to compare the sense of cold between the right
and left sides.
Vibratory testing is frequently used to detect nonorganic
sensory findings. Because vibration sense is carried by bone,
there should be very little difference between a stimulus
applied to the left of midline and the same stimulus applied
to the right of midline, especially over the frontal area. When
patients report inability to feel vibration on one side of the
forehead, it usually implies a nonorganic component to the
exam.
TESTS OF CORNEAL SENSATION
Assessment of response to corneal pain stimulation is
made by both subjective report and objective observation.
The patient’s report of differences in sensation after equal
stimulation of the two corneas may indicate unilateral reduction in ophthalmic division sensory function; however, only
the alert, relatively intelligent, and comfortable patient can
contribute this information reliably. More often, the examiner must rely upon objective signs, such as involuntary blink
reflex and head withdrawal. These are of particular value in
examination of the obtunded or semicomatose individual.
Both depth of coma and difference in corneal sensitivity can
be assessed by the patient’s involuntary response to corneal
stimulation. It should be noted that long-time contact wearers may have a substantial reduction in response to corneal
stimulation, but this should be symmetric.
Corneal sensitivity varies widely among normal persons.
Clinically, a mild depression in corneal sensitivity must be
judged by comparison of sensitivity between the two corneas. Before attributing such a difference to a trigeminal
nerve lesion, one must be satisfied that neither unilateral
ocular disease (e.g., herpetic keratitis) nor weakness of one
orbicularis oculi muscle (e.g., Bell’s palsy) is responsible
for a difference in response between the two eyes. When
testing corneal sensitivity in the presence of unknown or
suspected unilateral eyelid weakness, the comparison should
be made between blink responses of the normally innervated
eyelids when one cornea, then the other, is alternately stimulated.
A small piece of clean cotton or the end of a cotton-tipped
applicator drawn to a fine point can serve as a useful corneal