The
cranial nerves that can be readily tested are the trigeminal
(V), facial (VII), and spinal accessory (XI) (discussed
elsewhere).
Facial (VII)
The facial nerve is examined
by recording the latency and amplitude from a stimulus
at only one site along the course of the nerve. Nerve
conduction velocities are not calculated.
Stimulation site: Place the electrodes behind
the angle of the jaw, with the cathode posterior to
the earlobe and the anode behind. This placement stimulates
the nerve just before its enters the parotid gland.
Alternatively, you may place the cathode over the stylomastoid
foramen and the anode over the mastoid.
Ground: Usually the ground
is placed over the parotid area, but you may place it
on the chin or forehead also.
Recording sites: Place the
active recording over the orbicularis oris at the corner
of the mouth, over the orbicularis oculi on the outer
canthus of the eye, over the frontalis in the forehead,
or over the nasalis muscle on the nasolabial fold. Place
the reference electrode on the nose. Either a needle
or surface electrode may be used for recording.
The facial nerve may be evaluated
differently - by using the blink reflex, which will
be discussed with the trigeminal nerve (below).
Trigeminal
This nerve is evaluated by using
reflex activity and extrapolating information from it.
Blink reflex
A brief review of the anatomy will assure a better
understanding of this study.
As the sensory fibers of the Vth nerve enter the brain
stem, they establish three kinds of synaptic connections
with the VIIth nerve nuclei:
- One, a direct and monosynaptic
with the ipsilateral VIIth nerve nucleus.
- Another, indirect and polysynaptic with the contralateral
VIIth nerve nucleus.
- A third, also polysynaptic, again to the ipsilateral
VIIth nerve nucleus.
These connections are demonstrated
clinically by the fact that when the glabella is lightly
tapped with a reflex hammer or the finger, a brisk blinking
reaction is seen bilaterally. The blink reflex is the
electrical equivalent of this reaction referred to clinically
as the glabellar reflex.
Stimulation of the supraorbital
branch of the Vth nerve as it enters the skull through
the supraorbital foramen will result in contraction
of the orbicularis oculi muscles bilaterally.
The test is best performed by using
two channels on the cathode ray tube to study both sides
simultaneously.
One each side an active electrode
is placed over the orbicularis oculi muscle on the outer
canthus of the eye and the reference on the lateral
aspect of the nose. One ground is used and is placed
over the chin.
The Vth nerve is stimulated via
its supraorbital branch over the supraorbital foramen;
the sweep speed used is 10 msec/division and the gain
set at 200 µv/division. On the ipsilateral channel,
both direct and indirect responses are seen, the direct
of a short latency and mono or biphasic
configuration, the indirect of a long, usually variable,
latency and polyphasic configuration. On the contralateral
channel, only the indirect, long latency polyphasic
response is seen.
Blink Reflex Findings
- In unilateral Vth nerve lesions, all three responses
are equally affected.
- In unilateral VIIth nerve lesions, stimulation on
the same side of the lesion will result in delayed or
absent direct and indirect responses ipsilaterally but
a normal indirect response contralaterally. When the
nerve is stimulated on the healthy side, both the direct
and indirect responses are spared while the contralateral
indirect response is affected.
The blink reflex can be used in
the evaluation of toxic neuropathies and in comatose
patients and multiple sclerosis as a means of evaluating
brain stem functions.
Jaw jerk: This response
is produced by use of a percussion hammer, which triggers
the cathode ray sweep.
Stimulation site: The stimulus
is the percussion of the jaw, elicited as in the clinical
testing of this reflex.
Ground: The ground may be
placed on the chin or the nose.
Recording: Place the active
electrodes over the masseter muscles bilaterally and
the reference electrodes over the nose or forehead.
The response is bilateral, and
it is best to have two active channels on the CRT for
comparison.
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