Page 137 - Edited - Webster HEAD AND NECK - part 2-Merge PDF

HN 325

upon descending axons from the cerebral cortex. (It is “half” the movement required to shift lateral gaze
from one side to the other.)
Caloric testing: In simple terms, increased tonic vestibular activity in one ear "attracts the eyes' attention",
and "the gaze slowly wanders over to that side in order to see what is happening". Similarly, the eyes are
attracted to look at cold ears: pouring cold (~30°C) water into one external auditory meatus (HN 326)
causes endolymph convection currents in such a direction as to attract the eyes "slowly to investigate the
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intrusion" . (Using warm - ~44°C - water reverses the direction: the eyes slowly and stupidly wander over
to look at the relatively colder but unmolested ear.) In each case, the cortex eventually snatches back the
gaze, but then the process starts again - the vestibular system is a real nag. Clinicians perversely describe
horizontal nystagmus according to the direction of the quick component - which is the normal, cortically
controlled correcting movement - rather than the slow, which is the abnormal (the sluggard labyrinth,
asking the eyes to have a look at it ….). For example, a nystagmus with a fast right and slow left
component is called a right nystagmus. Usually the slow component is towards the pathological side
(come over and see my lesion …. and the eyes sneak over until the cerebral cortex, like a good parent,
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realises what is going on and snatches them back - see HN 326) .
"Doll's Eyes": Disruption of the connection between the medial longitudinal bundle and the cervical cord
or destruction/death of the midbrain (controlling convergent and vertical gaze movements) prevents co-
ordination of head and eye movements and results in the "doll's eyes phenomenon" - the direction of gaze
remains fixed irrespective of the appropriate direction of gaze in relation to the position of the head.

(See also Neuro notes Vol. I, pp. 112-113 for Ménière's disease).

6. THE GLOSSOPHARYNGEAL AND VAGUS NERVES

Pathology of these nerves is fortunately rare. Glossopharyngeal palsy: it becomes impossible to
elicit the gag reflex from the affected side, since the mucous membrane of that half of the posterior third of
the tongue and of the oro-pharynx is anaesthetic. The quality of "bitter" in tastes is also impaired. Death of
the medulla oblongata abolishes the gag reflex - the entire om-pharynx is anaesthetized and the nucleus
ambiguus, innervating the pharyngeal and laryngeal muscles, is destroyed.
"Vaso-vagal" (Stokes-Adams) attacks are syncope (fainting) associated with profound depression
of the heart rate, and thought to be due to some peculiarity of the vagus. Unilateral destruction of the
nucleus ambiguus (or, most commonly, of one superior, or especially the recurrent, laryngeal nerve)
produces paralysis of one vocal fold and causes dysphonia - in this case, a hoarse voice - and, in the case
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of the central lesion, difficulties with swallowing - dysphagia . The paralyzed vocal fold makes it difficult
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to cough properly, since the glottis cannot be closed (see Coughing, Volume I, p. 170) . In addition, the soft
palate is substantially paralyzed on the homolateral side (only tensor veli palatini is supplied by the
trigeminal nerve), so that when the patient is asked to say "AH" while the observer is examining the back of
his or her mouth, the soft palate fails to rise on the side of the lesion (HN 216, Fig. 3). Bilateral laryngeal
paralysis is life-threatening because both vocal folds adduct, shutting off the lower larynx and trachea from
the outside world.

7. THE ACCESSORY NERVE

The cranial accessory nerve derives from the nucleus ambiguus and is motor to the larynx (HN
60). The spinal accessory nerve arises from the upper three or four cervical segments and is motor to the
trapezius and sternocleidomastoid muscles (HN 60; 61). Complete paralysis of one trapezius leads to

3 Note: For caloric testing, the subject lies down with the head inclined at about 30° to the horizontal: this ensures that
the plane of the two lateral semicircular canals is vertical and therefore ideal for allowing endolymph flow
induced by convection. Notice that 'warm' and 'cold' are 37 ± 7°C.
4 Note: This is the easy way to understand and remember the two components of jerk nystagmus. It is, however, an
oversimplification, since vestibulo-ocular nystagmus can be elicited in the absence of cerebral cortex.
5 Note: Neurological dysphagia is relatively rare. Think of pharyngitis!
6 Note: This distinguishes a true neurological dysphonia from an hysterical or assumed one: such patients talk with a
low, hoarse voice, but quite happily cough normally on request.

\NewCMedPhysSc\27 HN 320 DisordCranNerves.