Pediatric emergency medicine trisk 0589 0589
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by vascular or neoplastic changes in the midbrain, increased intracranial pressure
(ICP), large anterior midline craniofacial tumors (e.g., nasopharyngeal
carcinoma), otitis media (OM) with involvement of the petrous portion of the
sphenoid (Gradenigo syndrome), and any abnormality that involves the cavernous
sinus. An abnormality of the sixth cranial nerve will cause a reduction in
ipsilateral abduction ( Fig. 28.2 ) resulting in a possible ipsilateral esotropia.
The fourth cranial nerve innervates the superior oblique muscle. It is the only
cranial nerve that completely decussates and has a dorsal projection over the
midbrain. This position renders the fourth cranial nerve particularly vulnerable to
blunt head trauma, one of the most common causes of fourth nerve palsy. The
fourth cranial nerve also has a relatively long intracranial course, which makes it
susceptible to increased ICP and parenchymal shifts caused by cerebral edema. It
also runs through the cavernous sinus. Fourth cranial nerve palsy may be
congenital but asymptomatic for several years during childhood until the brain is
no longer able to compensate. Acquired or congenital palsy of this cranial nerve
causes the eyes to become misaligned vertically (ipsilateral hypertropia). Patients
with congenital fourth cranial nerve paresis compensate by tilting their head to
the ipsilateral side, which allows for a rebalancing of the eye muscles such that
alignment may be achieved. Old photographs may demonstrate this tilt. Facial
asymmetry can also be seen after years of this compensatory tilting. Ophthalmic
consultation is usually needed to differentiate between congenital and acquired
palsy.
The third cranial nerve supplies the remaining four extraocular muscles. It is
involved with downgaze, upgaze, and adduction. Parasympathetic innervation to
the pupil (see Chapter 29 Eye: Unequal Pupils ) and innervation to the eyelid
muscle (levator palpebrae) are also carried in the third cranial nerve. A complete
third cranial nerve palsy results in an eye that is positioned down (from the
remaining action of the unaffected superior oblique muscle) and out (from the
remaining action of the unaffected lateral rectus muscle) with ipsilateral ptosis
and ipsilateral pupillary dilation ( Fig. 28.3 ). Because the third cranial nerve
divides into a superior and an inferior division just as it enters the orbit from the
cavernous sinus and because the fibers to individual muscles are segregated
within the nerve throughout its course, partial third cranial nerve palsies may
occur with or without ptosis and/or pupillary dilation. This may leave the patient
with complex strabismus, which is best left to the ophthalmology consultant. The
differential diagnosis of third cranial nerve palsies is summarized in Chapter 29
Eye: Unequal Pupils .
Muscle Restriction
