| CRANIAL NERVE IV PALSY SIGNS AND SYMPTOMS
The patient will present with complaints of vertical diplopia, which is
especially manifest as the patient tries to read. There may be an inability to look down
and in. There may also be horizontal diplopia, as a lateral phoria occurs due to the
vertical dissociation. The patient often has a head tilt contralateral to the affected
superior oblique muscle. The chin is often tucked downwards as well. There is frequently
concurrent hypertension and/or diabetes. The patient will present with a hyperphoric or
hypertropic eye on primary gaze. On alternate cover test, the hyper-deviation will
increase in contralateral gaze, reduce in ipsilateral gaze, increase on ipsilateral head
tilt, and decrease on contralateral head tilt. Visual acuity is unaffected and there is
very rarely pain. In bilateral cranial nerve IV palsy, the patient will manifest a
hyper-deviation which reverses in opposite gaze.
PATHOPHYSIOLOGY
The fourth cranial nerve nucleus is located in the dorsal mesencephalon.
From here, the nerve fibers then decussate and exit the brain stem dorsally into the
subarachnoid space. The nerve then courses around the brain to enter the cavernous sinus,
superior orbital fissure, orbit, and innervate the superior oblique muscle. Damage to the
fourth nerve nucleus or its fascicles within the brain stem will give a contralateral
fourth nerve palsy, along with the associated signs of light-near dissociated pupils,
retraction nystagmus, up-gaze palsy, Horner's syndrome, and/or internuclear
ophthalmoplegia. Bilateral fourth nerve palsies are possible as well. The main causes of
damage to the fourth nerve in this area are hemorrhage, infarction, trauma, hydrocephalus
and demyelinization.
The fourth nerve is especially prone to trauma as it exits the brain
stem and courses through the subarachnoid space. In contrast to third nerve palsies within
subarachnoid space, fourth nerve palsies are rarely due to aneurysm. The most common
causes of damage to the fourth nerve in this region are trauma and ischemic vasculopathy.
The most likely result from damage within subarachnoid space is an isolated fourth nerve
palsy.
Due to the large number of other neural structures that accompany the
fourth nerve as it travels through the cavernous sinus and superior orbital fissure, it is
unlikely that the patient will exhibit an isolated fourth nerve palsy due to damage within
these areas. More likely, there will be an associated palsy of cranial nerves III and VI.
Common causes of damage to the fourth nerve in these areas are herpes zoster, inflammation
of the cavernous sinus or posterior orbit, meningioma, metastatic disease, pituitary
adenoma, and carotid cavernous fistula. Trauma to the head or orbit can cause damage to
the trochlea, resulting in superior oblique muscle dysfunction.
MANAGEMENT
A fourth nerve palsy often presents suddenly, but may additionally result
from decompensation of a longstanding palsy. In order to differentiate these two types of
palsies, examine old photographs of the patient. A patient with a decompensated
longstanding palsy will present with a compensatory head tilt in old photos. Further,
patients with decompensated longstanding fourth nerve palsies will have an exaggerated
vertical fusional ability. Longstanding fourth nerve palsies typically are benign and no
further management is necessary.
In the case of complicated fourth nerve palsies, (i.e., those that
present with other concurrent neurological dysfunction), the patient should undergo
neuroradiological studies dictated by the accompanying signs and symptoms. In the case of
isolated fourth nerve palsies caused by recent trauma, the patient should undergo an MRI
or CT scan of the head to dismiss the possibility of a concurrent subarachnoid hemorrhage.
If the fourth nerve palsy is not associated with recent trauma, investigate for a history
of past trauma. If the fourth nerve palsy is due to previous trauma and has recently
decompensated, you can manage the diplopia with vertical prisms.
If the patient is elderly and has a fourth nerve palsy of recent origin,
perform an ischemic vascular evaluation to search for diabetes and hypertension. If the
palsy is caused by vascular infarct, it will spontaneously resolve over a period of three
to six months and the patient will not require further management beyond periodic
observation and either temporary occlusion or press-on prism therapy.
CLINICAL PEARLS
- Consider cases of true vertical diplopia to be a fourth nerve palsy until
proven otherwise. In children, nearly all cases of isolated fourth nerve palsy are either
congenital or traumatic in nature. In adults, approximately 40 percent of all isolated
fourth nerve palsies are traumatic, 30 percent are idiopathic, 20 percent are due to
vascular infarct, and only 10 percent are due to tumor or aneurysm.
- The vast majority of fourth nerve palsies are benign. When encountering a
sudden-onset isolated fourth nerve palsy, delay prescribing permanent prisms for at least
three months in order to allow the palsy to recover.
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