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.
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.
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.
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.