Fourth Nerve Palsy

Fourth nerve palsy is the most common cause of an isolated cyclovertical muscle palsy. Paresis of the fourth nerve can be congenital or acquired. Closed-head trauma is the most common cause of fourth nerve palsy in most series, and represents one third of all traumatic ocular motor palsies. Khawam and co-workers reviewed 3000 cases of strabismus, in which there were 40 cases of acquired superior oblique palsy. Sixty-eight per cent of the acquired superior oblique palsies were caused by closed-head trauma. Uncertain etiology was noted in 20 per cent, and the remainder of the cases stemmed from cerebrovascular accident, diabetes, brain tumor, ethmoiditis, or mastoiditis. Von Noorden reviewed 270 cases of superior oblique paralysis and found that 40 percent were congenital, 34 percent were traumatic, 23 percent were idiopathic and 3 percent were due to other causes such as vascular disease, tumors or myasthenia gravis.

Despite extensive clinical and laboratory testing, often no definitive etiology is identified in many cases of fourth nerve palsy. Harley reviewed the causes of paralytic strabismus in 121 children from birth to age 16, and found 67 per cent of fourth nerve paralysis to be of undetermined origin.

A fourth nerve palsy is initially an incomitant hypertropia, greatest in the adducted depression position of the involved eye. If the palsy continues, contracture of the ipsilateral inferior oblique occurs and the maximal hyperdeviation is found in the field of action of this muscle. Another sign of contracture of the ipsilateral inferior oblique muscle is overelevation of the adducted palsied eye. On the side in which the vertical deviation of the eyes is maximal, the hypertropia becomes incomitant in up- and downgaze.

Patients with unilateral fourth nerve palsy often present with torticollis to reduce diplopia. Head tilt to the side of the nonparetic eye is found in half of the patients, with unilateral palsy. Since the superior oblique muscle is a depressor and intortor, its tone is diminished by upgaze and by tilting the head to the shoulder opposite the palsied muscle, patients with unilateral superior oblique palsy can maintain binocular vision. The absence of a head tilt is usually attributed to amblyopia or extremely large amplitude of vertical fusion. Some patients tilt their head to the side of the paretic eye in order to increase the vertical deviation and to make it easier to ignore the second image.

Facial asymmetry has been associated with congenital superior oblique palsy. Typically, this asymmetry is manifested by midfacial hemihypoplasia on the dependent side opposite the affected superior oblique. The nose deviates toward the hypoplastic side and the mouth slants so that it approximates a horizontal orientation despite the torticollis. It is thought that the facial asymmetry is secondary to the compensatory head tilt which may lead to secondary gravitational effects, reduced blood flow through a compromised internal carotid artery or deformational molding of the face and skull during sleep. In muscular torticollis, once facial asymmetry develops, it may persist despite subsequent treatment. To prevent the facial asymmetry from developing, some authors recommend early surgery to correct the deviation.

Often an older patient presents with a new onset of diplopia secondary to a fourth nerve palsy. It is important to determine if the palsy has only recently developed or if it represents a congenital disorder that has decompensated. A newly acquired fourth nerve palsy may require further evaluation including a detailed neurologic examination and radiographic imaging. Several features may help to determine the acuteness of the deviation. A patient with a congenital superior oblique palsy will often have large vertical fusional amplitudes. If these amplitudes are measured and found to be large, or if the patient experiences only occasional diplopia in the presence of a large vertical deviation, the palsy is most likely longstanding or congenital. Examination of old photographs may show a compensatory head tilt or the presence of facial asymmetry, as described above, which would also signify a previous motility disorder. Lastly, patients with congenital superior oblique palsies may not experience subjective torsional diplopia whereas patients with an acquired palsy generally do complain of tilting of the second image.

Treatment Except for an occasional patient with vertical deviation of 10 prism diopters or less (when prism may be tolerated), most cases of superior oblique palsy require surgery. In general, surgery for superior oblique palsies should be directed to those muscles whose greatest action is in the field when the vertical deviation is the largest.


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