Congenital Trochlear Nerve Palsy

Trauma is the most common cause of acquired unilateral or bilateral trochlear nerve paresis.86,121,187,218,280,472–475,613 All traumatic fourth nerve pareses should be assumed to be bilateral until examination proves otherwise. The trochlear nerve may be damaged anywhere along its course by direct orbital trauma, frontal trauma, or an oblique blow to the head.

In severe brainstem damage, trochlear nerve paresis may be obscured by horizontal gaze abnormalities, and become apparent only when horizontal gaze begins to recover. As the trochlear nerves emerge from the dorsal surface of the midbrain, they are susceptible to damage from closed head trauma. The neurosurgical trauma involved in resecting a posterior fossa tumor can similarly injure one or both trochlear nerves.320 More anteriorly, the proximity of the trochlear nerve to the tentorial edge also makes it susceptible to injury in closed head trauma. A blow to the forehead may cause a contrecoup contusion of one or both nerves by impingement against the rigid tentorium.29 Traumatic avulsion here has also been described.227 Damage at this location is often bilateral, and trauma patients must be carefully examined for this possibility.540

Lindenberg333 has described a contrecoup contusion of the midbrain tectum at the caudal edge of the tentorial notch, when the forehead or skull vertex strikes a stationary object. Blows to the occiput or even falls on the buttocks may transmit forces that cause the cerebellum to be thrust against the tentorium from below, injuring the trochlear nerve.333 The fourth cranial nerve may also be injured by contusion or hemorrhage within the substance of the midbrain.333

Because the fourth nerve may be injured by remote trauma, reports of coexisting orbital floor fracture and trochlear nerve palsy are not surprising.91,99,269,284 The susceptibility of the fourth nerve to trauma that is not severe enough to produce either skull fracture or loss of consciousness may lead one to miss other underlying disease. Neetens396 has described three cases of basal intracranial tumors associated with trochlear nerve paresis following minor head trauma. It is unusual for the trochlear nerve to be the sole nerve damaged by cavernous sinus lesions, but it can be damaged in combination with other cranial nerves from lesions in the cavernous sinus. When orbital trauma causes superior oblique weakness, it may be impossible to know whether the injury involved the fourth nerve, the trochlea, or the superior oblique tendon.

Direct trauma to the superior-medial orbit can also produce a trochlear nerve palsy by laceration of the tendon, muscle, or by damage to the trochlea.25 Knapp285 coined the term “canine tooth syndrome” to describe the association of a mild Brown syndrome and trochlear nerve palsy caused by orbital trauma (Knapp type VII trochlear nerve palsy). Blunt trauma to the superior-medial orbit may also produce a Brown syndrome with no superior oblique weakness.28,35

Congenital trochlear nerve palsy is underdiagnosed, because many children are asymptomatic, and some affected infants may be thought to have congenital muscular torticollis.219 The vast majority of cases are nonfamilial, but several families with more than one affected member have been documented.219 Numerous cases of familial congenital trochlear nerve palsy have now been reported.23,49,62,256

Children with congenital trochlear nerve palsy typically come to medical attention because of a hypertropia in side gaze or an unexplained head tilt. In older children, congenital trochlear nerve palsy may present as acquired vertical diplopia. The diagnosis is based upon a history of head tilt beginning in infancy (as demonstrated by examination of family photographs) in a child with no specific inciting event, together with the following findings on examination, orbital MR imaging, and results of traction testing at the time of surgery. The following clinical findings are considered to be predictive of congenital trochlear nerve palsy:

Large Vertical Fusional Vergence Amplitudes

Normal vertical fusional vergence amplitudes are 2–3 prism diopters. Mottier and Mets389 studied 14 patients with congenital trochlear nerve palsy and found average vertical vergence amplitudes to be 16 prism diopters. We have seen adults with congenital trochlear nerve palsy who fuse up to 30 prism diopters of hyperdeviation. On examination, such a patient will initially seem to be orthotropic, but with prolonged occlusion, the measured vertical deviation will slowly increase as the examiner “chases it” with the prism bar. Symptomatically, older children note that, once they begin to see double, the images gradually spread apart. It is not unusual for patients with congenital trochlear nerve palsy to become symptomatic for the first time in their teenage or adult years. Whether such cases result from a gradual increase in the size of the deviation (perhaps related to ipsilateral superior rectus contracture) or from an age-related reduction in fusional vergence amplitudes is unknown.

Facial Asymmetry

Facial asymmetry is present in most cases of congenital trochlear nerve palsy, but it may also be seen in acquired cases that are longstanding.595 Wilson and Hoxie595 found facial asymmetry to be present in seven of nine patients with congenital trochlear nerve palsy. This facial asymmetry is thought to be secondary to chronic tilting of the head.595 Patients with facial asymmetry secondary to congenital trochlear nerve palsy have hemifacial retrusion with an upward

Fig. 6.10  Facial asymmetry in left superior oblique palsy. Note retrusion of the right side of the face with interpupillary axis downslanting to the right and mouth upslanting to the right

slanting of the mouth on the side of the head tilt (Fig. 6.10). The recognition of facial asymmetry associated with congenital trochlear nerve palsy can be facilitated by drawing one line through the center of both pupils and another line through the closed lips. In children with facial asymmetry, these lines converge and intersect toward the side of the shallow, more retruded side of the face (Fig. 6.10).595 In our experience, children with congenital trochlear nerve palsy characteristically have a detectable enophthalmos in the paretic eye due to absence of the translational force exerted by the normal superior oblique tendon.

This form of facial asymmetry must be distinguished from that associated with synostotic plagiocephaly, congenital muscular torticollis, and the nonspecific facial asymmetry that is common in normal individuals. Unlike synostotic plagiocephaly, which also involves the forehead, the facial asymmetry in the latter two conditions is confined to the midface. Because facial asymmetry in congenital muscular torticollis has been reported to resolve with continued facial growth, a similar regression is assumed, albeit unproven, to be possible following early treatment for congenital trochlear nerve palsy. The age of onset of facial asymmetry in congenital trochlear nerve palsy and the degree of potential resolution relative to the age at corrective surgery are as yet unknown. Its gradual development with a chronic head tilt suggests that strabismus surgery should not be unduly postponed when a congenital superior oblique muscle palsy is diagnosed in a young child. Patients with vertical diplopia associated with congenital trochlear nerve palsy do not complain of associated image tilt, whereas image tilt is noted in about 23% of patients with vertical diplopia from acquired trochlear nerve palsy.576 The absence of subjective torsion in congenital fourth nerve palsies presumably reflects the gradual development of complex pathophysiological and/or psychological adaptive mechanisms.122,576

The exaggerated superior oblique traction test, as described by Guyton,122 is more likely to show tendon laxity in congenital versus acquired cases.377 Plager436 used forced duction testing to demonstrate decreased superior oblique muscle resistance to rotation in 14 patients who carried the clinical diagnosis of congenital trochlear nerve palsy, while all ten patients with acquired trochlear nerve palsy had normal resistance to rotation. Helveston et al229 examined the superior tendon of 89 eyes of patients undergoing surgery for trochlear nerve palsy and found congenital trochlear nerve palsy to be associated with an abnormality of the superior oblique tendon in 87% of cases, as compared with 8% of cases with acquired trochlear nerve palsy. Abnormalities of the superior oblique tendon include absence, redundance, misdirection, and insertion into posterior Tenon’s capsule.482 Sato et al found that the amount of vertical deviation does not seem to correlate with the type of tendon abnormality found at surgery.482 An inherited anomaly confined to the superior oblique tendon could account for reports of familial congenital trochlear nerve palsy.219

MR imaging has shown that the anatomical abnormality is not limited to the tendon. The ipsilateral superior oblique tendon is often found to be small or absent on coronal orbital MR imaging in children with congenital trochlear nerve palsy.96,421,482 One study found an absence of the superior oblique tendon on MR imaging to be predictive of a larger primary position vertical deviation,482 while another found the clinical findings to be indistinguishable in children with present and absent tendons.513 Shokida et al513 found enlargement of the contralateral trochlear nerve palsy in some congenital cases. Because hypoplasia (in congenital cases) and atrophy (in acquired cases) are indistinguishable on MR imaging, the finding of a small superior oblique muscle cannot be used to classify the etiology of the palsy.421

There is suspicion that the structural abnormalities of the superior oblique tendon that characterize congenital trochlear nerve palsy could result from lack of innervation. Several studies have found polymorphisms in the ARIX gene in patients with congenital trochlear nerve palsy (some with an absent superior oblique muscle) that may be a genetic risk factor.242,256 They suggested that polymorphisms, in this homeobox-containing gene, may be responsible for some cases of congenital trochlear nerve palsy. The ARIX gene is known to be expressed in the brainstem nuclei for oculomotor and trochlear nerves387,617 and is the same gene that is responsible for affected families with CFEOM2.256

Wallace and von Noorden582 found the following examination findings in the patient with trochlear nerve palsy to be predictive of a congenitally absent tendon:

1. An associated horizontal deviation

2. Amblyopia

3. A large hypertropia in primary position (averaging 20.8 prism diopters)