4. Spread of comitance

5. Pseudo-overaction of the contralateral superior oblique muscle (due to ipsilateral superior rectus contracture resulting from a longstanding hyperdeviation)

In patients with intermittent exotropia and some degree of comitant hypertropia, examination of versions, forced head tilt testing, Double Maddox Rod testing, and fundus examination for torsion should be performed to look for signs of underlying trochlear nerve palsy alone.104 Cho and Kim104 found that small amounts of hypertropia can disappear with horizontal surgery alone.

The issue of whether the ocular motor synkinesis can contribute to the positive Bielschowsky Head tilt test, especially in congenital cases, requires further examination. Ohtsuki et al412 reported a case of trochlear nerve palsy with an unusually large Bielschowsky head tilt phenomenon and disproportionately small vertical deviation. Because the tilt improved after a small (3 mm) superior rectus recession, they attributed this phenomenon to a gain in the otolith ocular reflex affecting the vertical rectus muscles. It remains possible, however, that synkinetic innervation of the superior oblique muscle could explain this observation. While reported cases of ocular motor synkinesis generally tend to spare the trochlear nerve,174 Kothari et al296 reported primary superior oblique muscle levator synkinesis in a 7-year-old boy in whom gaze into the field of action of the superior oblique muscle caused the ptotic upper eyelid to retract. Other patients with congenital and acquired synkinesis involving the trochlear nerve have been reported.163,332,348,516

Synostotic Plagiocephaly

Synostotic plagiocephaly caused by stenosis of the ipsilateral coronal suture with subsequent deformation of the orbit has been shown to have a high association of vertical strabismus that mimics a trochlear nerve palsy corresponding to the side of the coronal synostosis.454 These patients have apparent overaction of the inferior oblique muscle, and an anatomical deformation of the position of the trochlea has been hypothesized as the cause.25 In contrast, children with congenital muscular torticollis and deformational plagiocephaly have an associated torticollis that is not ocular in nature.173

Hydrocephalus

Unilateral and bilateral trochlear nerve palsies are not uncommon in children with noncompressive hydrocephalus.

Because the finding of bilateral trochlear nerve palsy localizes to the superior medullary velum, the associated dilation

of the suprapineal recess is assumed to compress the trochlear nerves at their point of decussation.203 The association of bilateral trochlear nerve paresis with hydrocephalus is easily overlooked when signs of dorsal midbrain syndrome coexist.203

Idiopathic

The diagnosis of idiopathic trochlear nerve palsy is assigned to children who develop acute vertical diplopia with signs and symptoms of isolated trochlear nerve palsy, no history of recent head trauma, no signs of congenital trochlear nerve palsy, and no associated neurological abnormalities.455 Adults with idiopathic trochlear nerve palsies generally show spontaneous recovery over 4 months.121 This scenario in older adults is often attributed to microvascular infarction of the trochlear nerve.270 As discussed below, many patients who would previously have been classified as idiopathic, are now found to have schwannomas of the trochlear nerve on highresolution MR imaging.

The natural history (persistence versus resolution) of idiopathic trochlear nerve palsy in children is unknown. In our experience, the rarity of compressive lesions as a cause of isolated trochlear nerve palsy in children suggests that clinical observation of persistent trochlear nerve palsy may be appropriate, and that neuroimaging need only be performed if additional neurological signs develop.

Compressive Lesions

Because of the rarity of compressive trochlear nerve palsies in children, routine intracranial MR imaging is not indicated in uncomplicatedpediatricsuperiorobliquepalsies.86 However, neuroimaging of the brain often ends up being performed when orbital MR imaging is obtained to look for diminution in the size of the superior oblique muscle. Over the past decade, trochlear nerve schwannoma, a slow-growing benign tumor that is increasingly detected on high-resolution MR imaging, has been detected with greater frequency in children and adults with unilateral trochlear nerve palsy (Fig. 6.11).75,165,183,541 In the child with trochlear nerve palsy and clinical signs of neurofibromatosis 2, the possibility of trochlear nerve schwannoma warrants high-resolution MR imaging.75 In some patients, these tumors and their associated symptoms have been observed to resolve spontaneously, as these tumors are slowgrowing, and their associated symptoms occasionally resolve spontaneously.612

Other compressive causes of trochlear nerve palsy are rare. Krohel et al303 described a 9-year-old child who developed an isolated trochlear nerve palsy as the initial sign of