cause an isolated sixth nerve palsy even in the context of a quiet eye; patients must be asked about pulsatile tinnitus, which is often present. Some indirect fistulas remain stable or close spontaneously; however, both types of fistulas may be successfully treated with interventional radiologic techniques or radiosurgery. Angiographic studies are required to determine the location and configuration of the fistula, and a variety of thrombogenic materials (eg, coils, beads, or balloons) may be employed to eliminate the abnormal vascular flow.

Neuromuscular Junction Causes of Diplopia

Myasthenia gravis is the prototypical disease of the neuromuscular junction. It typically produces variable diplopia and ptosis with any pattern of pupil-sparing, painless ocular misalignment; conversely, it never produces sensory symptoms, pain, or autonomic or clinical pupillary dysfunction. Accordingly, myasthenia gravis belongs in the differential diagnosis of any such cases of diplopia (see Chapter 14).

Myopathic, Restrictive, and Orbital Causes of Diplopia

Eye movements may be restricted by congenital or acquired mechanical factors. Congenitally deficient neural innervation to extraocular muscles can also cause limited eye movements, which may have a restrictive component.

Thyroid Eye Disease

The most common cause of restrictive strabismus in adults is thyroid eye disease (TED). Any of the extraocular muscles may be involved, but the inferior and medial recti are most commonly affected. When the inferior rectus muscle is involved, an ipsilateral hypotropia typically occurs in primary position that increases in upgaze—the restrictive process pulls the eye down and limits supraduction (see Chapter 14, Fig 14-3). When the medial rectus is the affected muscle, an esodeviation typically occurs that increases on lateral gaze to the same side (the enlarged, “tight” medial rectus restricts abduction). The diagnosis of TED is straightforward if associated with proptosis, chemosis, eyelid retraction, and eyelid lag, but restrictive strabismus may be the only sign. Forced duction testing (see Fig 8-3) and measurement of intraocular pressure in different positions of gaze may provide information to support this diagnosis. Neuroimaging results for patients with TED typically reveal enlargement of the bellies of the extraocular muscles, with sparing of the tendons (see Chapter 4, Fig 4-20). For a more extensive discussion of TED, see Chapters 4 and 14, and BCSC Section 7, Orbit, Eyelids, and Lacrimal System.

Posttraumatic Restriction

Blowout fractures of the orbit often cause diplopia. The most typical presentation involves fracture of the orbital floor with entrapment of the inferior rectus muscle or its fascial attachments to the orbital tissues. This entrapment, best illustrated with coronal CT of the orbit, mimics the pattern of vertical strabismus often present in TED. Less commonly, the medial rectus muscle becomes entrapped (see

Chapter 2, Fig 2-15). Paretic strabismus from swelling also may occur in the acute phase. In either case, as swelling resolves, so may the diplopia. Hence, decisions about the need for surgery for orbital blowout fractures must be made judiciously. See also BCSC Section 7, Orbit, Eyelids, and Lacrimal System.

Post–Cataract Extraction Restriction

Binocular diplopia can result from injury to or inflammation within the inferior rectus or other muscles after retrobulbar injection for cataract or other ocular surgery. The onset of vertical diplopia just after surgery initially suggests nerve damage or myotoxicity from the local anesthetic. Over time, the initial paretic or myotoxic effect evolves into extraocular muscle fibrosis, leading to restricted eye movement; concomitantly, the involved eye transitions from hypertropic to hypotropic status, with hypotropia increasing in upgaze. (For more information on cataract surgery complications, see BCSC Section 11, Lens and Cataract, Chapter 8.)

Orbital Myositis

Idiopathic inflammation of 1 or more extraocular muscles typically produces ophthalmoplegia and pain, often with conjunctival hyperemia, chemosis, and sometimes proptosis. The pain may be quite intense and is accentuated by eye movements. If the inflammation is confined to the posterior orbit, the eye may appear to be white and quiet. Results of CT or MRI typically show enlargement of 1 or more of the extraocular muscles with tendon involvement, and the inflammation often extends into the orbital fat. Orbital myositis–related pain usually responds within 24 hours to systemic corticosteroid therapy, whereas diplopia may take longer to resolve. Orbital myositis is usually an isolated phenomenon but may be part of a systemic disease such as Wegener granulomatosis (ie, granulomatosis with polyangiitis), systemic lupus erythematosus, or sarcoidosis (see also Chapter 14 and BCSC Section 7, Orbit, Eyelids, and Lacrimal System).

Neoplastic Involvement

Infiltration of the orbit by cancer, especially from the surrounding paranasal sinuses, can impair eye movements because of either extraocular muscle infiltration or involvement of the ocular motor cranial nerves. Relative enophthalmos or associated eyelid “hang-up” on downgaze may accompany the diplopia. Occasionally, extraocular muscles may be the site of a metastatic tumor.

Brown Syndrome

Brown syndrome is a restrictive ocular motor disorder that produces limited upgaze when the affected eye is in the adducted position (Fig 8-14). This pattern of motility is usually congenital but can be acquired. In congenital cases, the cause is a short superior oblique tendon, which produces an ipsilateral hypodeviation that increases on upgaze to the opposite side (ie, impaired supraduction in adduction). Acquired cases result from damage or injury to the trochlea, which may cause a “click” that the patient can feel. Acquired Brown syndrome is usually found in patients with rheumatoid arthritis, idiopathic orbital inflammatory disease, or trauma, but it may, rarely, be a manifestation of a focal metastasis of a neoplasm to the superior oblique muscle.

Figure 8-14 Brown syndrome. Selected gaze positions of a 7-year-old who was referred for a 2-year history of the left eye “moving funny.” Visual acuity was 20/20 bilaterally. The patient’s eyes were straight in primary position. The motility evaluation revealed a left hypotropia in upgaze to the right (upper left photograph). This is the pattern of a congenitally short left superior oblique tendon that is characteristic of Brown syndrome.