[Brown's atavistic superior oblique syndrome: etiology of different types of motility disorders in congenital Brown's syndrome]

Abstract

Clinical features: The congenital Brown syndrome is characterized by a mechanical limitation of elevation in adduction, with an orthophoria in down gaze. Brown postulated a shortened superior oblique tendon sheath as the cause of the limitation but this was disproved by Parks et al. in 1975 and the origin of Brown syndrome remains unclear. In recent years, a congenital dysinnervation has been discussed; however, this does not explain the full spectrum of abnormalities and especially contradicts the unlimited depression in adduction seen in Brown syndrome.

Origin: Surgical exploration in Brown true typical cases reveals a fibrotic strand, typically located at the posterior margin of the superior oblique tendon. This strand originates from the trochlear area and has a common insertion with the superior oblique tendon posterior to the equator into the globe. It may represent an atavistic superior oblique muscle as described by Fink in various animals. They do not have a trochlea but a superior oblique muscle originating in the anterior superior nasal orbit.

Atypical brown syndrome: A fibrotic strand was also surgically revealed in two cases of atypical Brown syndrome. In the first case an elevation deficit-as in Brown true atypical cases-also present in abduction could be explained by an unusual insertion of the fibrotic strand anterior to the equator. The second case showed a fibrotic strand which was completely separated from the superior oblique tendon and inserted far posterior to the equator nasal to the superior rectus muscle. This finding had not been previously described and explained the total elevation restriction which was suddenly in > 30° adduction and the Y‑pattern exotropia which increased in adduction and decreased in abduction.

Treatment and follow-up: A 10 mm excision of the fibrotic strand from the insertion gives the best results from all procedures. The residual limitation of active elevation in adduction improved with gaze exercises mostly after more than 1 year.

Conclusion: The fibrotic strand, an atavistic superior oblique muscle, not only explains the typical Brown syndrome but also-by its variable insertion-different patterns of elevation deficits seen in atypical Brown syndrome. A 10 mm excision of the strand gives good functional results of abnormal head position (immediate in most cases) and even elevation in adduction (after 1 year in most cases).

Keywords: Dysinnervation; Fibrotic strand; Fibrotic strand insertion; Mechanical eye motility disorder; Surgical procedures.