Anomalous Extraocular Muscles with Strabismus

Discussion

Accessory extraocular muscles are very rare, having only been described in scattered case reports. The true prevalence is unknown due to the rare presentation. A few similar cases have been reported in the ophthalmic and pathologic literature.^1,2 To our knowledge, there are no reports in the radiologic literature.

Three broad types of accessory extraocular tissue have been reported including anomalous bands of muscle bridging 2 muscles; fibrous tissues adjacent to the muscles, which may attach to the globe; and muscles arising from the posterior orbit and inserting on the globe or extraocular muscles.¹

An interesting hypothesis of accessory musculature was proposed by Whitnall in 1911,³ asserting that these fibromuscular bands of tissue may represent an atavistic retractor bulbi muscle. This is a muscle found normally in reptiles, amphibians, and some ruminants but not in Homo sapiens, allowing retraction of the globe into the orbital cavity for protection.

Our patient did show retraction of the globe with attempted extraocular movements. Rather than an atavistic retractor bulbi muscle, our patient's findings could be considered as an accessory lateral rectus muscle. One argument against an accessory lateral rectus muscle is that this anomalous muscle did not appear radiographically to originate on the lateral rectus as described by Narasimhan et al.⁴ The anomalous muscle seen in our patient originated at the orbital apex at the optic foramen. Surgically, the muscle was extremely taut and inserted directly into the posterior globe, slightly inferolateral to the optic nerve.

An argument against this muscle being a vestigial retractor bulbi muscle is that it was an isolated distinct muscle and did not have pathologic features suggestive of skeletal muscle but rather had features typical of extraocular muscle.⁵ The described atavistic retractor bulbi muscles have either a cone shape or multiple tendon slips that surround the optic nerve region.⁶ Our patient lacked these findings.

We believe that this accessory muscle resulted from a disturbance of the mesodermal development of the extraocular muscles. Releasing this anomalous muscle from the globe relieved the globe retraction and significantly improved the exotropia.

Our patient coincidentally has Gorlin syndrome, which is due to a mutation at chromosome location 9q22·3-q31.⁷ This mutation causes loss of function of the PTCH1 receptor, of which sonic hedgehog protein is the ligand.^8,9 Gorlin syndrome is associated with subsequent excessive hedgehog signaling, one of the pathways that directs development during embryogenesis, and is associated with the increased risk of basal cell carcinoma and medulloblastoma seen in patients with this syndrome.⁹ No association has been made between Gorlin syndrome and accessory extraocular muscles, but animal research shows induction and promotion of muscle development due to hedgehog signaling.^10–13 One recent report found a patient presenting with exotropia similar to that in our patient, but the underlying cause was found to be an odontogenic keratocyst (a classic finding in Gorlin syndrome) of the alveolar ridge causing outward deviation of the globe.¹⁴

This case provides an example of a rare anatomic anomaly that has significant clinical pertinence. Awareness of such an entity can improve the search pattern of radiologists evaluating patients with atypical strabismus because this anomaly is amenable to surgical resection.