Unilateral ventral strabismus in an aquarium Silver Arowana (Osteoglossum bicirrhosum).

An adult silver arowana (Osteoglossum bicirrhosum) presented with a 2-month history of acute onset ventral strabismus of the right eye associated with a cystic structure overlying the dorsal aspect of the globe. Aspiration of the cyst, removal of redundant conjunctival tissue, and apposition of the surgical edges of the conjunctiva did not correct the strabismus. Magnetic resonance imaging of the head and histopathology of the eye did not reveal significant differences between affected and unaffected eyes except for a more posterior dorsal rectus muscle insertion site in the affected eye. This case report documents and describes a syndrome well known among arowana hobbyists as 'drop eye'.