CT scan characteristics of globe subluxation in thyroid orbitopathy

Purpose: Axial globe subluxation can complicate thyroid orbitopathy It is defined as an acute anterior displacement of the globe equator beyoTid the orbital rim, lid retraction behind the equator, and tethering of the optic nerve. If left untreated, vision may be threatened by exposure keratopathy and acute optic neuropathy The mechanism by which it occurs is obscure. We tried to-'explore the factors leading to globe subluxation in patients with thyroid orbitopathy. Methods: The medical charts, and axial and coronal CT scans of all patients with spontaneous globe subluxation due to thyroid orbitopathy, were reviewed. The patients were categorized according to their CT scans into type I or "upogenic" variant (increased orbital fat in the absence of enlarged extraocular muscles) or type II or "myogenic" variant (enlargement of extraocular muscles). Results: Four patients of approximately 4000 patients (0.1%) with thyroid orbitopathy presented with spontaneous globe subluxation. All of these patients had increased orbital fat without significant enlargement of the extraocular muscles. T.hus, they represent type I thyroid orbitopathy. Conclusions: Globe subluxation in thyroid orbitopathy requires compliance of the orbital soft tissues and extensibility of extraocular muscles. These are characteristics of type I orbitopathy. The increased fat content creates more compliance to the soft tissues, and the normal caliber of the muscles allows them to become more extensible. This permits the acute contraction of the eyelids posterior to the equator of the globe seen in the setting of globe subluxation. Patients with type I orbitopathy and extensive proptosis may be at greater risk to develop globe subluxation and should be monitored carefully and treated promptly.