Orbital computed tomographic characteristics of globe subluxation in thyroid orbitopathy

Objective: Axial globe subluxation may complicate thyroid orbitopathy. This acute event is defined by anterior displacement of the globe equator beyond the orbital rim, lid retraction behind the equator, and tethering of the optic nerve. The authors explored the factors leading to spontaneous globe subluxation in patients with thyroid orbitopathy. Design: A clinical review. Main Outcome Measures: The medical charts and axial and coronal orbital computed tomographic (CT) scans of all the patients with spontaneous globe subluxation were reviewed. The patients were categorized according to their CT scans into type I ('lipogenic' variant) or type II ('myogenic' variant). Results: Four (0.1%) of approximately 4000 patients with thyroid orbitopathy presented with spontaneous globe subluxation. All of these patients had increased orbital fat without significant enlargement of the extraocular muscles. Thus, they represent type-I thyroid orbitopathy. Conclusions: Globe subluxation in thyroid orbitopathy requires compliance of the orbital soft tissues and extensibility of the extraocular muscles. These are characteristics of type-I orbitopathy. The increased fat content results in more compliance of 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. Patients with type-I orbitopathy and extensive proptosis may be at relatively greater risk of having globe subluxation develop.