GRAVES DYSTHYROID OPHTHALMOPATHY (ORBITOPATHY)
- Imaging with computed tomography, magnetic resonance, or ultrasound can easily confirm the clinical impression of Graves dysthyroid ophthalmopathy.
- If an alternative diagnosis is possible, imaging will very likely help make that differential diagnosis more clear.
- Computed tomography and magnetic resonance imaging can be predictive of compressive optic neuropathy due to Graves dysthyroid ophthalmopathy.
- Computed tomography and magnetic resonance imaging are useful in treatment surveillance.
Graves disease is the most common cause of hyperthyroidism. It occurs far more commonly in women, and there is a familial predisposition. It is a potentially devastating disease with regard to its orbital manifestations that has several names, including dysthyroid ophthalmopathy, dysthyroid orbitopathy, and Graves dysthyroid ophthalmopathy (GDO). The latter designation will be used in this chapter.
This disease is also discussed and illustrated in Chapter 20 on autoimmune diseases.
Prevalence and Epidemiology
Graves disease is seen worldwide, with a marked predilection in women. It occurs primarily in adults between 30 and 50 years of age and is unusual in children. It is the most common cause of proptosis in adults.
Proptosis is a common clinical presentation for GDO. The proptosis is most commonly bilateral, although thyroid ophthalmopathy is also the most common cause of unilateral proptosis in adults. Visual loss and disordered eye movements, with diplopia, may also occur.
ANATOMY AND PATHOPHYSIOLOGY
The relevant anatomy of the orbit, eye, and optic nerve/sheath complex is discussed in detail in Chapter 44.
Pathology and Patterns of Disease
Graves disease affects the thyroid gland and orbits. It is caused by B- and T-lymphocyte reaction to thyroid-stimulating hormone (thyrotropin) receptors (TSHR) that cause hyperstimulation of the gland and therefore a reduced thyroid-stimulating hormone level. Orbital fibrocytes of the extraocular fat and connective tissue also (like the thyroid gland) have a high expression of TSHR. The resulting lymphocyte and macrophage reactive process affects the extraocular soft tissues and most dramatically the extraocular muscles.
GDO, for imaging purposes, may be divided into two categories based on whether or not there is associated optic neuropathy1,2 (Figs. 58.1–58.5). It may also be grouped by its likely acute/subacute phase (Figs. 20.11, 20.12, and 58.1–58.5) and later chronic fibrotic changes (Fig. 58.6).
Optic neuropathy is usually caused by compression of the optic nerve at the orbital apex by enlarged extraocular muscles1,2 (Figs. 58.3–58.5). This may be aggravated by the associated inflammatory reaction and vascular congestion also seen as part of the disease (Fig. 58.1). Effacement of 50.0% or more of the fat around the extraocular muscles correlates with a 66.7% chance of neuropathy, while lesser degrees of effacement of the fat planes strongly suggest that the cause of optic neuropathy will be unrelated to Graves disease.1,2 Patients with optic neuropathy may also have dilatation of the superior orbital vein due to vascular congestion and hydrops of the retrobulbar segment of the optic nerve/sheath complex.1 Increased orbital fat may be the only major manifestation of this disease (Fig. 58.2), but for the majority of cases, enlargement of the extraocular muscle(s) is the main imaging feature. When only one muscle is affected (6% of cases), it tends to be the superior muscle complex rather than the medial or the inferior rectus as previously reported1 (Fig. 58.2). The remaining cases demonstrate enlargement of more than one muscle. The superior oblique muscle is much less often affected, and the lateral rectus is affected less than the others. The disease is generally bilateral but often not to the same degree. At early stages of orbitopathy, changes may appear unilateral.
Chronically, the muscles may become atrophic, fat replaced, and fibrotic (Fig. 58.6).
Pathologically Altered Function
The disease causes proptosis that may cause the cornea and sclera to become dry and irritated because those structures have less coverage by the eyelids and tear pool.
The swelling of the muscles may lead to disordered eye movements. When the swelling of the muscles and surrounding soft tissues becomes substantial, a secondary optic neuropathy and loss of vision may occur.