in primary position and normal saccadic velocity in the direction of paresis should suggest restrictive orbitopathy, even in the absence of typical orbital signs and symptoms.

Painless vertical diplopia

Case: This 62-year-old engineer had a one-year history of painless vertical diplopia that remained undiagnosed despite a variety of neurodiagnostic tests. After some initial progression he felt that his double vision had remained stable. His medical history was unremarkable except for wellcontrolled hypertension. Neuro-ophthalmic examination showed a 30-diopter left hypertropia in primary position with limitation of right eye elevation that was most marked with the eye abducted (Figure 2.9A). Infraduction was full and saccadic velocities were normal, as was the remainder of the

examination. A CBC, ESR, acetylcholine-receptor antibody titers and thyroid function tests were normal, as was a brain MRI (Figure 2.9B).

What clinical features help to localize the source of this patient’s ocular motility disorder?

The normal saccadic velocity in the face of marked limitation of eye movement speaks strongly against supranuclear or cranial nerve dysfunction and points instead to either myasthenia or orbital restrictive disease (see Chapter 7, Farmer with an adduction deficit). The absence of variability over a one-year time period would be unusual for myasthenia. This pattern of motility disturbance (decreased elevation with the eye abducted) is typical of orbital restrictive disease, specifically due to loss of inferior rectus muscle elasticity. This is the most frequently affected extraocular

muscle in patients with thyroid eye disease, and therefore this pattern of motility disturbance is characteristic.

Based on the examination features described above, a dedicated CT scan of the orbits was obtained. This study showed a slightly plumper right medial rectus muscle compared to the left medial rectus and definite enlargement of the right inferior rectus muscle consistent with thyroid eye disease (Figure 2.10).

Discussion: This case illustrates an important point regarding the radiographic diagnosis of orbital disease. While brain MRI may detect orbital pathology in some patients, in other cases important findings will be missed if dedicated orbit views are not obtained. This is particularly true for abnormalities of the vertical eye muscles, as illustrated by this case. The horizontal rectus muscles are well visualized with axial views, which are often included in standard head scans, but coronal sections are needed to adequately evaluate the vertical muscles.

MRI is often considered preferable to CT because of its higher spatial resolution and superior soft tissue detail. Indeed, MRI is more sensitive than CT for inflammatory orbital disorders such as scleritis, optic neuritis and optic perineuritis (see below, Painful optic neuropathy) and is also superior for the detection of disease within the optic canal. However, CT scanning is a good alternative for demonstrating most orbital pathology and may be preferable in certain settings. CT provides excellent visualization of disorders of the extraocular muscles, such as thyroid eye disease. CT is preferable for all cases of trauma, for evaluating changes in the orbital bones and for demonstrating calcification, as in optic nerve sheath meningiomas and optic disc drusen. Whether MRI or CT is used, adequate orbital imaging includes both axial and coronal views.

This case also illustrates the limitations of thyroid function tests for the determination of Graves’ orbitopathy. Abnormal test results (e.g. elevated T4 levels and suppressed TSH or, occasionally, low thyroid levels and elevated TSH) are helpful when