Assessment Of Thickening Of Extraocular Muscles

Thyroid Eye Disease

The most common cause of extraocular muscle thickening, thyroid eye disease, usually entails multiple, bilateral, and asymmetrical muscle involvement (see Chapter 28). Clinically, the inferior rectus muscle is most often involved, followed by the medial, the superior, and the lateral rectus muscles. However, echo-graphically, the superior rectus/levator complex is most commonly enlarged followed by the medial, the inferior, and lateral rectus muscles.22 Microscopically, there is perivascular infiltration by lymphocytes and plasma cells with increased deposition of hydrophilic mucopolysaccharides in the muscle belly, sparing the tendon. Topographically, there is enlargement of the belly of the involved extraocular muscle with the acquisition of internal tissue echoes reflecting the his-tological changes.9,25 On A scan, the internal structure is slightly irregular with medium internal reflectivity (Figure 8.11).

Orbital Inflammatory Disease (Pseudotumor) and Lymphoma

Refer to the preceding section on pseudotumor and lymphoproliferative disease.

Metastatic Tumors

Metastatic lesions to the extraocular muscles are usually unilateral with invariably a single muscle involvement (see Chapter 24).26 There is a slow, progressive, painless increase in the size of the muscle with late onset diplopia (in contrast to the rapid, painful, acute myositis with early diplopia). Ultra-sonography shows echolucency in the belly of the muscle with low internal reflectivity similar in pattern to

FIGURE 8.11. (A) Dynamic A-scan examination of normal rectus muscle (M). 1, Probe is directed anteriorly toward the muscle insertion, which produces a small defect adjacent to the scleral spike. As the probe is angled more posteriorly (2-4), the sound beam shifts toward the muscle belly, which produces a wider defect that moves from left to right. (B) A-scan echogram of an enlarged rectus muscle (M) in thyroid eye disease showing slightly irregular internal structure with medium reflectivity.

FIGURE 8.11. (A) Dynamic A-scan examination of normal rectus muscle (M). 1, Probe is directed anteriorly toward the muscle insertion, which produces a small defect adjacent to the scleral spike. As the probe is angled more posteriorly (2-4), the sound beam shifts toward the muscle belly, which produces a wider defect that moves from left to right. (B) A-scan echogram of an enlarged rectus muscle (M) in thyroid eye disease showing slightly irregular internal structure with medium reflectivity.

the orbital inflammatory pseudotumor category but, in contrast, the tendon of the muscle is not involved.

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