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scopically, there is infiltration of the involved orbital tissues by inflammatory cells consisting of lymphocytes, plasma cells, and eosinophils. Echographic findings of pseudotumor depend on the involved tissue.

Scleritis

In scleritis the B scan shows an echolucent area between the anterior and the posterior scleral wall. In most cases it is associated with a linear echolucent area in the retroscleral space representing edema in Tenon's space (T-sign). On A scan, the high scleral spikes are wider than normal and invariably show abnormal thickening. In episcleritis, the similar A- and B-scan characteristics can be seen but in the immediate retroscleral space with a distinct echolucent rim between the sclera and the rest of the orbital tissues; a low reflective A-scan pattern indicates the edema-tous space that usually exists in this condition.

Myositis

In myositis there is usually a diffuse thickening of the involved muscle including the inserting tendon to the globe with echolucency on B scan and low reflectivity on A scan (Figure 8.7). Comparative assessment with other muscles, especially the counterpart of the other orbit, is quite revealing for the condition.

Orbital Pseudotumor An orbital pseudotumor is invariably a diffuse condition with significant low reflectivity from the internal structure of the involved area. Lacrimal gland in volvement is invariably unilateral and follows the same characteristic pattern but is localized to the area of the lacrimal gland. (Other lacrimal gland conditions are discussed under lacrimal gland lesions.)

Rhabdomyosarcoma

The most common primary malignant orbital tumor in children is rhabdomyosarcoma. It arises from un-differentiated mesenchymal cells that have the ability to differentiate into striated muscle cells. The patient presents with rapid, progressive proptosis. The tumor can involve any part of the orbit with a predilection to the superior portion. The echographic findings of rhabdomyosarcoma (Figure 8.8A,B) are quite similar to those of orbital inflammatory disease (pseudo-tumor).22 However, the age group, the clinical presentation, and the ultrasonography and CT findings are usually diagnostic of this condition.

Secondary Orbital Lesions

Lymphoproliferative Disease

Lymphoid tumors of the orbit occupy a wide spectrum of diseases ranging from the benign pseudolymphomas (pseudotumors) to the atypical lymphoid hyperplasias to the malignant lymphomas. Echographically (Figure 8.8C,E), lymphoid tumors share the same characteristics. The patient's age is important in the interpretation of the ultrasound findings especially with pseudotumor, rhabdomyosarcoma, and lymphoma, for which

part two: diagnosis of orbital tumors

FIGURE 8.7. Myositis. (A) Clinical photograph of a young woman shows right eye injection. (B) B-scan echogram shows enlarged right medial rectus muscle (upper and middle) with thickened insertion (lower) (arrows). A-scan echograms show (C) low reflectivity of the enlarged muscle belly (M) and (D) low reflective thickened insertion (arrow).

these findings are similar. Also, unilateral low reflective infiltrates in an adult is suggestive of pseudotumor while bilaterality supports a diagnosis of lymphoma.

Metastatic Tumors

The orbit, devoid of lymphatic channels, is reached by metastatic tumors via the hematogenous route.

The most common source of metastatic tumor to the orbit in adults is the breast followed by lung, prostate, skin melanoma, and gastrointestinal tract in decreasing order of frequency. In children, neu-roblastoma is the most common source of meta-static tumor to the orbit occurring in about 40% of cases. The disease may be bilateral and presents with a sudden onset of proptosis accompanied by lid ecchymosis (Figure 8.9).

FIGURE 8.8. (A,B) Rhabdomyoarcoma. (C-E) Lymphoproliferative disease. (A) Clinical photograph of a child shows left eye proptosis. (B) A-scan echogram shows diffuse, low-reflective lesion (L). Occasionally, there is active vascularity displayed on dynamic echogra-phy. On B-scan the lesion is echolucent with minimal sound at-

FIGURE 8.8. (A,B) Rhabdomyoarcoma. (C-E) Lymphoproliferative disease. (A) Clinical photograph of a child shows left eye proptosis. (B) A-scan echogram shows diffuse, low-reflective lesion (L). Occasionally, there is active vascularity displayed on dynamic echogra-phy. On B-scan the lesion is echolucent with minimal sound at-

tenuation. V, vitreous cavity; S, sclera; a, anterior and P, posterior surface spikes of the lesion. (C) Clinical photograph of an adult man shows right eye proptosis and downward displacement of the globe. (D) B-scan echogram shows large, diffuse echolucent orbital lesion (L), which had low internal reflectivity on A scan (E).

FIGURE 8.9. Metastatic carcinoma. (A) B-scan echogram displays a poorly defined diffuse lesion (L) with varying echogenicity. (B) On A scan, the internal structure is quite irregular with the internal reflectivity ranging from low23 to moderately high. The characteristic "V" pattern (arrow) results from a central zone of dense cellular infiltrates that become more lobulated toward the periphery (hence, the higher ascending limbs of the lesions).5'22

FIGURE 8.9. Metastatic carcinoma. (A) B-scan echogram displays a poorly defined diffuse lesion (L) with varying echogenicity. (B) On A scan, the internal structure is quite irregular with the internal reflectivity ranging from low23 to moderately high. The characteristic "V" pattern (arrow) results from a central zone of dense cellular infiltrates that become more lobulated toward the periphery (hence, the higher ascending limbs of the lesions).5'22

Mucocele

Mucocele is a cystic lesion filled with mucoid secretions and epithelial debris that arises from the paranasal sinuses. Orbital invasion occurs more commonly from either frontal or ethmoidal muco-celes. Mucoceles usually develop in adults and produce progressive proptosis, diplopia, or ptosis. Echographically, mucoceles present a distinct characteristic pattern almost pathognomonic of the condition (Figure 8.10).

FIGURE 8.10. Mucocele. (1) A-scan echogram shows a highly reflective anterior surface (anterior wall of the lesion, a) followed by an echolucent low-reflective internal structure representing the mucocele, M; B, orbital bone.24 Sound beam directed through intraorbital portion of the mucocele. (2) The beam is moved slightly and hits the edge of bone defect (red) and posterior wall of the sinus (blue). When scanning the lesion from the intraorbital to the intrasinus site, the posterior wall of the mucocele shifts from the intraorbital normal area into a deep intrasinus part (shifting posterior high reflective sinusoidal pattern). (3) Sound beam is directed entirely through bone defect. P, posterior bony wall of sinus.

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