Ординатура / Офтальмология / Английские материалы / Ultrasonography of the Eye and Orbit 2nd edition_Coleman, Silverman, Lizzi_2006

Figure 5.13 Ultrasonic characterization of patients referred for orbital examination. Schematic flow chart of orbital diagnosis, progressing from distinction of normal from abnormal orbits to differentiation of abnormalities into tumor types or inflammatory tissue, based on criteria of transmission and morphology.

For example, the contour of an orbital tumor may be smoothly rounded, sharply defined, and compressing adjacent normal structures. If the lesion has good sound transmission, its posterior wall will also be clearly evident. Internally, the lesion may be devoid of echoes, indicating no significant tissue interfaces within the lesion. These findings are characteristics of a fluid-filled cystic lesion, such as mucocele or dermoid tumor.

In other cases, a mass lesion may have a contour similar to a cystic lesion. However, its posterior boundary may be indistinct because of poor sound transmission through it, indicating a solid tumor. Low-amplitude echoes within the substance of a tumor indicate minor tissue interfaces, characteristic of a homogeneous solid tumor. These findings indicate a well-circumscribed solid tumor, for example, neurogenic tumor, lacrimal gland tumor, and some metastatic tumors. Location of the lesion within the orbit gives further clues to identification. A lesion of this type within the muscle cone and involving the optic nerve is

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probably one of the neurogenic tumors (glioma or meningioma). A similar lesion located in the upper temporal aspect of the orbit is more likely to be a lacrimal gland tumor.

Figure 5.14 A myopic globe can produce pseudoproptosis and a slightly reduced orbital volume, as

demonstrated in this 10-MHz B-scan. The globe outline can also be seen to be staphylomatous.

Figure 5.15 Schematic presentation of the division of orbital tumor patterns based on their outline and

transmission properties.

Figure 5.16 The location of a tumor in the orbit is an important aspect of acoustic evaluation. Tumors within the

muscle cone are particularly susceptible to ultrasonic detection.

An orbital tumor with an irregular contour suggests a different group of tumors. Angiomatous tumors show fingerlike protrusions extending into the orbital fat pattern, usually with a larger mass more posteriorly. The tumor contour may or may not be well defined. The internal structure of angiomatous tumors presents many dense acoustic interfaces from vessel walls and blood-filled spaces comprising the tumor. Ultrasonically, this heterogeneous interior appears as multiple, irregular, high-amplitude echoes throughout the mass, with little sound attenuation. This irregular contour and heterogeneous internal structure are distinctly different from the cystic and solid tumor patterns described previously.

Another irregular orbital tumor, having a more solid character, indicates a solid infiltrative tumor. Although the anterior lesion outline is jagged, it is also sharply defined and distinct. As with other solid tumors, low-amplitude internal echoes are present and sound attenuation is marked, making the posterior tumor margins indistinct. This pattern is associated with infiltrative tumors, particularly lymphomas and sarcomas. Metastatic tumors may also show these characteristics.

Idiopathic granulomas, or pseudotumors, often mimic invasive, solid tumors clinically and ultrasonically. Orbital hematoma may also appear ultrasonically as an infiltrative mass lesion, although it is nonneoplastic.

A history of trauma can, occasionally, be misleading, because some tumors may bleed with minimal trauma. A contrast MR should be considered for more

definitive imaging of most of these lesions. Follow-up serial ultrasonography can be a useful way to document resolution of the lesion, in the case of hematoma.

In our experience, the infiltrative tumor category is the most difficult to identify with certainty, using ultrasound, because inflammatory changes in the orbit, as described later, may produce a similar appearance, especially when edema involves the optic nerve and the sub-Tenon's space. Scarring from previous orbital surgery may also be confused with this category.

Classifying orbital tumors into the four categories noted earlier is not always possible, because tumors do not have a consistent presentation. However, the pattern and absorption characteristics outlined are consistent with the tumor types noted. Other ultrasonic techniques can augment the available information in these cases. Ballottement of the tumor with a contact probe placed against the eye will demonstrate compression of cystic or angiomatous lesions and resistance to compression with solid tumors. M-scan ultrasound or Doppler ultrasound provides a means of studying vascular pulsations within tumors. Dynamic or kinetic sector scanning will demonstrate motion of orbital structures with changes in gaze position and often aids in categorization.

Individual tumor types, the variations of ultrasonic patterns associated with them, and the reliability of ultrasonic diagnosis with each tumor type will be discussed more fully as separate topics.

INFLAMMATORY AND CONGESTIVE CHANGES

In contrast to orbital mass lesions, inflammatory and congestive processes tend to involve the normal tissues that are present in the orbit, causing subtle changes in them. Inflammatory changes may be classified, ultrasonically, as diffuse or as localized to a particular area or tissue in the orbit, depending upon the specific inflammatory process (Figure 5.17).

A generalized abnormal mottling of the orbital fat pad is indicative of a diffuse orbital inflammation, such as cellulitis. Echoes within the fat are more widely spaced than normal and are of high amplitude, giving the fat pad a more heterogeneous appearance without circumscribed borders. A similar acoustic appearance is seen in traumatic orbital hemorrhage. A localized area of such mottling within the fat is seen with an orbital abscess or focal granuloma. Other localized inflammatory findings involve specific orbital structures. Expansion of the sonolucent (hypoechoic) space between the fat pad

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and the orbital wall generally indicates enlargement of the extraocular muscles. The orbital wall may be accentuated in these cases, where the acoustic transmission characteristics of extraocular muscles are modified by the disease state. These findings are particularly characteristic of thyroid-related disease or endocrine exophthalmos, where edema and cellular infiltration of the muscles are present.

Figure 5.17 22-MHz scans of posterior pole with digital processing to enhance the boundary between the sclera, nerve, and Tenon's capsule, demonstrating the value of higher frequency ultrasound to demonstrate episcleritis.

Localized inflammation and edema may also involve the optic nerve or Tenon's space adjacent to the sclera. Accentuation of the optic nerve sheath without enlargement of the nerve itself is indicative of optic neuritis or edema—a distinctly different ultrasound presentation from enlargement produced by a tumor. These abnormalities usually resolve with regression of the inflammatory process. Although no pathologic specimens have been examined in the acute stage of retrobulbar neuritis, the ultrasonic and MR findings are consistent with inflammatory edema of the dural sheaths surrounding the nerve. Similar optic nerve findings may be seen with pseudotumor (idiopathic orbital inflammation), papilledema, or vascular anomalies.

The sub-Tenon's space surrounding the globe may become expanded posteriorly from edema associated with any type of orbital inflammation. Ultrasonically, this appears as a sonolucent area adjacent to the globe wall and connecting to the optic nerve outline. Other inflammatory signs are usually present in the orbit along with this finding. It may also accompany some orbital tumors, particularly granulomatous tumors or lymphomas. In a normal orbit, this becomes a potential space and, as such, is not acoustically evident.

ORBITAL TUMORS

ACOUSTICALLY “CYSTIC” TUMORS

Acoustically “cystic” orbital tumors are those that have a rounded regular outline and good acoustic transmission. It should be emphasized again that acoustic transmission is a relative phenomenon, and the ability to demonstrate transmission through orbital structures depends on a knowledge of the patterns obtained with one's own equipment and transducers and with careful calibration of receiver gain.

Mucocele

Mucoceles are cysts lined by paranasal sinus mucous membranes. They enlarge slowly because of continued secretion and desquamation of lining cells and frequently cause erosion of the bony walls of the sinus by pressure. They expand in the direction of least resistance, often into the orbit through the medial wall and floor of the frontal sinus, forcing the globe in the opposite direction. Common clinical presentations are pain and periorbital swelling, diplopia, and proptosis. Invasion of the orbit by mucoceles of the paranasal sinuses is an important cause of unilateral exophthalmos. In series compiled by radiologists, orbital mucoceles are the most common cause of unilateral exophthalmos, with a figure as high as 15% in the series compiled by Zismor et al. (18). In series compiled by ophthalmologists, orbital mucoceles are a less common but still significant cause of unilateral exophthalmos. In Reese's clinical series of 230 cases, 3% of unilateral exophthalmos was a result of orbital invasion by mucoceles (19). In children, Shields et al. (20) note that mucoceles are the most common secondary cysts that occur in children.

Ultrasonography gives accurate information as to the location and size of an orbital mucocele. Secondary changes, such as compression of retrobulbar fat and indentation of the posterior pole of the globe, are well shown by ultrasonography (21).

Mucoceles ultrasonically appear rounded and smooth, or even spherical in contour (Figure 5.18). They are well demarcated from the surrounding normal orbital tissues. Mucoceles demonstrate a sharply defined, rounded anterior acoustic border that indents the retrobulbar fat. They show definite acoustic hollowness, and A-scans through them show few or no internal echoes (Figure 5.18). On B-scan, the interior of a mucocele appears as a solid, black cavity, that is, a sonolucent space like the globe itself. Little sound energy is absorbed in these cystic structures, and, consequently, ultrasonic radiation can penetrate through the mass, clearly outlining its posterior extent and the orbital walls and orbital apex.

Although MR diagnosis is reliable in orbital mucoceles, CT may better confirm the extent of bony erosion,

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particularly in the case of paranasal sinus involvement (20). B-scan ultrasonography may add information as to the soft tissue configuration of the orbit and facilitate the choice of surgical approach.

Figure 5.18 A 10-MHz B-scan of a mucocele of the orbit appearing as a sonolucent area, much like the globe

itself.

Dermoid Cysts

Dermoid cysts are relatively common tumors of aberrant ectodermal tissue. They usually occur in children (22). When dermoid or other epithelial cysts involve the bony orbital wall, they may be diagnosed on x-ray, CT, or MR (23). However, even in this situation ultrasound is valuable, because it can often demonstrate the exact limits of the orbital mass (24,25). In cases without bony involvement (up to 16% in one series) (26), ultrasound is particularly useful in diagnosing these tumors.

Figure 5.19 10-MHz B-scan images of a cavernous hemangioma (H) of the orbit. These tumors are well encapsulated and usually in the muscle cone. The internal A-scan echoes and MR are shown in Figures 5.20, 5.21 and 5.22.

Cavernous Hemangiomas

Hemangiomas are one of the most common of all orbital tumors. Reese (19) described the proportion of hemangiomas to all orbital masses as 12% in a clinical study of 230 consecutive cases and as 15% in a histopathologic study of 877 cases. In a more recent series, children in a study of 1,264 patients have the proportion of cavernous hemangiomas as 6%, the third most common behind lymphoid tumor, 11%, and pseudotumor, 11% (27). The cavernous hemangiomas of adults are by far the most common orbital intraconal tumor (22).

The size and location of cavernous hemangiomas are well demonstrated with B-scan ultrasonography (Figure 5.19). All cavernous hemangiomas that we have examined were located in the muscle cone.

The group of acoustic characteristics of all cavernous hemangiomas includes: (a) a rounded, regular outline; (b) a sharply defined and rounded anterior acoustic border; (c) good demarcation from surrounding structures; (d) low-to-moderate acoustic absorption (i.e., fair-togood sound transmission); and (e) appearance alteration with varying transducer frequencies.

Cavernous hemangiomas show a sharply defined anterior acoustic border as a result of the abrupt transition in acoustic velocities between retrobulbar fat and the fluid-filled tumor. These tumors are ultrasonically well

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demarcated from surrounding, normal orbital structures, also resulting from the marked acoustic discontinuity. Secondary changes of the globe, such as flattening of the posterior pole, are also demonstrable.

Figure 5.20 Combined 10-MHz A- and B-scan image of the same tumor shown in Figure 5.19. A-scan shows

moderate reflectivity from the tumor.

Cavernous hemangiomas (Figure 5.20) are mainly blood-filled vessels that are characterized by good sound transmission but can contain low-amplitude internal echoes from the vessel walls (Figure 5.21). Sound transmission is, of course, better than acoustically solid lesions with high sound absorption (such as optic nerve tumors) but is not as high as in the completely acoustically hollow lesions with minimal sound absorption (such as mucoceles). Because sound absorption is low in cavernous hemangiomas, ultrasound energy is able to penetrate, so that the posterior extent of the tumor and the outline of the wall and apex of the orbit are often demonstrated (25,28).

Figure 5.22 Sagittal T1-weighted view shows a well-encapsulated lesion hypointense to fat, and axial MR image with contrast shows enhancement of the cavernous hemangioma shown in Figures 5.19, 5.20 and 5.21. (Courtesy of Michael Kasim, MD, New York, NY.)

Cystic Lymphangiomas

Lymphangiomas of the orbit are discussed in more detail in the section on angiomatous tumors. The usual presentation of lymphangioma of the orbit is diffuse; however, some are cystic. Hemorrhage into a lymphangioma cavity may cause a hemorrhagic cyst to form. In these cases, and as discussed previously, the ultrasonic pattern is an orbital mass lesion with a rounded outline and good sound transmission through the tumor, permitting outlining of its posterior wall (Figures 5.23 and 5.24).

ACOUSTICALLY “SOLID” TUMORS

Acoustically “solid” orbital tumors have a rounded outline, as do cystic-appearing orbital tumors; however, sound transmission is poor through these masses, and multiple internal echoes are present.

Tumors of the Optic Nerve

Neurogenic tumors are relatively common causes of unilateral exophthalmos. Reese (19) described the proportion of neurogenic tumors to total orbital mass as 11% in a clinical study of 230 cases and as 10.6% in a histopathologic study of 877 cases. Silva (35) found 32 neurogenic tumors in a series of 300 consecutive cases of orbital masses, a proportion of 10.9%. Shields et al. (27) found 8% neurogenic tumors in a series of 1,264 patients. This group of tumors may require a neurosurgical approach for optimum removal and, thus, differentiation from other orbital tumors is clinically important. This differentiation is aided by B-scan ultrasonography.

Secondary changes, such as flattening of the posterior pole of the globe and papilledema, can be shown (Figures 5.25 and 5.26) with B-scan ultrasound.