DOI: 10.1148/radiol.2273010758
(Radiology 2003;227:718-719.)
© RSNA, 2003
The Optic Nerve Tram-Track Sign1
Uday S. Kanamalla, MD
1 From the Department of Diagnostic Imaging, Temple University School of Medicine, 3401 N Broad St, Philadelphia, PA 19140. Received April 9, 2001; revision requested May 29; final revision received August 12, 2002; accepted August 28. Address correspondence to the author (e-mail: ukanamal@temple.edu).
Index terms: Meninges, neoplasms, 139.366 • Nerves, optic, 144.92 • Signs in Imaging
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APPEARANCE
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The tram-track sign is most evident on contrast material–enhanced transverse computed tomographic (CT) or fat-suppressed T1-weighted magnetic resonance (MR) images of the orbit. On these images, the optic nerve appears as a negative defect in relation to the surrounding enhancement in the area of the optic nerve sheath on either side (Figure) (1–7).
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Transverse contrast-enhanced fat-suppressed T1-weighted (repetition time msec/echo time msec, 600/20) spin-echo MR image shows optic nerve sheath meningioma. Optic nerve (arrow) is seen as central linear hypointensity in comparison to enhanced meningioma on either side, producing the tram-track sign.
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EXPLANATION
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The optic nerve tram-track sign is most commonly described in relation to optic nerve sheath meningiomas. Meningiomas tend to cause segmental or diffuse circumferential thickening of the optic nerve sheath (1–4). Once intravenous contrast material has been administered, the optic nerve can be seen on CT or MR images as an unenhanced central linear structure (negative defect) surrounded by the enhanced meningioma. On transverse or sagittal images this produces a tram-track sign, which is composed of two enhanced areas of tumor separated from each other by the negative defect of the optic nerve. The corresponding finding on coronal images is a doughnut configuration (1). Though it is less common, the tram-track sign may be evident at unenhanced CT when there is linear calcification of the optic nerve sheath meningioma.
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DISCUSSION
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The tram-track sign has been described as a feature to help distinguish an optic glioma from an optic nerve sheath meningioma (1,6,7). In optic nerve sheath meningiomas, the tumor sheath demonstrates increased attenuation compared with the optic nerve at contrast-enhanced CT. Optic gliomas, however, arise from glial cells within the optic nerve and remain closely associated with the optic nerve regardless of the pattern of growth, so that there is no clear separation between the tumor and the nerve at imaging. The optic nerve/sheath complex becomes homogeneously enlarged, without a definable hypoattenuating central optic nerve (8).
Optic nerve sheath meningiomas arise from meningothelial cells of the arachnoid situated along the optic nerve sheath. Histologically, the cells of the meningioma are usually of the meningotheliomatous type, but they may occasionally be of the transitional type (9,10). The early subdural growth causes the tumor to surround the nerve. Since the nerve is only surrounded and not totally obscured by the tumor, it can usually be identified as a "negative defect" within the tumor. Meningiomas that arise along the course of the optic nerve may do so within the orbit (optic nerve sheath meningioma), within the optic canal (intracanalicular meningioma), or at the intracranial opening of the optic canal (foraminal meningioma). They occur most commonly in women (up to 80% of cases) in the 3rd, 4th, and 5th decades of life. They may also be seen in children with neurofibromatosis type 2, in whom the tumors may manifest bilaterally.
The predominant clinical feature of optic nerve sheath meningioma is insidious onset of progressive visual loss, with proptosis occurring later. Papilledema and optic atrophy are common accompaniments. Retinal examination shows characteristic features of pale disk edema and optociliary veins (11–13). Disk edema is due to a combination of impaired venous outflow and ischemia in the swollen nerve. Enlarged retinociliary anastomoses, also called "optociliary veins," represent dilated connections between the two major divisions of the ocular circulation, namely, the ciliary circulation and the central retinal vessels. Their presence indicates a lesion in the anterior perioptic meninges and reflects the chronicity of the lesion.
Meningiomas that involve the optic nerve sheath are typically in plaque form and grow linearly along the nerve sheath. Three distinct growth patterns have been ascribed to optic nerve sheath meningiomas (1,3). These include (a) tubular enlargement, in which there is uniform expansion in the cross-sectional area of the nerve and sheath; (b) fusiform enlargement, in which there is a spindle-shaped increase in the cross-sectional area; and (c) excrescent enlargement, in which there is an outgrowth emanating from or attached to the sheath. Calcification may be demonstrated in optic nerve sheath meningiomas in 20%–50% of cases.
At CT, osseous changes may be present in the region of the optic nerve canal, such as bone erosion and occasionally hyperostosis (1–3). Johns et al (2) have described three distinct levels of attenuation in optic nerve sheath meningiomas at contrast-enhanced CT: (a) the low-attenuation optic nerve, (b) the higher attenuation meningiomatous tumor mass, and (c) the even higher attenuation parallel linear enhancing areas adjacent to the optic nerve. The additional linear enhancement adjacent to the optic nerve is thought to be related to the linear spread of the tumor along the subarachnoid space, which occurs once the tumor has extended from its original location in the subdural space. Other masses in the retrobulbar region do not usually exhibit this type of perineural spread and therefore would not create such a striking contrast between the tumor mass and the optic nerve adjacent to it.
MR imaging of optic nerve sheath meningiomas reveals isointensity or slight hypointensity on T1-weighted images and isointensity or slight hyperintensity on T2-weighted images relative to the optic nerve (4,5). Enhancement at MR imaging is intense and is particularly evident with fat saturation techniques.
The tram-track sign is nonspecific in that it can be seen in conjunction with other orbital diseases, including orbital pseudotumor, perioptic neuritis, sarcoidosis, leukemia, lymphoma, metastases, perioptic hemorrhage, and Erdheim-Chester disease (1–4,14–20). In addition, enhancement of the periphery of an optic nerve, which is not enlarged, may be a normal finding and probably represents normal dural enhancement. Orbital pseudotumor can closely mimic meningiomas through enhancement of the optic nerve sheath. The clinical picture is often different, however, in that a patient with pseudotumor usually presents with a painful red eye, proptosis, and impaired ocular mobility. Also, a very distinguishing characteristic of pseudotumor is its marked sensitivity to oral steroid therapy. There are often additional features at imaging, such as inflammatory stranding of the retrobulbar fat, uveoscleral thickening and enhancement, and orbital muscle thickening. Perioptic neuritis secondary to demyelination, which is a finding in multiple sclerosis, infections (measles, mumps, syphilis), and inflammatory diseases such as sarcoidosis, also closely mimics the imaging appearance of meningiomas. The leptomeninges are involved more severely than the optic nerve, and at CT or MR imaging the enhanced and inflamed meninges obscure the lucent optic nerve. Although uncommon, carcinomatous implants can be deposited in the optic nerve sheath and thereby resemble a meningioma. Perineural hemorrhage, whether spontaneous or traumatic, obscures the optic nerve and produces marked attenuation at unenhanced CT; the acute hemorrhage is not enhanced and thus appears different from a meningioma. Leukemia and lymphoma can infiltrate the optic nerve sheath to produce the tram-track sign, but more commonly both the leptomeninges and the optic nerve are infiltrated so that the optic nerve cannot often be distinguished from the tumor at CT or MR imaging. Erdheim-Chester disease (systemic xanthogranulomatosis) is a rare, multisystem non–Langerhan cell histiocytosis. Orbital manifestations range from infiltration of the retroconal fat and optic nerve sheath to large retrobulbar masses. The orbital process can be differentiated from optic nerve sheath meningioma by its multisystem involvement, including skeletal, brain, pulmonary, and abdominal structures.
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FOOTNOTES
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A trainee (resident or fellow) wishing to submit a manuscript for Signs in Imaging should first write to the Editor for approval of the sign to be prepared, to avoid duplicate preparation of the same sign.
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REFERENCES
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