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Neuroradiology Case of the Week

Case 48

Ravinder Sidhu, MD, Akio Hiwatashi, MD, Henry Z Wang, MD, PhD,
and P-L Westesson, MD, PhD, DDS

Clinical Presentation: A 15-year-old female presented with acute severe pain and diminished vision in right eye.

Radiological Findings: Post-contrast T1-weighted axial MR image revealed thickened right optic nerve in its distal half region along with enhancement, near its insertion to the right globe (Fig. 1). The coronal T1-weighted MR image showed thickened and hyperintense right optic nerve (Fig. 2). The remainder of the intraocular structures including muscles was normal. MR of the head was unremarkable.

Diagnosis: Right optic neuritis/pseudotumor

Discussion:  The orbital pseudotumor is defined as non-specific, non-neoplastic inflammatory process of the orbit without identifiable local/systemic cause. The disorder was first described by Birch-Hirschfield in 1905 [1]. This is a diagnosis of exclusion based on history, the clinical course of the disease, and the response to steroid therapy, laboratory tests, and biopsy in a limited number of cases. There is a group of disease entities that can mimic pseudotumors such as lymphoid tumors, thyroid orbitopathy, sarcoidosis, and other granulomatous diseases.
    Each of these abnormalities, or at least components of each, has been included under the umbrella term pseudotumor at sometime during the last few decades. Currently the term pseudotumor should be reserved for idiopathic orbital inflammatory syndrome.
    Idiopathic orbital inflammatory syndrome accounts for 4.7% to 6.3% of orbital disorders and the disease is more prevalent in adults than pediatric population. The pathogenesis of the disease remains elusive but several lines of evidence point to immune mediated processes as the likely underlying mechanism. Orbital pseudotumor may have protean clinical manifestations. The most common being unilateral, sudden onset ocular pain, proptosis and impaired/loss of vision. Some presentation of idiopathic orbital inflammatory syndrome may mimic conditions such as orbital cellulitis [2].
    Idiopathic orbital inflammatory syndrome is usually confined to orbit, rarely may extend intracranially. However, when confined to orbit, may be multifocal, involving more than one structure. Most authors recognize it as, (a) myositis, (b) dacrycystadenitis, (c) periscleritis, (d) perineuritis, or (e) diffuse group. The diffuse group refers to the patients in whom involvement of orbital fat predominates. It is the diffuse variety which simulates lymphomatous infiltration, and biopsy is helpful in establishing the diagnosis in such cases [3].
    In the subgroup of perineuritis, there is inflammation of the sheath surrounding the optic nerve. In addition to idiopathic orbital inflammatory syndrome, optic neuropathy may be seen in number of conditions such as multiple sclerosis, optic nerve sheath meningioma, autoimmune disease (systemic lupus erythematosus), post viral infections (herpes, chicken pox, rubella, etc.), and infective conditions as syphilis/toxoplasmosis/Lynne disease. Radiation optic neuropathy is another rare cause. It is important to diagnose the underlying cause of the optic neuritis since prognosis and treatment varies for each condition [4].
    Acute pain is an important feature of pseudotumors. Multiple sclerosis usually presents with gradual painless loss of vision. Immune markers with other systemic presentation would negate the diagnosis of pseudotumors. Viral neuritis is usually seen 10-14 days after the primary illness. The index case was found negative for antinuclear antibodies and there was no history of preceding illness. Since patient presented with acute pain with impaired vision without any systemic involvement, hence a diagnosis of optic neuritis /pseudotumor was considered.
    CT may show some enlargement of optic nerve, usually with some degree of enhancement. MR imaging will depict the thickening of the optic nerve. There may be few streaky densities in contiguous orbital fat. Post contrast fat-suppressed, T1-weighted MR images may be the best technique to demonstrate optic neuritis as seen in index case. Contrast enhancement is often subtle or present in short segment of nerve, particularly in intracanalicular portion of the nerve.
    Steroids show a dramatic effect in the treatment of acute cases of pseudotumors and usually reverse the changes completely.

References:

1. Stevens JL, Rychwalski PJ, Baker RS. Pseudotumor of the orbit in early childhood. Journal of American Association for Ophthalmology and Strabismus.1998;2:120-123.
2. Weber AL, Romo LV, Sabates NR. Pseudotumor of the orbit. Clinical, pathologic, and radiologic evaluation. Radiol Clin North Am 1999;37:151-168.
3. Curtin HD. Pseudotumor. Imaging in ophthalmology Part I. Radiol Clin North Am 1987;25:583-599.
4. Mafee MF, Goodwin J, Dorodi S. Optic nerve sheath meningioma: role of MR imaging. Radiol Clin North Am 1999;37:195-202.