Ординатура / Офтальмология / Английские материалы / Imaging of Orbital and Visual Pathway Pathology_Muller-Forell_2005

weighted and hyper-intensity on T2-weighted images (Figs. 7.61, 7.62). 3D-CISS sequence (Casselman et al. 1996) demonstrates a high sensitivity for even small structures, its application is helpful in the differential diagnosis, especially from suprasellar congenital epidermoid cysts (Osborn 1994b).

Epidermoid cysts probably arise from inclusion of ectodermal epithelial elements at the time of neural tube closure (Vion-Dury et al. 1987), accounting for 0.2%–1% of all primary intracranial tumors, 7% of

them in the suprasellar region (Osborn 1994b).Imaging of these well-delineated, tumor-like cystic lesions is not always able to differentiate them from arachnoid cyst, especially since the MRI signal intensities are similar to CSF in every conventional sequence. However, the high resolution capability of a 3D-CISS sequence is able to define the irregular, lobulated surface (“cauliflower-like”) of the lesion, and additional DWI confirms the diagnosis (Fig. 7.63) (Fitzek et al. 1998).

7.3

Optic Tract and Area

7.3.1 Neoplasms

7.3.1.1 Ependymoma

Intracranial ependymomas (WHO grade II) are slowly growing tumors, which exhibit variable morphologic features and biological behavior. They arise from the ependymal layers of the ventricles and include subependymoma, myxopapillary ependymoma (both WHO grade I), and anaplastic ependymoma (WHO grade III). Ependymomas account for 3%–9% of all neuroepithelial tumors (Wiestler et al. 2000a), with intracranial manifest-

ation preferentially in children and young adults, most commonly in the infratentorial compartment, in contrast to a predominantly intraspinal involvement in adults (Mork and Loken 1977; Atlas 1996; Prayson 1999). These tumors occur at any site along the ventricular system with preference for the fourth but also the lateral ventricle. Supratentorial intraparenchymal ependymoma may occur particularly in children or young adults, originating from embryonic ependymal remnants (Naidich and Zimmerman 1984; Wiestler et al. 2000a).Although the histological grading of ependymoma is rather low (WHO II), the clinical outcome is poor in young children (below 3 years),incomplete tumor resection, and anaplastic histo-pathological features, whereas it is about 50% for supratentorial localization and appearance in young adults (Pollack et al. 1995; Horn et al. 1999).

Intracranial Pathology of the Visual Pathway

Imaging Characteristics. On MRI, these quite distinctly circumscribed lesions present with heterogeneous signal intensities, low on T2-weighted and slightly hypointense on T1-weighted images reflecting the partly cystic, partly calcified, necrotic and hemorrhagic areas of the mass (Fig. 7.64) while solid tumor parts may show BBB disruption. Because of CSF dissemination, usually found in conjunction with recurrence at the primary site, MRI is the method of choice for the accurate determination of the relationship to adjacent structures along the CSF spaces (Burger and

Scheithauer 1994; Atlas 1996).

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7.3.1.2

Primary CNS Lymphoma (Malignant Lymphoma)

The incidence of primary CNS lymphoma has recently increased worldwide from 0.8%–1.5% to 6.6% of primary intracranial neoplasms, mainly thought to be a consequence of patients with acquired immune deficiency syndrome (AIDS) (Miller et al. 1994). Current studies report a decreasing incidence in these patients but an increase in the immunocompetent population (Nasir and DeAngelis 2000). Primary CNS lymphomas are defined as extranodal malignant

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lymphomas arising in the CNS in the absence of obvious lymphoma outside the nervous system at the time of the diagnosis, and should be differentiated from secondary manifestation of systemic lymphoma (Paulus et al. 2000). The mean age of manifestation in immunocompetent patients is the sixth to seventh decade, whereas the median age of incidence in immunocompromised patients (organ transplant recipients, congenitally immunodeficient or HIV patients) ranges from 10 to 39 years (Paulus et al. 2000). The most common location is supratentorial; involvement of the occipital lobe is seen in only 3%. Although multiple locations are observed in up to 50%, primary leptomeningeal involvement is seen in only 8% of patients (Grove and Vyberg 1993). The clinical presentation is unspecific, with mostly focal neurological deficits or neuropsychiatric symptoms (Paulus et al. 2000). As an exquisite sensitivity to steroids exists, these drugs should be withheld until tissue is obtained for diagnosis by stereotactic biopsy (Nasir and DeAngelis 2000).

Imaging Characteristics. Classic imaging findings show multiple, sometimes solitary masses involving deep gray matter structures, periventricular regions, and the corpus callosum (Jack et al. 1988). Hyperdense on CT or isointense to gray matter on all MRI sequences, they present with moderate perifocal edema, generally less marked than in glioblastoma or metastasis, and a diffuse contrast enhancement (Figs. 7.65, 7.66). The detection of enhancement along perivascular, leptomeningeal spaces (Fig. 7.67) should put CNS lymphoma at the top of the differential diagnosis, together with sarcoidosis and toxoplasmosis (see Fig. 7.87). In rare cases of CNS lymphoma, an additional dural involvement of the skull base, cavernous sinus, and orbit is reported (Fig. 7.67) (Jaiswal et al. 2000). Biopsy is needed,as imaging characteristics of cerebral involvement especially in AIDS patients may mimic any pathological lesion, and even the serostatus may not discriminate CNS lymphoma from toxoplasmosis. One highly sensitive and specific finding for cerebral lymphoma is the detection of Epstein-Barr virus DNA in CSF (Atlas 1996; Miller et al. 1998).

7.3.2

Nonneoplastic Lesions

7.3.2.1

Vascular Lesions

Although cerebral vascular lesions, clinically presenting as stroke, often manifest in a similar manner as other acute functional disorders of the brain, the underlying pathology includes a heterogeneous group of cerebrovascular disorders. The four major types of stroke are cerebral infarction (80%) caused by arterial vessel occlusion or as a result of inflammatory vascular disease, spontaneous subarachnoid (5%) or intracerebral hemorrhage (15%), and venous occlusion (Bradac 1990; Hankey and Warlow 1991).It is beyond the scope of this book to discuss the different clinical presentations, pathology, etiology, prognosis, and treatment of these diseases in detail (and the reader is referred to special textbooks), but some of the most important pathophysiological and diagnostic imaging criteria with regard to the visual pathway will be briefly reviewed.

7.3.2.1.1

Cerebral Infarction

Most patients presenting with the clinical symptomatology of a stroke with the sudden onset of neurological deficit as hemiparesis or hemiplegia suffer from atherosclerosis, causing macroangiopathic vascular lesions (mostly of the carotid bifurcation) with infarction of a cerebral territory (Figs. 7.68–7.71).The main arterial supply of the optic tract and optic radiation arises from the territory of the posterior cerebral artery (PCA). The PCA gives rise to the medial and posterior choroidal arteries which are in hemodynamic balance with the anterior choroidal artery, arising from the ICA and supplying the optic tract. Although considerable variation exists, the PCA mainly supplies the inferior temporal lobe and the occipital lobe (van der Zwan et al. 1992). As in acute cerebral ischemia and infarction, the blood flow is significantly diminished, but the

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