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

6.4

Optic Nerve

The optic nerve represents an approximately 45–50 mm long cerebral fiber tract with a diameter of 4–6 mm, covered by the three layers of the brain: the pia, arachnoidea, and dura (see Sect. 2.5). It is topographically divided into four parts. The intraocular part is located in the posterior part of the globe and does not contain a myelin sheath.Myelination starts at the lamina cribrosa, the beginning of the intraorbital segment, where the optic nerve is embedded in orbital fat. The optic nerve is surrounded by a muscular cone and tendons (Zinn’s circlet) at the orbital apex and leaves the orbit proper at the optic canal, where the third part of the optic nerve begins. The optic canal represents the location of the transition of the optic nerve from its extracranial to the intracranial course. The optic canal and the duraperiosteum sheath of the optic nerve form a nonelastic osteofibrous canal where compression of traumatic or tumor origin, may cause an abrupt or slow decrease in the oxygen supply by the axons of the nerve itself,resulting in visual deficiency or complete visual loss. A differentiation of the fourth,the intracranial (prechiasmal) part of the optic nerve, is useful under conditions when pathologic lesions involve the optic nerve only in its intracranial course from the optic canal to the chiasm, which is primarily caused by adjacent tumors or by the optic nerve itself (Rauber and Kopsch 1987).

6.4.1 Tumors

6.4.1.1

Optic Nerve Glioma

Subclassified by their origin, gliomas of the optic nerve, histologicallyprovenas(juvenile)pilocyticastrocytoma, are located intraocularly on the optic disc, intraorbital and intracranial, the latter with or without involvement of the chiasm (Hollander et al. 1999). Optic nerve gliomas are benign, slow-growing tumors of low radiation sensibility, accounting for 66% of all primary optic nerve tumors, and 4% of all orbital space-occupying lesions.They are more common than optic nerve sheath meningiomas (4:1) and have a female preponderance (Alvord and Lofton 1988; Hollander et al. 1999). Although solitary optic nerve gliomas are found, the incidence in NF1 is rather high,as 10%–38% of patients with optic nerve glioma have NF1, and approximately 50% of patients with NF1 harbor optic nerve glioma, a bilateral disease considered to be pathognomonic for

NF1 (Alshail et al.1977; De Potter et al.1995; Hochstrasser et al. 1988; Brodsky 1993; von Deimling et al. 2000). In NF1, intraorbital tumor growth may be stable for years, but intracanalicular and intracranial growth with involvement of the chiasm, hypothalamus, and optic tracts is seen as well (Hollander et al. 1999; Barkovich 2000). Patients with NF1 may harbor additional cerebral astrocytoma, myelin vacuolization, dysplasia of the cerebral vasculature with stenosis, caused by intimal proliferation and sphenoid bone dysplasia (Aoki et al. 1989; Barkovich 2000) (see Sect. 5.4.1).

In children,the incidence of symptomatic optic pathway tumors is less than 7%, especially when the tumor is restricted to the optic nerve (Listernick et al. 1994; Barkovich 2000),but in recent studies ophthalmologic examination showed some degree of visual dysfunction in about 47% of patients (Balcer et al. 2001). It has further been observed that intracranial postchiasmal involvement was found more often in children with non-NF type 1 optic glioma than in children with NF1 (Listernick et al. 1995). Not only do those patients present symptoms earlier than those with intraorbital optic nerve glioma, they also present with a high incidence of hydrocephalus (Dutton 1994; Listernick et al. 1995). Visual impairment symptomatology includes afferent papillary defect caused by astrocytic proliferation and nerve fiber compression in about 75%, although various fibers persist over a long period of time, thus preserving vision in some cases (Goodman et al. 1975; Dutton 1994). Visual field loss is seen in 63% of patients, regardless of tumor location (Dutton 1994), while optic nerve atrophy is detected in about 59% of patients, regardless of age (Dutton 1994). Although the most prominent symptom in intraorbital optic nerve glioma is axial proptosis (94%), the discrepancy between the rather space-occupying orbital lesion and the rather discrete proptosis with only minimal motility impairment is pathognomonic (Dutton 1994).

Imaging techniques reveal an enlargement of the optic nerve without calcification but of different shapes, as it can present as tubular (Fig. 6.168), fusiform (Fig. 6.169), or lobulated, kinked, and buckled (Figs. 6.168, 6.170),the last mentioned probably due to an additional elongation (Jakobiec et al.1984; De Potter et al.1994). Thickening of the dura represents a perineural growth with tumor extension into the arachnoid space, highly characteristic of juvenile optic glioma (De Potter et al. 1994). Coronal CT shows the bony anatomy, the erosion and/or widening of the optic canal (Fig. 6.169), although superior contrast resolution and clear morphological demonstration of the lesion in axial,coronal, and parasagittal views makes MRI the method of choice

Orbital Pathology

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Fig. 6.168a–c. A 2-year-old boy with NF1. Diagnosis: bilateral optic nerve glioma. CT: a Axial contrast-enhanced image demonstrating bilateral tumor growth with emphasis to the left not only in the orbit, but also intracranially, distal to the dilated optic canal. MRI: b Corresponding T1-weighted native view. c Parasagittal view parallel to the course of the left optic nerve visualizing tumor growth also in the prechiasmal optic nerve. (With permission of Müller-Forell and Lieb 1995b)

in evaluating the intraorbital, the intracanalicular, and especially the intracranial extent of optic nerve glioma (Brown et al. 1987; Sartor 1992). T2-weighted images demonstrate homogeneous high signal intensity of the affected nerve in contrast to the low signal of the contralateral unaffected optic nerve (Fig. 6.170), characterizing the intraneural growth pattern with expansion of fibrovascular trabeculae by intra-axial astrocytic proliferation with little cystic degeneration (Seiff et al.1987). The peripheral hyperintense portion surrounding the linear core of the isointense signal representing the compressed nerve characterizes the perineural growth

of optic nerve glioma (Figs. 6.170, 6.171). It represents not only widening of the subarachnoid space by proliferating astrocytes (Stern et al. 1980), but also perineural arachnoidal gliomatosis, a myxoid proliferation of glial cells and blood vessels combined with arachnoidal hyperplasia,suggestive for patients with NF1 (Seiff et al. 1987; Brodsky 1993; De Potter et al. 1994). T1weighted images should be obtained preand post-con- trast injection, the latter with a fat suppression technique, as it best delineates subtle BBB disruption in the optic nerve itself (Figs. 6.170–6.172) and/or the chiasm (Fig. 6.171) and anterior visual pathway. The lack of

signal enhancement in the periphery of the tumor may represent perineural arachnoid gliomatosis and/or an ectatic subarachnoid space around the affected nerve, but arachnoidal gliomatosis with reactive proliferation of the fibrovascular arachnoid trabeculae is characterized by enhancement (Fig. 6.171).

Additional intrinsic cerebral lesions seen in patients with optic nerve glioma in NF1 are astrocytomas, with a preferred location in the brainstem (Hochstrasser et al. 1988; Pollack et al. 1996). Other infraand supratentorial white matter changes, seen only on T2-weighted images that are multiple, without any mass effect or contrast enhancement, correspond to regions of myelin vacuolization, areas of separation of the layers of the myelin sheath, as they spiral around the axon (DiPaolo et al. 1995). These lesions have characteristic locations (cerebellar white matter, pons, midbrain, internal capsule, splenium, and globus pallidum) and are seen in patients about 3 years old. The lesions increase in number and size until 10–12 years of age, with a subsequent decrease, becoming invisible in patients in their 20s, probably as a result of repair and remyelination (Sevick et al. 1992; Terada et al. 1996).

The differential diagnosis of optic nerve glioma should include optic nerve sheath meningioma (see Sect. 6.4.1.2), demyelinating optic neuritis (see Sect. 6.4.2.1), sarcoidosis, idiopathic inflammation of the optic nerve (Figs. 6.183, 6.184) (see also Sect. 6.4.2.2), lymphoma (Figs. 6.180, 6.181), medulloepithelioma, metastasis (Fig. 6.182) (Hollander et al. 1999), or involvement of the optic nerve in mucopolysaccharidosis (Fig. 6.179). Although optic nerve sheath meningiomas are found in children and young adults (Walsh 1975), the age of presentation and gender (preference for girls/women) should be considered in addition to imaging characteristics in the differentiation from optic glioma. While the described pathologic processes produce a thickened, enlarged, and enhancing optic nerve and sheath complex, consideration of additional clinical symptoms and findings may help in the accurate interpretation of the observed factors.

Management of these patients with optic nerve glioma is still being discussed controversially. A clear trend toward increased mortality in patients with intracranial extension, especially with hypothalamic involvement, is obvious (Dutton 1994). It is wellknown that the long-term survival rate and prognosis are excellent when the tumor is confined to the optic nerve. Conservative management (Fig. 6.172) is to be established in view of the fact that only 21% of these patients show tumor progression (Dutton

1994). Surgery should be restricted to patients with severe proptosis, a blind painful eye, or when extension along the intracranial portion of the optic nerve threatens the chiasm (Albert and Puliafito 1979;

Hoyt and Baghdassarian 1969; Dutton 1994). Radiotherapy does not appear to alter the ultimate prognosis even in cases where tumor involvement includes the hypothalamus and third ventricle (mortality rate: 28%), since CNS complications of radiotherapy, especially in very young children, might be severe (Dutton 1994).

6.4.1.2

Optic Nerve Sheath Meningioma

Meningiomas of the optic nerve sheath are primarily benign, slowly growing neoplasms that arise from the arachnoid cap cells within the optic nerve sheath complex, or from an intraorbital extension of sphenoid wing meningioma (Mafee et al. 1999a). Despite histological differences [most of them are meningotheliomatous (syncytial), fibroblastic, or transitional meningioma] (Marquardt and Zimmerman 1982; Mafee et al. 1999a), the most important clinical feature is their tendency to recur (Mafee et al. 1999a). Optic nerve sheath meningiomas account for 10%– 33% of orbital meningiomas and show a female predominance (twice as often in women as in men), consistent with the presence of female hormonal receptors in most meningiomas (Grunberg et al. 1991; Block et al. 1996). Patients with optic nerve sheath meningioma (mean age: approx. 40 years) present with slowly progressive, painless visual loss and proptosis (Jakobiec et al. 1984; Mafee et al. 1999a). Transient visual obscuration is not uncommon as an initial symptom. Ophthalmoscopy may show optic disc pallor and/or disc swelling at the site of the lesion (Jacobiec et al. 1984; Pless and Lessell 1996), and opticociliary venous shunts (compensation of obstruction of the central retinal vein), highly suggestive of indolent optic nerve sheath or spheno-orbital meningioma (Mafee 1992). Visual impairment and proptosis depend on the location of the tumor, as visual impairment at an orbital apex location is more severe than at a more distal location, where proptosis leads to clinical symptomatology (Henderson 1973), although preservation of the central visual field may be stable for years.

A well-defined tubular thickening and/or calcification of the optic nerve complex is seen on CT (Figs. 6.173, 6.174) (Mafee et al. 1999a). Due to the high vascularization of meningiomas and the absence of a blood-brain barrier (BBB), marked contrast

enhancement is seen with both imaging techniques. Optic nerve sheath meningioma may present as diffuse or eccentric thickening of the nerve but the tramtrack sign (Fig. 6.173), originally called specific, may be seen also in optic nerve lymphoma or optic neuritis (Mafee 1992, 1996).

MRI is the method of choice in the diagnosis of optic nerve sheath meningioma, as it provides a high sensitivity and specificity, even though it is associated with a lower sensitivity for calcification. Especially in fat-suppressed, gadolinium-enhanced images, the presentation of an isointense signal of the often compressed optic nerve (Fig. 6.175), which is clearly differentiated from the homogeneous, marked, or moderate signal enhancement of the primarily isointense tumor, is significant (De Potter et al. 1995; Ortiz et al. 1996; Mafee et al. 1999a). Optic nerve sheath meningiomas present with three patterns (De Potter et al. 1995): as diffuse thickening of the optic nerve sheath complex (Figs. 6.175, 6.176), fusiform swelling (Fig. 6.177), or a globular eccentric tumor (Fig. 6.178). This gives

rise to the question of whether ectopic intraorbital arachnoidal cells play a role in meningioma formation, since the differential diagnosis from neurofibroma or cavernous hemangioma is difficult in the presence of a marked dislocation of the optic nerve (Newman and Jane 1991; Mafee 1992). The great value of MRI is emphasized in cases of intracranial extension via the optic canal (Figs. 6.175, 6.176), where even small and thin en-plaque growth of the tumor matrix is distinctly visible, enabling bilateral involvement or optic chiasm affection to be ruled out.

6.4.1.3

Miscellaneous (MPS Type VI, Metastasis, Cerebral Pseudotumor, Fibrous Dysplasia)

6.4.1.3.1 Mucopolysaccharidosis Type VI

The mucopolysaccharidoses (MPS) are a family of six inherited, autosomal recessive disorders, dominated by the storage of glycosaminoglycan (previ-

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