Optic Pathway Gliomas
The most important primary brain abnormality in NF1 is the OPG (25
). OPG is seen in 15% to 20% of individuals with NF1; almost all present before age 6 years. Less than one-half of these tumors cause vision loss; very few cause precocious puberty (57
). Chiasmatic and retrochiasmatic lesions carry the highest risk for progression and visual loss (56
); however, imaging features that predict development of symptomatic OPG have not been identified (60
). Early identification of OPG by “screening MRI exams” (i.e., MRI exams in asymptomatic NF1 children) may lead to improved visual outcomes (58
Figure 6-1 Optic pathway glioma in NF1. A. Contrast-enhanced sagittal T1-weighted image shows a mass (arrows) infiltrating the chiasm and the hypothalamus and extending into to the lower third ventricle. B. Axial T2-weighted image reveals posterior extension (arrows) of the OPG from the chiasm into the optic tracts to the level of the lateral geniculate body. C. Postcontrast axial T1-weighted image shows incomplete enhancement of the tumor within the optic tracts (arrows).
The tumor can be isolated to a single optic nerve or can involve both optic nerves; the chiasm and the optic tracts can also be involved (Figs. 6-1
). Primary involvement of the optic nerve is more common in the NF1 population, while sporadic OPG (in patients without NF1) more commonly involves the chiasm and optic radiations (61
). Irrespective of the distribution, the clinical course of NF1-associated OPG is more indolent than in the sporadic OPG (61
Optic pathway tumors (and other tumors that arise commonly arise in the diencephalon, near the floor of the third ventricle in NF1 patients) are most commonly of low histological grade; most are pilocytic astrocytomas. In some NF1 patients, these tumors can spontaneously involute over time (Fig. 6-5
). Therefore, aggressive therapy is usually not indicated unless symptoms progress or radiologic evidence of progression is noted. Uncommonly, optic pathway tumors can be highly malignant, demanding a very aggressive treatment approach (59
Tumors of the optic nerve infiltrate and expand the nerve, producing a fusiform mass. Two architectural forms can be distinguished. The intraneural form
is a diffuse expansion of the optic nerve itself without
significant subarachnoid tumor (Fig. 6-2C
). The perineural form
is tumor infiltration of the subarachnoid space (astrocytic proliferation), creating a rim of tumor around a relatively unaffected nerve; at times, both forms can coexist (Fig. 6-3B
). Postcontrast MR using fat suppression will distinguish the two architectural forms of optic nerve gliomas. When the nerve is diffusely infiltrated, enhancing tumor fills the optic nerve sheath, whereas infiltration of the subarachnoid spaces shows a rim of enhancing tumor around a minimally enhancing optic nerve. Marginal cases (characterized by development of mild thickening of the optic nerves or by transient or mild enhancement of the optic nerves) are commonly observed; it is not clear whether these truly warrant a diagnosis of OPG (60
). Finally, optic nerve tortuosity (without abnormal thickening or enhancement) is commonly observed in NF1 and should not be mistaken for an optic nerve glioma (60
Figure 6-2 Optic pathway glioma with extensive infiltration. A. Sagittal T1-weighted image shows a mass extending from the chiasm and hypothalamus (arrows) extending toward the mammillary bodies and the fornices. The pons is slightly enlarged and shows abnormal low T1 signal. B. Axial T2-weighted image shows T2 hyperintense tumor has infiltrated into the medial basal ganglia/internal capsules (white arrows) and the fornices (black arrow). The tumor has extended posteriorly to the level of the lateral geniculate bodies. C. Axial T2-weighted image at a lower level reveals diffuse tumor infiltration, manifested as hyperintensity of the pons and middle cerebellar peduncles (long white arrows). Both optic nerves (short white arrows) are enlarged with homogeneous low T2 signal.
Although tumors of the intraorbital optic nerves can be evaluated with CT, MR is the preferred imaging modality for evaluating the entire CNS in NF1 due to the increased information and lack of exposure of the child to ionizing radiation. Assessment of the precise diameter of the intraorbital optic nerves may be hampered by chemical shift artifact resulting from the location of the nerve adjacent to orbital fat, but use of fat-suppression techniques eliminates the chemical shift artifact and allows the nerve to be excellently evaluated, along with the intracranial optic nerves, optic chiasm, optic tract, and optic radiations. T1 and fatsuppressed T2-weighted axial and coronal sequences, using less than or equal to 3 mm slice thickness, should be obtained through the globes, optic nerves, and optic chiasm and followed by fat-suppressed contrastenhanced T1 axial and coronal images. A routine brain scan should follow to look for involvement of the optic tracts and other areas of the brain. Fluid-attenuated inversion recovery (FLAIR) and T2-weighted images show involvement of the optic tracts as high signal intensity extending posteriorly, often to the level of the lateral geniculate bodies; the internal capsules and adjacent basal ganglia are commonly infiltrated (Figs. 6-1
). Further extension beyond the optic pathways can also be identified in some patients: superiorly into the hypothalamus, fornices, and
septum pellucidum; laterally into the temporal lobes; posteriorly into the optic radiations; and inferiorly into the cerebral peduncles and the brainstem (Fig. 6-2
). Rarely, tumor can extend into the lateral ventricles.
Figure 6-3 Optic nerve tumor with infiltration of the optic nerve sheath. A. Postcontrast axial T1-weighted image with fat suppression shows enlarged, enhancing the left optic nerve (long white arrow) and tumor infiltration into the dilated perineural spaces (low T1 signal bordered by a thin rim of enhancement [short white arrows]). The right optic nerve (black arrows) shows a smaller tumor. B. Oblique sagittal T2-weighted image of the left optic nerve with fat suppression reveals thickening and elongation of the nerve. The sheath around the nerve shows marked dilatation and hyperintense signal (white arrows).
Figure 6-4 Multivoxel spectroscopy of optic pathway tumor in NF1. A. Axial FLAIR image shows tumor infiltration in the chiasm/hypothalamus (long white arrow) and into the optic tracts (short white arrows). B. Color map of the choline to creatine ratio (Ch:Cr) from a long echo (TE = 270) multivoxel spectroscopy reveals the highest ratio of choline to creatine (red color) at the level of the chiasm/hypothalamus. Significant Ch:Cr elevation is also evident in the thalami bilaterally (yellow color). C. MR spectra from four different ROIs (numbered in image B). Highest (Ch:Cr) is evident in box 1, at the level of the chiasm/hypothalamus. Ch:Cr levels are elevated in the thalami (boxes 3, 4) compared to the putamen (box 2); this suggests the presence of microscopic tumor infiltration in the thalamus, despite the normal FLAIR signal.
Figure 6-5 Spontaneous partial involution of a diencephalic tumor. A and B. Axial T2 FLAIR (A) and contrast-enhanced T1 (B) images of a 5-year-old reveals a mass in the inferior left diencephalon with intense enhancement (long arrows, A, B); nonenhancing, presumed vacuolation is evident in the right globus pallidus (short arrow, A). C. Axial FLAIR image 3 years later (without intervening treatment) shows only a small T2 bright residual abnormality, immediately behind the anterior commissure (arrow), which could represent vacuolation or residual tumor (lesion did not enhance). T2 abnormality previously evident in the right globus pallidus shows near-complete resolution.
Care should be taken not to mistake enlarged subarachnoid spaces within the optic nerve sheaths (the result of ectasia of the dura surrounding the optic nerves) for optic nerve tumors. The enlarged perioptic spaces have signal intensity identical to cerebrospinal fluid (CSF) and do not enhance after contrast administration. Care should also be taken not to confuse extension of OPG into the optic tracts (and beyond) with the characteristic T2 bright foci encountered in pediatric patients with NF1 (see next section). Differentiation of the two pathologies can be very difficult; features of OPG infiltration include contiguity, mass effect, low T1 signal on precontrast images, enhancement after contrast administration, and significant elevations of choline (at MR spectroscopy) (Fig. 6-4
Other Gliomas and Tumor-Like Conditions
Astrocytomas are more common in NF1 than in the general population (25
); they can arise anywhere in the CNS. Pilocytic astrocytomas (Figs. 6-6
) are most common, but other low-grade (71
) and higher-grade tumors (70
) also occur. Almost any part of the CNS can be affected; outside of the optic pathway/diencephalon, the most common site is the brainstem (Fig. 6-7
). It is of interest to note that the brainstem tumors occurring in NF1 seem to differ biologically from brainstem tumors in other patients. In contrast to the predominance of pontine tumors in the general population, the medulla is the most common site of brainstem tumor origin in NF1, followed by the mesencephalon (particularly the periaqueductal region/tectum) and the pons. This difference in location may partly explain the indolent course and vastly better long-term outcome of most brainstem tumors in NF1 (73
). Many tumors do not require therapeutic intervention (52
); some may regress spontaneously (77
). Thus, management of brainstem tumors in children with NF1 is generally not aggressive unless clinical progression is seen—isolated radiologic progression is considered less serious. Other common locations for astrocytomas in children with NF1 are the cerebellum (Fig. 6-6
), the corpus callosum, and the cerebral hemispheres (Fig. 6-8
Chronic, nonprogressive enlargement of the pons, medulla, and middle cerebellar peduncles is occasionally observed in children with NF1 in association with, and likely the result of, extensive myelin
vacuolation (Fig. 6-9
). This hamartomatous enlargement should not be misdiagnosed as a brainstem glioma; differentiating features from infiltrating gliomas include normal or nearly normal signal in T1-weighted images, only modest (and usually heterogeneous) signal increase on T2 images, absence of contrast enhancement, and lack of progression on follow-up scans. The T2 signal abnormalities partly regress during adolescence, though the brainstem remains enlarged. For more discussion of brainstem tumors, see Chapter 7
Figure 6-6 Development of a pilocytic astrocytoma in an area of extensive myelin vacuolation. A. Axial T2-weighted image of a 5-year-old child reveals multiple characteristic foci of T2 hyperintensity (arrows) in the deep cerebellar white matter and middle cerebellar peduncles. B and C. Axial T2-weighted (B) and postcontrast sagittal T1-weighted image (C) obtained 4 years later. A large rim-enhancing T2 hyperintense mass (arrows) has developed within the deep right cerebellar white matter, with surrounding edema.
Figure 6-7 Midbrain juvenile pilocytic astrocytoma in a 14-year-old with NF1. A. Postcontrast T1-weighted image shows a sharply marginated, intensely enhancing mass (white arrow) in the dorsal inferior midbrain. B. Axial FLAIR image demonstrates the well-circumscribed hyperintense mass (white arrow) to have minimal surrounding edema. C. Single voxel long echo (TE = 270) spectroscopy of the tumor reveals a very high Ch:Cr (3.49; normal < 1.5) and presence of lactate (double arrows). These MRS findings, worrisome for a malignant tumor, are commonly observed in pilocytic astrocytomas, the likely diagnosis in this patient. N-Acetylaspartate to creatine ratio (NAA:Cr) is decreased (1.68; normal > 2). The NAA:Cr is higher than expected for a glial neoplasm, likely secondary to contamination of the voxel by surrounding (normal) brain tissue as the voxel (2 × 2 × 2 cm size) is slightly larger than the tumor.
Figure 6-8 Grade 2 (fibrillary) glioma in the left temporal lobe of a 10-year-old with NF1. A. Axial T2-weighted image shows a tiny, hyperintense lesion (white arrow) in the white matter of the posterior left temporal lobe. B. Axial T2-weighted image a year later shows an ovoid, well-defined T2 bright focus in the same location (arrow), representing the enlarged tumor.
Hamartomatous enlargement of the hypothalamus is occasionally seen in children with NF1 (Fig. 6-10
), usually not associated with precocious puberty or gelastic seizures (79
). The lesion typically lies within the walls of the hypothalamus in the lower third ventricle (intrathalamic/sessile configuration). Like hypothalamic hamartomas in children without NF1, they are nearly isointense to gray matter on T1- and T2-weighted images and do not enhance (80
); they should not be confused for a chiasmatic or hypothalamic glioma.
Figure 6-9 Probable hamartomatous enlargement of the brainstem. A. Sagittal T1-weighted image reveals diffuse enlargement of the pons and, especially, medulla (white arrows); the signal intensity is normal. B. Axial T2-weighted image shows enlargement and heterogeneously increased signal in the dorsal right pons and middle cerebellar peduncle, with slight mass effect on the fourth ventricle (arrow).
In a small but significant number of patients with NF1, hydrocephalus is present (81
). The site of obstruction of CSF flow is usually the aqueduct of Sylvius, resulting from benign aqueductal stenosis, from extensive vacuolation of the tissues surrounding the aqueduct (see next section), or from tumors (astrocytomas or hamartomas) of the adjacent tectum or tegmentum of the mesencephalon. The diagnosis of a tectal glioma is relatively simple using MR. In contrast to benign aqueductal stenosis (in which the proximal aqueduct is dilated and the tectum displaced superiorly and thinned, see Chapter 8
), gliomas enlarge the tectum and narrow (or completely obliterate) the aqueduct.
Occasionally, the tectum may appear short and thick in aqueductal stenosis because of mass effect on the rostral aspect of the tectum by a dilated suprapineal recess (see Chapter 8
). In these cases, the patient should be reevaluated after ventricular decompression.
Figure 6-10 Hypothalamic hamartoma in a 15-year-old boy with NF1. A. Sagittal T1-weighted image shows a gray matter intensity mass (short white arrows) in the floor of the third ventricle, posterior to the chiasm (long white arrow), and anterior to the mammillary bodies (black arrow). B. Coronal T2-weighted image reveals bilateral gray matter intensity mass (black arrows) in the floor and left lateral wall of the hypothalamus.
White Matter Abnormalities and Characteristic Hyperintense T2 Lesions
White matter abnormalities are evident in many NF1 children. Many patients have an increased volume of cerebral white matter, typically associated with a large midsagittal corpus callosum area (Fig. 6-11
); these findings are more pronounced in patients with macrocephaly (˜40% of those with NF1 (86
)) and are associated with a lower overall IQ (87
). Advanced MRI diffusion analysis of the white matter of NF1 patients reveals a decreased fractional anisotropy (FA) and an increased apparent diffusion coefficient (ADC). Radial diffusivity is disproportionately increased, suggesting that looser packing of axons, with or without myelination changes, may contribute to the increased volume of cerebral white matter (88
). Abnormalities of apoptosis (leading to redundancy of fibers connecting the cerebral hemispheres) have also been postulated as an explanation of the increased white matter volume (87
Figure 6-11 Sagittal T1-weighted image shows an abnormally thick corpus callosum (white arrows). The ventral pons (black arrows) is prominent.
MR imaging often reveals characteristic hyperintense T2 lesions in pediatric patients with NF1, located in the brainstem, cerebellar white matter, basal ganglia (especially the globus pallidus), thalamus, internal capsule, corpus callosum, and, occasionally, corona radiata (Figs. 6-12
) but not in the centrum semiovale or the subcortical white matter. These lesions are characteristically multiple; they have little or no mass effect (unless extensive confluence is evident, in which case mild mass effect may be detected, Fig. 6-9
) and do not elicit vasogenic edema; their T1 signal appears iso- or slightly hypointense compared to uninvolved white matter; and they do not enhance after intravenous administration of paramagnetic contrast. Intermediate echo (TE = 144 ms) proton MR spectroscopy shows nearly normal N
-acetylaspartate (NAA)/creatine (Cr) and moderately increased Cho/Cr (69
). These lesions are rarely identified in the first 2 years of life but develop/become visible from late infancy to about age 12 years (Fig. 6-12
); new lesions almost never develop after age 15 years. The lesions start to regress starting late in the first decade of life (90
) and slowly disappear; they are almost never seen in patients over the age of 20 years (Fig. 6-13
). In the peak age group (6-12 years), they are seen in up to 90% of NF1 children (39
). A pathologic analysis has shown that these regions contain areas of myelin vacuolation, where the layers of myelin become separated as they spiral around the axon (94
). As might be expected, water diffusivity is increased in these areas of T2 hyperintensity to a greater degree than in other areas of the brain (95
); anisotropy is normal, but transverse eigenvalues are increased (96
). Multishell diffusion MRI studies have shown that the water contained in the T2 hyperintense lesions behaves as intracellular water that has obtained extracellular-like properties, thus supporting the intramyelinic edema appearance of vacuolation (97
The discoveries that the oligodendrocyte myelin glycoprotein gene is imbedded within the NF1
) and that its protein product (neurofibromin) is required for Schwann cell myelination (18
) further support dysgenesis of myelin as the cause of the white matter abnormalities in NF1. The vacuolization appears to develop, therefore, in regions where the myelin is inherently abnormal; presumably, the return of normal T2/FLAIR signal on subsequent exams reflects myelin
repair or remyelination (98
). The increased diffusivity of white matter (even normal-appearing white matter) in NF1 (99
) further supports the concept of abnormal myelin as an underlying cause.
Figure 6-12 Evolution of characteristic T2 bright signal abnormalities in pediatric NF1. A. Axial T2-weighted images through the posterior fossa of a 14-monthold reveals a few ill-defined foci of increased T2 signal within the deep cerebellar white matter in the middle cerebellar peduncles (arrows). B. Corresponding axial T2-weighted image in the same patient at age 3 years. There are now extensive foci of T2 abnormality within the deep cerebellar white matter, the middle cerebellar peduncles, and the pons (arrows). The volume of the involved tissue is mildly increased, resulting in mild distortion of the fourth ventricle.
In view of the frequency with which the characteristic T2 hyperintense foci are encountered in children with NF1, neither close imaging follow-up nor biopsy is indicated when they occur in the characteristic locations, are not significantly hypointense on T1-weighted images, and lack associated mass effect, edema, and enhancement. However, a growing astrocytoma of the basal ganglia, pons, or cerebellum, when small, may be indistinguishable from one of these lesions on noncontrast MR—especially in cases when there are numerous, confluent T2 hyperintense foci in expected locations. If a lesion has any suspicious characteristics, it is recommended that the patients have a follow-up contrast-enhanced MR scan, including diffusion-weighted imaging (DWI), perfusion imaging, or proton MR spectroscopy, 6 months to a year after the first study. A tumor will typically show progressive enlargement and mass effect on adjacent structures and often enhances (Fig. 6-6
); it may also have locally reduced diffusion in its solid portion (95
), have increased blood volume (101
), and show elevated choline with decreased creatine and NAA on proton spectroscopy (69
Figure 6-13 Partial resolution of basal ganglia signal abnormalities in NF1. A. Axial T2-weighted image at age 7 years shows multiple bilateral hyperintense abnormalities (white arrows) in the globi pallidi and internal capsules. B. Axial T2-weighted image 4 years later shows significant regression of the abnormalities.
T1 hyperintensity is observed in the globi pallidi of patients with NF1, with a different radiologic appearance than the characteristic T2 hyperintense foci: the hyperintensity on T1-weighted images develops after the T2 prolongation appears and persists after T2 prolongation disappears, a time course that suggests that the T1 shortening represents delayed or reactive formation of myelin (Fig. 6-14
). The distribution of the T1 bright signal can be diffuse within the T2
abnormality or be located along its periphery/rim. (Calculation of T1 values shows that the frontal white matter, caudate nuclei, putamina, and thalami also have some T1 shortening (86
), but it is not apparent to visual inspection in these other areas.) In some patients, the T1 shortening is observed at the time of a first MRI, prior to exposure to gadolinium-based contract agents. However, many of the NF patients with appreciable T1 bright signal changes in the globus pallidus also have bright T1 signal in their dentate nuclei and have undergone multiple contrast-enhanced MRI exams; the T1 bright signal changes in some of these cases, therefore, may be a reflection of gadolinium deposition in the brain (107
), rather than a reflection of an intrinsic abnormality of the myelin.
Figure 6-14 T1 and T2 prolongation in the basal ganglia in NF1 of an 18-year-old. A and B. Noncontrast sagittal T1- (A) and axial T2 (B)-weighted images show a lesion in the left globus pallidus with hyperintensity on both T1 and T2 images (arrows). There was no associated mineralization or calcification on susceptibilityweighted images or CT images (not shown).
The overwhelming majority of these characteristic T2 bright lesions resolve over time. However, as with all small, nonenhancing foci of T2 prolongation in the brain, the biological behavior of white matter lesions in NF1 cannot be predicted with complete accuracy
by imaging alone. Benign-appearing lesions can degenerate into neoplasms (108
), and neoplastic-looking lesions can shrink or disappear (63
). Also remember that focal lesions developing in the cerebral cortex or in the subcortical or deep white matter do not represent characteristic T2 bright lesions of pediatric NF1
); if present, these should be followed by sequential imaging, as they may potentially represent lowgrade neoplasms.
Occasionally, the hippocampi of pediatric patients with NF1 can show diffusely increased signal on T2-weighted images, often with increased volume (Fig. 6-15
); this finding can be unilateral or bilateral and is not progressive; it persists throughout childhood and adolescence (93
), in contrast to the characteristic T2 lesions described above that regress during later childhood and adolescence. The nature of the hippocampal abnormality is not known. NF1-related hippocampal T2 changes should not be confused with hippocampal sclerosis (in which case the hippocampal volume is decreased) or with neoplastic infiltration.
Dysplasia of the cerebral vasculature is increasingly recognized in children with NF1; it develops at a young age, with prevalence of up to 6% (109
). The incidence in the symptomatic NF population appears to be higher, with a predicted range of 7% to 15% (112
). The known significant incidence of vascular abnormalities in the cerebral vasculature and in other parts of the body makes vascular dysplasia a distinct clinical entity. Any child with NF1 who has seizures, mental retardation, paralysis, or severe headaches may have a vascular dysplasia, particularly in the absence of a brain tumor or hydrocephalus. However, clinically, a majority of NF1 patients with vascular dysplasia do not have focal deficits as a result of their arteriopathy by the time the dysplasia is diagnosed with MRI. Intracranial vascular dysplasias can result as a complication of irradiation for optic nerve or optic chiasm gliomas (presumably radiation arteritis in these cases; see discussion in Chapter 3
Common vascular abnormalities include stenoses, occlusions, ectasia, moyamoya disease, and fusiform aneurysm formation. Most commonly, the dysplasia results from intimal and smooth muscle proliferation, with resultant stenosis or occlusion; they can progress overtime (Fig. 6-16
) and may require revascularization surgery (109
). The carotid (common or internal) artery, proximal middle cerebral
artery, or proximal anterior cerebral artery is the most common site of involvement (Figs. 6-16
). Moyamoya phenomenon with marked enlargement of the lenticulostriate arteries (which function as collateral vessels) is seen in many such patients (113
). (Moyamoya is discussed further in Chapter 12
.) Fusiform arterial dilation, cerebral aneurysms, and arteriovenous fistulas are also described, although less commonly (109
Figure 6-15 High-resolution coronal FSE T2-weighted image reveals hyperintense T2 signal of both hippocampal formations (white arrows) (compare signal to that of adjacent temporal cortex). The right hippocampus is brighter and larger than the left.
Figure 6-16 Progressive NF1-related vasculopathy. A. Axial T2-weighted image at age 18 months shows normal-appearing signal voids in the basilar and proximal anterior and middle cerebral arteries—note the normal right MCA (arrow). B. Axial T2-weighted image 2 years later shows diminished size of the right MCA signal void (white arrow). C. Coronal reconstructed MIP image from a 3D TOF MRA reveals a lack of flow-related enhancement in the right MCA (arrows), consistent with an acquired complete occlusion (or very severe stenosis).
Vascular dysplasias can be difficult to detect on standard CT or MR images; with MR, close inspection of the arterial signal voids (Figs. 6-16
), especially the distal carotid arteries and the circle of Willis (to detect narrowing of the cavernous or supraclinoid carotids and proximal middle and anterior cerebral arteries), is essential in patients with NF1 to avoid delays in clinical detection of these lesions; note that the intracranial vessels are poorly seen on FLAIR images, so acquisition and inspection of T2-weighted spin-echo images is strongly encouraged when imaging children with NF1, particularly those with declining function
. As many vascular lesions are subtle and easily overlooked, some authors promote the routine use of MR angiography to better delineate intracranial vasculopathy (17
). If vascular dysplasia is suspected, MR (or CT) angiography should be performed; conventional, catheter angiography is usually reserved for a presurgical workup. Angiography generally shows either severe stenosis or occlusion of the involved cerebral vessels (Fig. 6-17
); arterial dysplastic lesions are also demonstrated.
Calvarial and Orbital Abnormalities
Other cranial/intracranial manifestations of NF1 include deficiencies of the sphenoid wing and of the occipital bone along the lambdoid suture. Sphenoid bone dysplasia
allows herniation of the temporal lobes into the orbit (Fig. 6-18
). The pulsations of the temporal lobe may be transmitted through the globe, thus being visible externally as pulsatile exophthalmos (or enophthalmos secondary to atrophy of orbital contents). The globe may be dysplastic, enlarged (buphthalmos), or hypoplastic. Sphenoid wing deficiency is nearly always associated with PNs in the orbit or periorbital regions (Fig. 6-18
). Recent work indicates that the abnormalities of the bony orbit may progress over time, suggesting that these are not simple dysplasias of bone, as previously postulated, but may in part result from erosions by the adjacent neurofibromas (Fig. 6-18
) or optic nerve tumors (118
). Occipital bone dysplasia is usually not clinically significant; it is easily recognized with CT imaging. With MR imaging, it presents as distortion and focal outward bulging of the cerebellar hemisphere deep to the bone defect; this can be confusing until the bone defect is recognized.
Figure 6-18 Dysplasia of the sphenoid wing with associated plexiform neurofibroma. A. Axial T2-weighted image shows an extensive neurofibroma (white arrows) infiltrating the superior orbit and the left temporal fossa (the V1 distribution). The greater wing of the sphenoid bone is dysmorphic, resulting in anterior extension of the contents of the left middle cranial fossa. B. Axial T2-weighted image at a slightly lower level shows the inferiorly displaced, proptotic left globe. Neurofibroma is also evident within an enlarged pterygopalatine fossa (V2 distribution, white arrows).
Neurofibromas and Plexiform Neurofibromas
Another cause of intracranial complications in NF1 is excessive growth of craniofacial PNs. They tend to progress along the nerve of origin (usually small, unidentified nerves) into the intracranial space, causing distortion and compression of the brain. They most commonly arise from branches of the trigeminal nerve, with occasional intracranial involvement to the level of the cavernous sinus. In the orbit, they act as masses, causing impaired ocular movements and exophthalmos (Fig. 6-19
). Orbital neurofibromas commonly arise in the region of the orbital apex or superior orbital fissure (in the distribution of the first division of the trigeminal nerve); careful evaluation of the images
often reveals extension into the cavernous sinus, nasopharynx, or pterygomaxillary fissure. The neck is another common location for neurofibromas, especially along the course of the vagal nerve, with an estimated occurrence of 25% to 30% in patients with NF1 (Fig. 6-20
). Neck masses, their appearance, and their differential diagnoses are discussed in Chapter 7
Figure 6-19 Rapid progression of orbital plexiform neurofibroma. A. Axial fat-suppressed T2-weighted image of an 8-month-old reveals a dysplasia of the left greater wing of the sphenoid with resultant enlargement of the left middle cranial fossa. Mild soft tissue thickening is evident in the left cavernous sinus (black arrow) and in the lateral left orbital extraconal soft tissue (white arrows) indicative of a V1 distribution plexiform neurofibroma. The left globe is mildly proptotic. B and C. Corresponding axial fat-suppressed T2 (B) and contrast-enhanced T1 (C) images 2 years later show marked progression in the size of the plexiform neurofibroma within the cavernous sinus (black arrows) and in the pre- and postseptal soft tissues of the left orbit (white arrows). Left globe proptosis has significantly worsened.
Solitary neurofibromas have slightly greater signal intensity than does skeletal muscle on T1-weighted sequences. On T2-weighted sequences, the lesions have variable signal intensity. Most commonly, the periphery of the lesions tends to be of high T2 signal intensity with respect to muscle, whereas the center of the lesions is often of low signal intensity (121
); this has been referred to as the “target sign” (Figs. 6-20
). The central area of decreased T2 intensity is probably related to the known dense central core of collagen within these lesions (122
). Collagen has a low mobile proton density and therefore is of low signal intensity on T2-weighted images. Enhancement after administration of paramagnetic contrast is variable, although at least a portion of the tumor usually enhances (usually the central aspect, corresponding to the central T2 hypointense (“target”) components (see Fig. 6-24
Figure 6-20 Slow progression of an extensive plexiform neurofibroma. A. Coronal inversion recovery fat-suppressed T2 image at age 5 years reveals several infiltrative T2 hyperintense masses (numbered 1 to 3, seen to connect on other images) in the paravertebral, deep cervical, and upper thoracic soft tissues typical of a plexiform neurofibroma. B. Corresponding coronal fat-suppressed T2 image 12 years later shows marked enlargement of the plexiform neurofibroma. The largest component measures approximately 40 mm in diameter. It has preserved central T2 dark/target characteristic (evident in all components of the mass); findings are not suggestive of a malignant transformation.
PNs tend to be of low attenuation on CT and generally show little enhancement after administration of intravenous contrast.