often present. Umbilical hernia, malformed ears, mental retardation, and hypogenitalism in male patients are all common associations.

Greig Syndrome

Greig cephalopolysyndactyly syndrome (GCPS) is characterized by multiple limb and craniofacial anomalies. GCPS is an autosomal-dominant inherited disorder caused by heterozygous mutation or deletion of GLI3.

Trigonocephaly with metopic or sagittal synostosis is a distinctive presenting feature of GCPS. Preand postaxial polydactyly and cutaneous syndactyly of hands and feet are common. Corpus callosal dysgenesis and mild cerebral ventriculomegaly are recognized associations.

Meningeal Anomalies

Anomalies of the cranial meninges commonly accompany other congenital malformations such as Chiari 2 malformation. Lipomas and arachnoid cysts are two important intracranial abnormalities with meningeal origin. Arachnoid cysts were considered in detail in Chapter 28. We therefore conclude our discussion of congenital anomalies by focusing on lipomas.

Lipomas

The 2016 WHO places intracranial lipomas in the mesenchymal, nonmeningothelial CNS neoplasms under "Other Mesenchymal Tumours," but also notes that "whether these various lesions (i.e., lipomas and complex lipomatous lesions) are neoplasms or malformative overgrowths is yet to be determined."

We include lipomas in this chapter rather than in the discussion of intracranial neoplasms because of their frequent association with other congenital malformations.

Fat—adipose tissue—is not normally found inside the arachnoid. Therefore, any fatty tissue inside the skull or spine is abnormal. Because fat deposits commonly accompany congenital malformations such as callosal dysgenesis (41-23) or tethered spinal cord, imaging studies should be closely scrutinized for the presence of additional abnormalities.

Terminology

So-called ordinary lipoma is the most common of all soft tissue tumors and is composed of mature adipose tissue. "Complex lipomatous lesions" may contain other mesenchymal tissues such as striated muscle and have sometimes been referred to as choristomas.

Etiology

Intracranial lipomas are uncommon lesions whose etiology remains poorly understood. Two explanations have been offered.

Lipomas were once thought to be congenital anomalies. This theory postulates that lipomas arise as malformations of the embryonic meninx primitiva (the undifferentiated mesenchyme). The primitive meninx normally differentiates into the cranial meninges, invaginating along the choroid fissure of the lateral ventricle. Maldifferentiation and persistence of the meninx was thought to result in deposits of mature adipose tissue, i.e., fat, along the subpial surface of the brain and spinal cord and within the lateral ventricles.

Recent fluorescence in situ hybridization (FISH) and comparative genomic hybridization (CGH) studies have identified clonal cytogenetic aberrations in

1311

(41-26) Autopsy case demonstrates subpial lipoma attached to quadrigeminal plate. (Courtesy E. T. Hedley-Whyte, MD.)

(41-27) (L) Autopsy shows suprasellar lipoma .

(R) Coronal section shows lipoma attached to hypothalamus. (J. Townsend, MD.)

(41-28) Low-power photomicrograph shows normal fat cells . Prominent vessels course through lesion. (Courtesy E. Rushing, MD.)

1312

nearly 60% of ordinary systemic lipomas. The 12q13-15 region is the most commonly involved site.

Pathology

Location. Nonsyndromic lipomas are usually solitary lesions that can be found in virtually any location in the body, including the CNS. Nearly 80% of intracranial lipomas are supratentorial, and most occur in or near the midline. The interhemispheric fissure is the most common overall site (4050%) (41-24). Lipomas curve over the dorsal corpus callosum, often extending through the choroidal fissures into the lateral ventricles or choroid plexus (41-25).

Between 15-25% are located in the quadrigeminal region, usually attached to the inferior colliculi or superior vermis (4126). Approximately 15% are suprasellar, attached to the undersurface of the hypothalamus or infundibular stalk (4127). About 5% of lipomas are found in the sylvian fissure.

(41-29A) Axial NECT scan shows well-delineated, hypodense (-75 HU) lipoma attached to the quadrigeminal plate. Some calcification is present . (41-29B) Axial T1WI shows that the hyperintense lipoma is attached to the quadrigeminal plate without a distinct medial border.

(41-29C) FSE T2WI shows that the lipoma remains hyperintense (because of J-coupling) and cannot be distinguished from the adjacent CSF in the quadrigeminal cistern.

(41-29D) T1 C+ FS demonstrates that the lipoma suppresses completely and does not enhance.

Approximately 20% of lipomas are infratentorial. The cerebellopontine angle cistern is the most common posterior fossa site (10%).

Size and Number. Lipomas are generally solitary lesions that vary from tiny, barely perceptible fatty collections to huge bulky masses. Most are less than 5 cm in diameter.

Gross Pathology. Lipomas appear as bright yellow, lobulated soft masses. They usually adhere to the pia and underlying parenchyma. At least one-third encase adjacent vessels and/or cranial nerves (41-24).

Microscopic Features. Lipomas are composed of mature, nonneoplastic-appearing adipose tissue with relatively uniform fat cells (41-28). Patchy hyalinization and calcification can be present.

(41-30A) Sagittal T1WI shows aqueductal stenosis , curvilinear lipoma with thin posterior body, and absent splenium of corpus callosum .

INTRACRANIAL LIPOMAS: ETIOLOGY AND PATHOLOGY

Etiology

•Two theories

○Maldifferentiation of embryonic meninx primitiva

○Genetic aberration

Pathology

•Usually solitary

•Supratentorial (80%)

○Interhemispheric fissure (40-50%)

○Quadrigeminal (15-25%)

○Suprasellar (15%)

•Infratentorial (20%)

•Gross appearance: lobulated, yellow

•Microscopic: mature, nonneoplastic adipose tissue

Clinical Issues

Epidemiology and Demographics. Lipomas are relatively rare, accounting for less than 0.5% of intracranial masses. They can be found in patients of all ages. There is a slight female predominance.

Presentation. Lipomas are rarely symptomatic and are usually incidental findings on imaging studies. Headache, seizure, hypothalamic disturbances, and cranial nerve deficits have been reported in a few cases.

Syndromic intracranial lipomas occur in encephalocraniocutaneous lipomatosis (see Chapter 39) and Pai syndrome (cutaneous lipomas and facial clefts).

Natural History. Lipomas are benign lesions that remain stable in size. Some may expand with corticosteroid use.

(41-30B) Coronal T1 C+ scan demonstrates the interhemispheric lipoma . (Courtesy A. Maydell, MD.)

Treatment Options. Lipomas are generally considered "leave me alone" lesions. Because they encase vessels and nerves, surgery has high morbidity and mortality.

Imaging

General Features. Lipomas are seen as well-delineated, somewhat lobulated extraaxial masses that exhibit fat density/signal intensity.

Two morphologic configurations of interhemispheric fissure lipomas are recognized on imaging studies: a curvilinear type (a thin, pencil-like mass that curves around the corpus callosum body and splenium) and a tubulonodular type (a large, bulky interhemispheric fatty mass). Dystrophic calcification occurs in both types but is more common in tubulonodular lesions.

CT Findings. NECT scans show a hypodense mass that measures -50 to -100 HU. Calcification varies from extensive—nearly two-thirds of bulky tubulonodular interhemispheric lipomas are partially calcified—to none, generally seen in small lesions in other locations (41-29A) (41-31A). Lipomas do not enhance on CECT scans.

MR Findings. Lipomas follow fat signal on all imaging sequences. They appear homogeneously hyperintense on T1WI and become hypointense with fat suppression (41-29). Lipomas exhibit chemical-shift artifact in the frequencyencoding direction.

Signal on T2WI varies. Fat becomes hypointense on standard T2WI but remains moderately hyperintense on fast spin-echo studies because of J-coupling. Fat is hypointense on STIR and appears hyperintense on FLAIR. No enhancement is seen following contrast administration.

1314

On SWI, lipomas show hyperintensity surrounded by a lowsignal intensity band along the fat-water interface that is more prominent than seen on T2* GRE sequences.

Other CNS malformations are common. The most frequent are corpus callosum anomalies. These range from mild dysgenesis (usually with curvilinear lipomas) (41-30) to agenesis (with bulky tubulonodular lipomas) (41-25) (41-31).

Differential Diagnosis

Although fat does not appear inside the normal CNS, it can be found within the dura and cavernous sinus. Metaplastic falx ossification is a normal variant that can resemble an interhemispheric lipoma. Dense cortical bone surrounding T1 hyperintense, fatty marrow is the typical finding.

The major differential diagnosis of an intracranial lipoma is an unruptured dermoid cyst. Dermoids generally measure 20-40

HU, often calcify, and demonstrate more heterogeneous signal intensity on MR.

INTRACRANIAL LIPOMAS

Clinical Issues

•Less than 0.5% of intracranial masses

•Usually found incidentally, "leave me alone" lesions

Imaging

•NECT: -50 to -100 HU

○Ca++ rare except in tubulonodular lesions

•MR: "just like fat"

○Other intracranial malformations common

○Often surrounds, encases vessels/nerves

Differential Diagnosis

•Dermoid cyst

•Falx ossification