Intracranial Lipoma Embryologic Origin

Theories regarding the pathogenesis of intracranial lipomas have evolved since the first report of a chiasmatic lipoma by Meckel in 1818.23 A contemporary consensus holds that intracranial lipomas result from the "abnormal persistence and maldifferentiation of the meninx primitiva during the development of the subarachnoid space."20 This theory, first proposed by Verga in 1929 and well summarized and further supported by Truwit and Barkovich in 1990,20 accounts for several of the distinctive features of intracranial lipomas: the subarachnoid, cisternal locations of intracranial lipomas; the commonly associated parenchymal malformations (see Table 37-1); and the incorporation of, and respect for, intracranial vessels and nerves.

Table 37-1

Anomalies Associated with Intracranial Lipomas

Agenesis or hypogenesis of surrounding neural tissue (e.g., corpus callosum) Frontal bone defects; facial dysplasia or hypoplasia Absent, tortuous, or dilated blood vessels; aneurysms Cranial nerve absence or duplication; unusual branching patterns of nerves

(Modified from Wilkins RH, Rengachary SS: Neurosurgery, ed 2. New York, McGraw-Hill, 1996.)

Table 37-2

Genetic Syndromes that Manifest Intracranial Lipomas

Genetic Syndrome

Bannayan

Gorlin-Goldenhar

Encephalocutaneous lipomatosis

Frontonasal dysplasia

Neurofibromatosis variants

Associated Features

Macrocephaly, hemangiomas

(intracerebral and bony) Mandibular hypoplasia, macrostomia, upper vertebral anomalies, and epibulbar dermoids Soft scalp masses with overlying alopecia, papular skin lesions over face and eyes, and progressive intracranial calcifications Midline craniofacial anomalies, hypertelorism, cranium bifidum, cleft lip or nose, and mental retardation Café-au-lait lesions, neurofibromas, Lisch nodules, plexiform neuromas

The meninx primitiva, first described by Salvi, is a mesenchymal derivative of the neural crest. Between days 32 and 44 of embryogenesis, the meninx dissolves, giving rise to the subarachnoid spaces.20 The dissolution of meninx primitiva occurs in a stepwise fashion, regressing first in the region of the ventral brainstem (eventually to form the prepon-tomedullary cisterns), then dorsally (sites of the future per-imesencephalic and dorsomesencephalic cisterns), and finally cephalad (to form the supratentorial cisterns).20,23 This genetic patterning, in conjunction with the predilection for lipoma formation at areas of flexion or at sites of redundant meninx, accounts for the observed distribution of intracranial lipomas.

The vast majority (80% to 95%) of intracranial lipomas are found along or near the midline.6 Approximately 50% of all intracranial lipomas are found within the pericallosal cistern; 20% in the ambient, quadrigeminal plate and chiasmatic cisterns combined; 12% in the cerebellopontine angle (CPA) or internal auditory canal (IAC); and 7% along the convexities of the brain. Other types and locations include rare intradural cervical lipomas, which can spread into the posterior fossa,

Figure 37-1

Sagittal, T1-weighted magnetic resonance image demonstrates the characteristic bright signal image of a curvilinear, pericallosal lipoma (arrow) associated with partial agenesis of the posterior corpus callosum. (Image courtesy of the Neuroradiology Division, Department of Radiology, University of Colorado Health Sciences Center.)

Figure 37-1

Sagittal, T1-weighted magnetic resonance image demonstrates the characteristic bright signal image of a curvilinear, pericallosal lipoma (arrow) associated with partial agenesis of the posterior corpus callosum. (Image courtesy of the Neuroradiology Division, Department of Radiology, University of Colorado Health Sciences Center.)

and multiple intracranial lipomas. Lipomas have also been described in the choroid plexus, interpeduncular cistern, septum pellucidum, and pineal region.6,15,19,23 Lipomas of the choroid plexus are usually extensions of callosal lipomas, and lipomas of the interpeduncular cistern often contain bone within their matrix. Lipomas containing bone are referred to as osteolipomas.

Pericallosal lipomas account for approximately half of all intracranial lipomas, with two distinct subtypes of pericallosal lipomas recognized based on their configuration: the tubulo-nodular and curvilinear. The pathogenesis outlined by Truwit and Barkovich20 helps account for the features of these two subtypes. Tubulonodular lipomas are generally found anteriorly within the pericallosal cistern. Because they form early in embryogenesis, they are usually large, round, or cylindrical and associated with more severe anomalies of the corpus callosum, frontal lobes, and surrounding soft tissue and bone.6,23 In contrast, curvilinear lipomas occur more posteriorly, are usually smaller than tubulonodular lipomas, and form thin, ribbon-like structures (Figure 37-1). These lipomas are more often asymptomatic and are unassociated with severe anomalies. Curvilinear lipomas are thought to form later in embryogenesis, and therefore they do not grow as large and are less likely to disturb callosal or frontal lobe formation. Other anomalies of the neighboring parenchyma, soft tissue and bone, blood vessels, cranial nerves, and various body parts may be associated with intracranial lipomas, depending on their location or as a part of a genetic syndrome.

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