Lipomyelomeningocele: Pathology, Treatment, and Outcomes

A Review

Christina E. Sarris, B.S; Krystal L. Tomei, M.D., M.P.H; Peter W. Carmel, M.D; Chirag D. Gandhi, M.D.

Disclosures

Neurosurg Focus. 2012;33(4):e3 

In This Article

Embryology

The pathology of congenital spine and spinal cord defects is best understood through knowledge of embryological development. Central nervous system development initiates in the third week in a process known as neurulation. During primary neurulation, the ectoderm overlying the notochord proliferates, forming the neural plate. The lateral edges of the neural plate soon elevate to form the neural folds. As development continues, the neural folds continue to elevate and approach each other in the midline, fusing to form the neural tube. This fusion begins in the cervical region and proceeds in both the cephalic and caudal directions.[25,58] Secondary neurulation is the process of development of the caudal cell mass that forms the caudal-most portion of the neural tube, forming the spinal segments below L-2. Following neural tube closure, the epithelial ectoderm separates from the neural ectoderm, a process known as disjunction. The epithelial layers fuse to create skin covering the neural tube, and mesenchyme migrates between the neural tube and skin to form the meninges, neural arches of the vertebrae, and paraspinal muscles.[64] In the third month of development, the spinal cord extends the entire length of the embryo. However, as development continues, the vertebral column and dura lengthen more rapidly than the neural tube, and the terminal end of the spinal cord shifts to a higher vertebral level.[58] In a whole-spine imaging study by Kesler et al.,[35] the conus medullaris in all 100 children studied terminated between the lower third of T-12 and the middle of L-2, with the mean level at the lower third of L-1 and a mode at the L1–2 disc space. No child presented with the conus medullaris below the middle third of L-2. Pinto et al.[56] observed that the inferior tip of the conus medullaris resides at or above the L-2 level in 95.12% of the sample studied, with the greatest number (41.5%) at the L-1 level. While there is normal variation in the vertebral level of termination, the "adult" level is reached approximately 2 months postnatally.[4] If there are abnormalities in any of these developmental stages, a spinal dysraphism, or defect in closure of the neural tube, can result (Table 1).

Spinal dysraphisms can be classified as either open or closed dysraphisms (Table 2). Open spinal dysraphisms include meningocele, myelomeningocele, myeloschisis, encephalocele, and anencephaly. All involve exposure of nervous tissue and/or meninges to the external environment. Closed spinal dysraphisms such as lipomyelomeningocele, diastematomyelia, and spina bifida occulta have no exposed neural tissue[2] and are accompanied by cutaneous markers in 43%–95% of cases,[8,22,23,34,40,45,55,64] and include lesions such as subcutaneous masses, capillary hemangioma, dimples, and hairy nevus.[14,68] These cutaneous markers may present with closed spinal dysraphisms because of the chronological association of neural tube closure with separation of neural and epithelial ectoderm during embryological development.[14] These cutaneous markers can be used to recognize cases in an asymptomatic neonate. Guggisberg et al.[23] suggested that a combination of 2 or more congenital midline skin lesions is the strongest marker of closed spinal dysraphism. Kriss and Desai[40] observed that only atypical dimples were found to be associated with a high risk for spinal dysraphism, characterized by high placement on the back (> 2.5 cm from the anus), large size (> 5 mm), and appearance in combination with other lesions. Other high-risk cutaneous markers were raised lesions such as tails, masses, hairy patches, hemangiomas, and the presence of multiple skin lesions.

Several types of closed spinal dysraphisms result from embryological abnormalities during primary neurulation. Those that arise from premature disjunction result in fusion of the spinal cord with fatty elements, the most common of which is a lipomyelomeningocele.[68] When premature disjunction occurs, the epithelial ectoderm detaches prematurely from the neural ectoderm, allowing mesenchyme to contact the inner portion of the developing neural tube.[50] The mesenchyme is induced by the dorsal surface of the closing neural tube to form fat, and this prevents proper neurulation. The extent of the fatty tissue is limited laterally by the neural ridge because the ventral surface of the neural plate induces the mesenchyme to form meninges. This results in a junction between meninges and fat at the neural ridge, and thus the lipoma extends posteriorly through the meningeal and bony defect and into subcutaneous tissues in the extradural space. The neural placode-lipoma interface, which is the connection between the spinal cord and the lipoma, can lie outside of, within, or at the edge of the spinal canal. In contrast to a lipomyelocele where the neural placode-lipoma interface is located within or at the edge of the spinal canal, lipomyelomeningocele is characterized by a placode-lipoma interface located outside the spinal canal.[64]

Tethered cord is inherently associated with lipomyelomeningocele as the lipoma tethers the cord to the adjacent dura and soft tissue. Syringomyelia may occur in 20%–25% of patients with a tethered cord.[5] Lipomyelomeningocele can also be associated with other abnormalities. In a study of 97 patients with lipomyelomeningocele, Hoffman et al.[27] reported an association with genitourinary tract anomalies (4.1%), split cord malformations (3.1%), associated dermal sinuses (3.1%), dermoid or epidermoid cysts (3.1%), diastematomyelia (3.1%), terminal hydromyelia (3.1%), anal stenosis (1.0%), and Down syndrome (1.0%). Kanev et al.[34] also reported associated anomalies in a series of 80 patients with lipomyelomeningocele, including scoliosis (8.75%), amniotic band extremity deformity (7.5%), sacral dysgenesis (5.0%), anterior anal displacement with stenosis (2.5%), and hydromyelia (2.5%). There is an increased incidence of Chiari malformation Type I in patients with lipomyelomeningocele as compared with the general population, with 13% of a 54-patient series with Chiari malformation also having lypomyelomeningocele.[66]

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