Definition and Classification

The literature regarding LMM is very confusing. The term lipomyelomeningocele has mistakenly been grouped into the spectrum of all lipomas of the lumbosacral spine and is often misleading because it implies herniation of neural elements through a spina bifida defect into a meningeal sac. Unlike myelomeningoceles or meningoceles, however, there is neither neural nor meningeal tissue found outside the spinal canal. The neural elements remain within the spinal canal, and there is typically an associated subcutaneous lipomatous mass that protrudes through a midline bony defect, displaces the dura, and infiltrates and tethers the spinal cord.^5,6 In 1982, Chapman classified three anatomic variants of LMM according to the relationship of the lipoma–spinal cord interface: dorsal, caudal, and transitional.⁷

Lipomas that insert onto the dorsal surface of the conus medullaris are classified as dorsal-type LMMs (Fig. 215-1). In this variant there is typically a substantial subcutaneous fat component attached to the underlying spinal cord via a fibrolipomatous stalk of varying thickness. The posterior half of the lower spinal cord at the site of fusion of the neural fold is unfused (i.e., partial dorsal myeloschisis is present), and the stalk attaches directly to the exposed alar and basal cord regions, just posterior and medial to the posterior columns and dorsal to the central canal.⁸ The lipoma can be located either in the midline (68%) or eccentrically (32%) and can extend into the central canal and expand it for variable distances.⁹ Along the lateral interface of the attachment of the lipoma to the spinal cord, the dura and pia are fused. Sensory nerve roots emerge just anterior to this lateral line of fusion, which can be traced circumferentially. There is often no evidence of leptomeninges posteriorly, and no sensory or motor roots are found within the actual substance of the lipoma.

FIGURE 215-1 Oblique cross-sectional representation of a dorsal-type lipomyelomeningocele. The lipoma attaches to the dorsal aspect of the conus and is continuous with the subcutaneous mass via a fibrolipomatous stalk (not pictured). The lipoma-cord interface is posterior to the dorsal root entry zones, and the spinal cord caudal to the interface is normal. Note the presence of a thickened fatty filum terminale.

In caudal- or terminal-type LMM, the lipoma–spinal cord interface is located at the caudal end of the conus, almost like a continuation of the spinal cord itself (Fig. 215-2). The remaining lipomatous mass may then lie entirely within the spinal canal or extend dorsally through a defect in the dura and bone into the subcutaneous space. The fatty tumor may either replace or surround the filum terminale, or a separate filum may lie anteriorly. The nerve roots may lie ventral to the lipoma or pass through the mass itself. A caudal-type LMM is often asymmetrical and involves more of the cord and nerve roots on one side, and unlike the discrete lipoma-cord interface of a dorsal-type LMM, the lipoma-cord interface in a caudal-type LMM is diffuse.⁸

FIGURE 215-2 Oblique cross-sectional representation of a caudal-type lipomyelomeningocele. The lipoma attaches to the caudal aspect of the conus, and the lipoma-cord interface is diffuse and vertically oriented.

In addition to the dorsal and caudal LMM forms originally described by Chapman, transitional forms between the two variants may exist (Fig. 215-3). Frequently in these cases, the more cephalad portion is of the dorsal type with posterior rootlets emerging from the cord just ventral to the line of fusion of the lipoma to the cord and leptomeninges. Caudally, however, as the line of fusion is displaced ventrally, nerve roots emerge from the anterolateral portion of the lipoma mass. Magnetic resonance imaging (MRI) can often be helpful in defining the type and may be useful in determining the operative approach. The transitional form is considered by some to simply represent a continuum between the other two forms.^7,8,10,11

FIGURE 215-3 Oblique cross-sectional representation of a transitional-type lipomyelomeningocele. The lipoma-cord interface extends from the rostral end of the myeloschisis to the conus. The lipoma-cord interface is dorsal to the dorsal root entry zone rostrally. As the lipoma is followed caudally, the dorsal roots enter at or above the interface.

According to Pang (personal communication), a fourth classification of LMM not previously described in the literature is the chaotic form. This type of LMM contains “unruly internal structures that do not obey the embryologic logic of either dorsal or transitional lipoma.”

Embryology

Congenital lumbosacral lipomas develop as a result of disturbances in the precise sequence of events encompassing the embryologic stages of primary and secondary neurulation, which occur between postovulatory day (POD) 25 and 48.⁸ Closure of the posterior neuropore marks the completion of the first phase (primary neurulation) of neural tube formation, at which point the fetal spine is covered by ectoderm but the lumbar, sacral, and coccygeal segments have not yet developed.¹² The caudal end of the neural tube and the notochord combine into a large aggregate of undifferentiated cells in a process called condensation. This caudal cell mass extends to the level of the tail fold and represents a solid neural cord containing neurons, neural crest, glial cells, and ependymal cells. In a process called canalization, a series of small vacuoles within the mass begin to coalesce and enlarge to create an ependymal-lined tube in continuity with the rostral central canal of the previously formed neural tube.¹³ This process takes place between POD 27 and 29 and leads to formation of the ventriculus terminalis.^14,15

The third phase in formation of the caudal neural tube involves regression of the structures previously derived during canalization. During retrogressive differentiation, the embryologic tail disappears, whereas the filum terminale, coccygeal ligament, and ventriculus terminalis of the conus remain. This process is thought to complete by POD 48 to 52.^{12,13,15–17} Because cell rests within the caudal cell mass have totipotent characteristics, a small clone of lipomatous cells developing in the region would not necessarily inhibit the growth of surrounding structures. The bony vertebrae develop subsequently, and the sacrococcygeal segments also undergo regressive changes to decrease the number of segments originally present.¹⁵ As a result, vertebral maldevelopments are found in conjunction with the neural defects arising during this period.

McClone and Naidich proposed that the process of dysjunction, in which the closed neural tube detaches from the cutaneous ectoderm, occurs prematurely in patients with LMM. Consequently, a dorsal cleft is maintained and paraxial mesenchyme tissue has access to the prospective lumen of the neural tube, thereby further preventing its closure. The luminal surface of the ectoderm induces mesenchymal cells to differentiate along a path that results in adipocyte formation, whereas the outer surface of the incompletely closed neural tube causes the mesenchyme to form the piaarachnoid and dura.^18,19 Other tissues have been found in the substance of an LMM, including striated muscle, cartilage, nerve cells, ependyma, and even cerebellum.⁶ Similar to other spinal dysraphisms, a lumbosacral lipoma that tethers the spinal cord at the upper lumbar level probably occurs at a slightly earlier embryologic period than does a lesion that tethers the cord lower down in the spine.⁸ Several authors have found that patients with LMM can have other associated dysraphic conditions of the spinal cord, including split cord malformation, terminal syringomyelia, neurenteric and intraspinal dermoid/epidermoid cysts, a tight filum terminale, and tethering tracts.^20,21

Clinical Findings

Cutaneous Signs

Although patients with LMM can have a spectrum of cutaneous anomalies, infants are seen most often with a cutaneous sign that suggests a potential underlying abnormality. The most common finding (>90% of patients) is a subcutaneous fatty mass covered with skin that is located in or near the midline of the lumbosacral spine (Fig. 215-4). This mass is not tender and appears to be in continuity with the normal subcutaneous tissues. Although the overlying skin is typically intact, LMMs can also be associated with a hairy patch (hypertrichosis), hemangioma, dermal sinus, skin dimple, skin tag, atretic meningocele, or caudal appendage (Table 215-1).^20,22 The most superficial portion of the mass typically consists of lobulated and unencapsulated subcutaneous fat, which becomes more fibrous as the deeper structures are approached. The stalk of the fibrous fatty tissue can then be found to penetrate the dura and is associated with the spinal cord, conus, cauda equina, or filum terminale, as described earlier. Additional malformations may also be present, including cloacal exstrophy, bifid uterus, duplicated vagina, anterior sacral meningocele, anomalies of the lower limbs, and imperforate anus.¹⁵ It is rare for patients with LMM to have intellectual impairment or associated hydrocephalus, Chiari malformations, or other brain anomalies.^15,23,24

FIGURE 215-4 Lateral (A) and dorsal (B) views of a patient with a lipomyelomeningocele manifested as a subcutaneous fatty lipoma and cutaneous hemangioma.

TABLE 215-1 Lumbosacral Cutaneous Manifestations Found in Patients with Lipomyelomeningocele

SERIES	HOFFMAN ET AL.,20 1985 (N = 97)	KANEV ET AL.,22 1990 (N = 80)
Soft tissue mass	97	80
Skin dimple	26	14
Hemangioma	24	9
Hypertrichosis	1	11
Skin tag/tail-like appendage	5	6
Atretic or denuded skin patch	1
Dermal sinus hypopigmentation	3	3

Evaluation