What are the types of Meningomyelocele (spinal defect)?

Types of Meningomyelocele (Spinal Dysraphism)

Meningomyelocele is not classified into distinct "types" per se, but rather exists as part of a spectrum of spinal dysraphic malformations that share similar embryologic origins and clinical features. The term "meningomyelocele" (MMC) specifically refers to the most severe form of open spinal dysraphism, but it is important to understand the related malformations that are often discussed in the same context 1.

Classic Meningomyelocele (MMC)

MMC is the archetype of neural tube defects and represents a localized failure of primary neurulation. 1

• The malformation contains a placode of neural tissue attached peripherally to the surrounding skin, with the placode always visible on the skin surface 1
• The underlying cerebrospinal fluid (CSF) elevates the placode on a dome or sac; if the thin tissue tears, CSF escapes and the malformation becomes flat 1
• By definition, MMCs are open malformations with exposed neural tissue 1
• Neurologic impairment is generally related to larger size and more cranial location of the defect 1

Associated Malformations with MMC:

• Hydrocephalus occurs in 70% of cases 1
• Chiari type II malformation is present in 98% of cases 1
• Syringomyelia affects 40% to 80% of patients 1
• Spinal cord tethering occurs in virtually all cases 1

Related Spinal Dysraphic Malformations

Lipomyelomeningocele (Spinal Lipoma)

This is a malformation where a fatty subcutaneous mass extends into the vertebral canal and ends as an intramedullary spinal cord mass. 1

• Most have a visible fatty mass, sometimes with associated capillary malformation, infantile hemangioma, and/or dimple 1
• A minority have no associated skin manifestations 1
• The fat is readily identified as hyperintense on T1-weighted MRI sequences 1

Myelocystocele

Myelocystoceles present as full-thickness, skin-covered lumbosacral masses with three distinct MRI components. 1

• Component 1: A dilated terminal spinal cord 1
• Component 2: An even more largely dilated distal dural sac (described as "sac within a sac") 1
• Component 3: An adjacent spinal cord lipoma 1
• Children may be normal or have variable neurologic deficits 1

Meningocele

A meningocele is an isolated full-thickness skin sac filled with CSF and lacking central nervous tissue. 1

• On MRI, the sac contains only fluid without visible neural tissue 1
• Frequency is one-tenth that of MMCs 1
• Affected children generally have no neurologic deficits 1
• Despite traditional teaching, careful dissection may disclose a fibrous tract connecting the sac lining to the spinal cord, potentially causing tethering 1

Atretic Meningocele

This rare malformation consists of a small, flat area of dysplastic skin with characteristic appearance. 1

• Described as "scarified," "cigarette paper," or "cigarette burn" appearance 1
• The lesion is flat or slightly indented, sometimes painful to touch 1
• Often surrounded by hyperpigmented skin or salmon-colored capillary malformation 1
• Contains an underlying fibrous tissue tract (meningocele manqué) passing from a small fascial defect 1

Fatty Filum Terminale

An enlarged filum (defined as >2 mm in cross-sectional diameter on MRI) that is often infiltrated with fat. 1

• Considered by some to be a forme fruste of terminal lipoma 1
• Most common cutaneous manifestation is a fawn's tail or hairy patch on the back 1
• Other manifestations include capillary malformations or subcutaneous masses 1

Clinical Pearls

Common pitfall: The term "double myelomeningocele" refers to the rare occurrence of MMC at multiple spinal levels, not a distinct type 2. This is an extremely uncommon presentation requiring complex surgical management.

Important distinction: Lipomyelomeningocele, myelocystocele, and enlarged filum terminale all share the common feature of fat within the spinal cord and/or filum terminale and may represent different manifestations of the same embryogenetic process 1.