Classification of Lipomas
Lipomas of the Conus Medullaris
A classification system that is valuable reflects the anatomical features and/or natural history of a lesion or disorder so as to facilitate an organized and logical therapeutic approach to its treatment. Chapman, alone  and with Davis  has described such a system for lipomyelomeningoceles that broadly divides them into two distinct variants and a transitional form. This system is of particular value to the surgeon as it enables him or her to anticipate the location of neural structures, meningeal coverings, and the site of tethering in relation to the fatty mass.
The dorsal variant (Figure 5) is a lipoma that arises through a fascial defect and attaches directly to the dorsal aspect of the caudally descended conus medullaris; all nerve roots emerge from the ventral or lateral surface of the neural tissue and lie in the subarachnoid space. The lateral nerve roots are sensory, and the more medial nerve roots are motor roots. Critical to the surgical approach is the junction formed by the lipoma and the spinal cord complex, as well as the position of the dural attachment in relation to that complex.
Drawing of a dorsal lipomyelomeningocele. The lipoma is fixed to the dorsal surface of the conus medullaris. The nerve roots emerge from the spinal cord immediately anterior to the zone where the lipoma, dura, and conus meet. The associated fatty accumulation penetrates a bone and fascial defect and is in direct continuity with a large subcutaneous fat accumulation. Often the filum is thickened and infiltrated by fat. Adapted with permission from Oakes.
The caudal variant (Figure 6) is a lipoma that exits the area of the terminal filum so that the cord becomes progressively larger caudally. In this form of lipomyelomeningocele, the nerve roots do transgress the lipoma. Many of these nerve roots are thought to be non-functional and can be sacrificed after stimulation. Although this caudal variation is difficult to reform into a tubular structure, the cut end of the lipoma may retract sufficiently cephalad to lessen the likelihood that retethering will occur postoperatively.
Drawing of a caudal lipomyelomeningocele. Fat extends from within the central canal of the cord through the caudal end where it intermixes with exiting nerve roots. A transverse fibrous band is often present at the level of the last intact lamina. Release of this band may result in dural expansion. Adapted with permission from Oakes.
Transitional Forms. The transitional form (Figure 7) has elements of both dorsal and caudal variants. Viable nerve roots pass through significant amounts of lipoma before exiting. Typically these are asymmetrical and associated with a rotational component of the spinal cord. The process of distinguishing among these three types of lipoma is usually straightforward; however, in the literature scant regard is often given to the differences. Specifically lipomas of the conus medullaris and those of the terminal filum are frequently considered as a single entity, an assumption that is not justified on the basis of long-term follow-up studies in children with such lesions.
Drawing of a transitional lipomyelomeningocele. This lesion demonstrates components of both dorsal and caudal variants of lipomyelomeningoceles.
Lipomas of the Terminal Filum
There is currently no widely used classification system for lipomas of the terminal filum nor is there one described in the literature. Because they have only become commonly diagnosed since the advent of MR imaging, much remains to be learned about these lesions. As such, the natural history of filum lipomas is incompletely understood. As noted earlier a meaningful classification system will either correlate with natural history and/or facilitate a therapeutic approach.
To create a classification system it is reasonable to begin by distinguishing those lesions in which the conus medullaris is at the normal age-specific position from those in which the conus medullaris is abnormally low lying. Another reasonable criterion for distinguishing these lesions would be the presence of pain or neurological signs (symptomatic or asymptomatic). Thus, these lesions could be broadly classified into one of four patterns: 1) fatty filum with descended conus medullaris and symptoms, 2) fatty filum with descended conus medullaris and no symptoms, 3) fatty filum with normal conus medullaris position and symptoms, and 4) fatty filum with normal conus medullaris position and no symptoms. The relative importance of other issues such as thickness of the filum, relative amount of fat, or presence of spina bifida occulta is less clear.
Neurosurg Focus. 2001;10(1) © 2001 American Association of Neurological Surgeons
Cite this: Spinal Lipomas - Medscape - Jan 01, 2001.