Tethering and Mechanical Sheer
Throughout the 1970s the goal of surgical treatment of lipomyelomeningocele was to reduce the mass associated with the lipoma and thereby improve the associated cosmetic defect. At that time surgeons understood that manipulation of the intradural portion of the defect was associated with an unacceptable risk of neurological decline. With greater study over a longer period of time, it has been learned that the connection between fibrofatty elements of the mass and the underlying conus medullaris serve to fix the conus medullaris and not allow it to ascend normally (Figure 4). This mechanism is described as the tethered cord syndrome an is considered central to the pathophysiology of lipomas of both the conus medullaris (lipomyelomeningocele) and the filum (fatty filum). It is now recognized that an initial cosmetic operation is a risk factor for delayed postoperative neurological deterioration.[10,36]
The fibrofatty mass of a dorsal lipomyelomeningocele fixes the distal cord and prevents its normal ascension. Note the infiltration of underlying neural elements with the fatty fibrous tis-sue. With growth the cord gradually comes under tension, which is thought to be central in causing the progressive neurological dete-rioration that is the hallmark of the TCS.
Currently two hypotheses are favored to account for the observed neurological deterioration in the setting of a tethered cord: 1) that the tethering process results in a mechanical tension across the cord, which directly imparts neurological insult and 2) that the tension that develops adversely affects cord perfusion and causes an ischemic injury.
The elegant studies reported by Yamada, et al., have provided significant insight into the causes and effects of excessive cord stretching. In studies in a cat model of spinal cord tethering and stretching the authors have demonstrated that isotonic traction of the spinal cord results in different degrees of elongation in different cord segments. As such distal segments were most affected. These changes were mirrored by reduced oxidative metabolic values and evoked potentials. These findings are in keeping with the clinical observation of selective involvement of the lower extremities in clinical TCS. Additionally these investigators showed that severe traction of the spinal cord was associated with depression of spinal evoked potentials. Evoked potentials recovered after traction was released; however, interneuron potentials only partially recovered. The diminution of interneuron potentials appeared linked to changes in oxidative metabolism of the neurons. In additional studies, in this same report in which spectrophotometry of the cytochrome a, a3 was used to quantitate metabolic activities in a cat model of TCS, the results suggested that traction shifted the baseline redox ratio to a more reduced state. This finding implies either diminished adenosine triphosphate use or reduction of oxygen supply.
Neurosurg Focus. 2001;10(1) © 2001 American Association of Neurological Surgeons
Cite this: Spinal Lipomas - Medscape - Jan 01, 2001.