Under normal conditions the spinal cord is restrained within the vertebral canal by the dentate ligaments and the filum terminale. The latter is a band of fibroconnective tissue that allows some displacement of the conus medullaris during small flexion-extension movements. In this way, the filum terminale fixes and stabilizes the spinal cord while absorbing impacts and excessive movements on the spinal cord.
If there is any cause that limits the mobility of the spinal cord (e.g., fatty infiltration of the filum or lipomas of the conus medullaris that can produce traction and tension on flexion and extension stretches of the spine), abnormal stress on the conus (the end of the spinal cord) can occur, resulting in tethered cord syndrome.
Causes of tethered cord
There are two hypotheses that attempt to explain the neurological deterioration that can occur in an anchored conus medullaris. On the one hand, mechanical tension caused by traction on the medulla by the anchoring elements, whether these are a lipoma, a thickened filum terminale or fibroconnective bands. On the other hand, ischemia caused by blood supply defects resulting from tension on the medullary microvasculature.
The variability in the circulation of the cerebrospinal fluid (CSF) that bathes and protects the spinal cord, and other individual factors such as vascularization, the size of the spinal cord and its position with respect to the canal or anchor point, could determine the differences in symptomatology between similar patients.
In most cases, the tethered cord is secondary to a congenital malformation that produces an “anchorage”, an impossibility of movement of the spinal cord within the vertebral bone canal, thus resulting in microtrauma and blood supply defects due to stretching of the vessels that nourish the spinal cord. This deficit, if maintained over time, may eventually lead to spinal cord injury.
The most frequent causes of congenital malformations that produce an anchored medulla are medullary lipomas and dermal sinuses, although there is a great variety of them in the whole spectrum of spina bifida occulta (diastematomielia, “limited dorsal myeloschisis”, etc.).
Some patients operated on an anchored cord may develop a new anchorage (“re-anchorage”), produced by an internal fibrous scar, late after the first operation. It is the second most frequent cause of operation for tethered cord, although fortunately it is not a frequent complication.
Neurosurgery for tethered spine
In cases of spina bifida occulta with spinal cord tethering, such as conus medullaris lipoma, dermal sinus or diastematomyelia, treatment should be surgical if symptoms arising from it are present. In some cases, preventive surgical treatment is recommended, even in asymptomatic patients, to prevent future complications that could result from spinal cord injury caused by the tether.
Surgical treatment of spina bifida occulta and tethered cord generally has a good prognosis. But both the indication for surgery and its risks and potential benefits should be discussed with the patient and/or family members on a case-by-case basis.
The aim of surgery is the prevention of neurological deterioration or its progression if there were previous symptoms. As a general rule, all symptomatic cases, or previously operated cases presenting new symptoms or progression of the previous deficit (“re-anchoring”), should be operated on.
The general principles include resection of the lipoma mass, complete unanchoring of the spinal cord, preservation of the neurological tissue, and reestablishment of the anatomical planes (including tubulation of the neural plate and repair of its coverings).
The indication for surgery in the case of lipomas must take into consideration two variables: the type of lipoma, and whether it is symptomatic or not. There are cases more favorable than others to achieve an exeresis with desanclaje without the appearance of complications. Thus, it is easier to make the decision to indicate surgery in patients with a caudal or filum terminale lipoma than in those with a complex lipoma.
The experience of the neurosurgeon in these cases has a fundamental influence in achieving satisfactory results.