Pathology of Tethered Cord Syndrome With a Normally Positioned Conus in Children
Inelastic filum terminale: For a clinician to be able to diagnose TCS by MRI, there should be defined criteria for appreciating a filum as being tight. One arguably objective finding is a dorsally displaced conus and filum on MRI. This issue is quite controversial. Such patients may have a pathologically inelastic filum terminale so that with various ranges of motion, abnormal tension is placed upon a conus, even when it is at a normal level.
Attenuation of filum terminale with pelvic flexion: Breig (2) showed that the distal spinal cord becomes attenuated with flexion of the pelvis and that this biomechanical feature causes insult to the spinal cord during motion and increase in stature. However, these studies were performed in cadavers, which have lost a great deal of their viscoelastic characteristics.
Decreased oxidative metabolism: Others have found that the distal cord tends to be hyperemic (indicating stress/tension) following sectioning of fila in patients with TCS, both with or without an abnormally positioned conus. Yamada et al. (39) and Tani et al. (29) eloquently showed in a cat model that the tethered spinal cord produces changes in the redox activity of cytochrome a,a3 with a decrease in oxidative metabolism, presumably from mitochondrial anoxia, which may explain the postoperative hyperemia.
Tethered cord syndrome with a normally positioned conus
Some question about what is normal conus position: The term ‘tethered’ usually indicates a caudally positioned conus medullaris. Most definitions of normal cord termination levels are based on the anatomical measurements of Barson (1), who, quite intuitively, concluded that his findings in embryos may be somewhat inaccurate due to the hyperextension of the specimens during dissection.
Reports of resolution of neurogenic bladder: Many series (10-13, 28, 31) have suggested that incontinence in children with normal lumbosacral MRIs and indications of a neurogenic bladder on cystometrograms may be successfully treated by simple sectioning of the filum terminale. These series have had patients with both normal and fat-infiltrated fila. Bladder function in these series showed improvements of 58%, 44%, 59%, and 67% at 1, 26, 13, and 20 months, respectively. Most patients in these series demonstrated little if any external signs of occult spinal dysraphism (cutaneous anomalies, club foot, and others). The authors’ experience has shown that most of these patients have other signs of occult spinal dysraphism such as bony malformations or cutaneous stigmata. Urological signs alone should caution the surgeon and result in an aggressive search for other causes, especially in a child. The authors stress the rarity of a normally positioned conus and signs of a tethered spinal cord.
TCS still diagnosis of exclusion when conus in normal position: Clinical and imaging criteria still do not exist by which one can distinguish between patients who are likely to respond from untethering surgery (true TCS) and ones who are not (incontinence from other causes, including idiopathic). Therefore, TCS with a normally positioned conus remains a diagnosis of exclusion and should be considered only when all other potential causes of the symptoms are ruled out.
Adipose tissue: Adipose tissue may be identified with histological analysis within the filum terminale in patients with a conus in normal position. Adipose tissue was found in 92% of patients by either MRI or histological studies (15).