Letter to the Editor: Filum terminale in tethered cord syndrome

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TO THE EDITOR: I have read with great interest the clinical article by Thompson et al. (Thompson EM, Strong MJ, Warren G, et al: Clinical significance of imaging and histological characteristics of filum terminale in tethered cord syndrome. J Neurosurg Pediatr 13:255–259, March 2014). The importance of this well-documented paper stems from their assertion that surgical intervention is highly effective in reversing the signs and symptoms of properly diagnosed (that is, “true”) tethered cord syndrome (TCS)—at least 90% of their cases.

The importance of accurate TCS diagnosis is further emphasized by earlier assertions that surgery is ineffective in

TO THE EDITOR: I have read with great interest the clinical article by Thompson et al. (Thompson EM, Strong MJ, Warren G, et al: Clinical significance of imaging and histological characteristics of filum terminale in tethered cord syndrome. J Neurosurg Pediatr 13:255–259, March 2014). The importance of this well-documented paper stems from their assertion that surgical intervention is highly effective in reversing the signs and symptoms of properly diagnosed (that is, “true”) tethered cord syndrome (TCS)—at least 90% of their cases.

The importance of accurate TCS diagnosis is further emphasized by earlier assertions that surgery is ineffective in patients with signs and symptoms and even anatomical abnormalities that are similar to those of TCS but attributable to other causes.1,7 Therefore, in considering treatment options, it is essential to avoid diagnostic misinterpretations and to distinguish true TCS from TCS-mimicking disorders.10

To enhance understanding as a basis for TCS diagnosis, Thomson and colleagues explored clinical and anatomical abnormalities in pediatric TCS patients with an emphasis on MRI and histological studies of the filum terminale (FT) and on the statistics of multiple clinical findings. Their study confirmed that the filum in these patients was filled by fibrous or fibroadipose tissue (the latter always the component of fat). The fila in these patients lacked normal elasticity and were exerting a traction effect on the spinal cord.

My concern is directed at their introductory statement that the pathophysiology of TCS is unknown. In fact, much is known about TCS pathophysiology and such knowledge enhances our ability to properly diagnose TCS and to define treatment. The key fact is that neurological deficits in patients with true TCS result from a lesion due to tethering (that is, stretching) of the spinal cord with the lesion localized cephalic to the level of the inelastic FT.2,8,10

Insights into TCS pathophysiology have been derived from anatomical and physiological studies.12,19,21 For example, links between embryology and the development of spinal dysraphism are supported by findings of tufts of hair in the lumbar area, an elongated cord and thickened filum,5 as well as aberrant nerve fibers in the filum. These findings suggest additional clues that can be used to initiate a differential diagnosis between TCS and TCS-mimicking disorders.10 Further, our findings of a posteromedially displaced filum and conus on MRI9,13,17 also assist in confirming a TCS diagnosis, as does intraoperative stretch testing of the filum.14,18 Although reflectance dual wavelength spectrophotometry for oxidative metabolism study10,12,21 is not practical for practicing neurosurgeons because of the extra technological time required during surgery, additional information about TCS pathophysiology has been derived from experimental and clinical studies, which have shown impaired oxidative metabolism in the tethering-induced stretched region of the spinal cord and its reversal after untethering.14,18,21

With these insights into TCS pathophysiology, it has been possible to categorize patients with spinal dysraphism and occult TCS (modified from Yamada et al.15). Category 1 is considered true TCS. This category includes patients in whom the caudal end of the spinal cord is attached by an inelastic structure such as a fibrous or fibroadipose filum, sacral myelomeningocele (MMC), or caudal lipomyelomeningcele (LMMC).3,15 Neurological dysfunction seen in patients in Category 1 derives from pathophysiology attributable to stretching of the conus and/or lumbar segments of the spinal cord.

Category 2 is considered partial TCS. This category includes patients with a small dorsal or transitional LMMC,3,15 those with a small MMC attached to the dorsal aspect of the conus or lowest lumbar segments, and those with fibroadipose filum and conus.4,14 In these patients, neurological dysfunction cephalic to the lesion is reversible, whereas dysfunction in the affected segments is only moderately reversible or nonreversible,13 the latter because of fibrous or fibroadipose invasion into the cord parenchyma.

Category 3 includes patients with a TCS-mimicking disorder that results from a large MMC, LMMC, or dermoid, which covers most of the lumbosacral area. Because of extensive fibrous or fibroadipose invasion into the cord, no neurological reversibility is expected in these patients. Pang and colleagues' chaotic LMMC corresponds to this category.6 According to our experience, the spinal cord after removing a large lumbosacral LMMC showed no redox response from reflectance spectrophotometry,11 indicating that this is an example of a TCS-mimicking disorder.

Category 4 includes patients with a large MMC or LMMC associated with a barely developed lumbosacral cord. Studies show that there is no functional reversibility in these patients.

In summary, the study by Thompson et al. adds important insights into the diagnosis of TCS and should be well considered by those providing clinical management of this disorder.

References

Disclosures

The author reports no conflict of interest.

Response

We are grateful for Dr. Yamada's thoughtful and supportive commentary regarding our paper. His contribution to our best understanding of the pathophysiology of TCS based on changes to oxidative metabolism in the distal spinal cord represents a major advance in the field. Nevertheless, a number of theoretical and practical questions remain.

In his letter, Dr. Yamada outlines 4 categories of TCS or TCS-mimicking anatomy on the basis of a correlation with the proposed underlying pathophysiology. Importantly, these physiologically based categories reflect a significant overlap in anatomical presentation, precluding definitive preoperative categorization based on MRI-defined anatomy alone. Similarly, we have found an imperfect correlation between the presence of TCS and the imaging-derived anatomy of the FT (thickness, fat content, and cord position). Filum terminale histology in patients with presumed TCS also demonstrates a spectrum of abnormality, as reported in our study.

In sum, TCS—whether determined prospectively by imaging and clinical characteristics or retrospectively by FT histology—lacks precise diagnostic boundaries. Patient selection for surgery, while uncertain, is very important to ensure that appropriate interventions result in defined clinical benefit. Further progress in this area will result from the use of validated outcome measurement tools specific to TCS as well as the accumulation of prospective, high-quality registry data.

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Article Information

INCLUDE WHEN CITING Published online January 8, 2016; DOI: 10.3171/2015.4.PEDS15204.

© AANS, except where prohibited by US copyright law.

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References

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