Prone lumbar MRI in patients with occult tethered cord syndrome

If the occult tethered cord syndrome (OTCS) due to a tight filum terminale is occult, how is the diagnosis made? Tethered cord syndrome due to a tight filum terminale (the non-occult variety) has been classically diagnosed by appropriate clinical signs and symptoms and radiographic evidence of the conus medullaris ending below the lumbar 2 vertebrae.* In 1990, Khoury et al. at the University of Toronto described the tethered cord syndrome in children with abnormal urodynamic studies in which the conus medullaris ended above L2, a “normal” position. Thus, since cord tethering could not be confirmed radiologically, the disorder was viewed as occult.

While a number of recent studies are contributing to our understanding of this disorder, development a radiographic method to confirm a tight filum in OTCS has been difficult. In the January 2013 issue of the Journal of Neurosurgery Spine, Nakanishi and colleagues from the Department of Orthopaedic Surgery at Hiroshima University in Hiroshima, Japan describe a method of assessing elasticity of the filum by determining its position on lumbar MRI performed in both the supine and prone positions.

The study includes 14 patients with clinical signs and symptoms of TC and 12 asymptomatic subjects serving as controls. The clinical presentation in the OTCS patients consisted of “incontinence, urgency and/or frequency of urination beyond 5 years of age” and signs of neurogenic bladder such as bladder hyperreflexia. Other findings supportive of the diagnosis of OTCS, including spinal stiffness, low-back pain, and motor or sensory dysfunction in the legs were present to variable degrees.

Axial MR images from T12 to S3 where performed on the patients and controls. Ratios based on the position of the filum and the nerve roots in relation to the anterior and posterior ends of the subarachnoid space where calculated on both the supine and prone T2 axial images. The fila were found to be “significantly posterior” in the spinal canal on the prone MRI in OTCS patients when compared to controls:

Our results show that the dot of the terminal filum was significantly posterior at the L3-4, L4, L4-5, and L5 levels on prone MRI in patients with OTCS.

In addition, the nerve roots in the OTCS patients were significantly anterior on the prone MRI “suggesting a difference in elasticity between the terminal filum and cauda equina in patients with OTCS.”

An additional finding of the study was the presence of thin fila in the majority of the OTCS patients. Classically, tight fila, as visualized during surgery, are expected to be thicker than normal. However, in 10 of the 14 surgical patients in this study, the fila were “no more than 2 mm in diameter, and no fatty tissue was detected on MRI studies or on microscopic findings during surgery.”

The Japanese Outcome Association score improved in all 14 patients following section of the filum terminale. In closing, Nakanishi and colleagues suggest:

Since this method is simple and noninvasive, it should be considered when a patient has clinical symptoms or signs of OTCS and it may prove helpful in diagnosis or surgical indication.

Take home message: In this study, the position of the filum as seen in prone T2 axial MR images in patients with occult tethered cord syndrome was significantly posterior within the spinal canal while the nerve roots were significantly anterior when compared to asymptomatic controls. Almost ¾ of the fila were 2 mm or less in size.

John Oró, MD
February 5, 2013

* some authors now consider an “abnormal” position of the conus as below the L1-L2 disc space