Filum Terminale Sectioning: A Technical Appraisal

Sectioning of filum terminale is a standard procedure performed in the entire spectrum of tethered cord syndrome. Although it may seem a standard procedure, it needs a conscious approach towards the decision making and implementation of the procedure. The filum terminale is a terminal extension of the conus till coccyx. It is a connective tissue stalk formed due to caudal cell mass retrogressive differentiation and regression as part of secondary neurulation. The filum is mostly nonfunctional. However pathological filum terminale (thickened or fatty infiltration) can cause neurological symptoms, primarily due to loss of its elasticity and consequent restriction in the vertical movement of the spinal cord.[1,2] This results in increased stretch, tension and traction on the conus.[3,4] Excessive traction on the spinal cord causes impaired spinal cord perfusion, oxidative metabolism, reduced glucose metabolism, and mitochondrial failure with corresponding electrophysiologic changes and neurologic dysfunction. Congenital etiologies, such as myelomeningocele, or acquired etiologies, such as scar formation are common causes. They can result in widespread neurologic, urologic, musculoskeletal, or gastrointestinal abnormalities. The pathophysiologic and electrophysiologic changes can recover following detethering which then translate into neurologic improvement. Although the decision to section the filum is relatively straightforward in most symptomatic cases, it can be challenging in asymptomatic patients or in patients with normal positioning of the cord. In a developing country like India with diverse geographical and socioeconomic conditions, the use of modern technology and ideal surgically conducive situation may not always exist uniformly. Hence judicious attention to anatomical, clinical, radiological and accurate macroscopic identification of thickened filum is paramount and cannot be underestimated. The filum terminale length as well as the diameter from the conus till the sacral hiatus decreases proportionately from cranial to caudal as noted in cadaveric studies. Neural stimulation techniques can help identify filum by exclusion. The termination of dural sac ranges from S1 to S3 but in majority of cases, it ends at S2 level.[5] The filum terminale fuses with the dura mater at S2 level. (10% fused off midline, 15% fuse above S1). In case it is not at S2 level, it is still less than one vertebral body. Hence, the exposure of filum terminale should be done with its epicenter at the distal most end viz. at S2 level. The bifid spinous process in small children and a larger interlaminar space facilitate a minimal invasive approach. An interlaminar exposure at L5S1 should be performed and the dural tube is exposed in the midline taking care prevent any inadvertent injury to underlying nerve roots.[6] The end of the thecal sac at S2 level should be in view at all times. The thecal sac is opened at L5 S1 and S2 level. The thecal sac is dilated and turgid in most cases due to long standing tethering and is exemplified by the fact that CSF gushes out under high pressure like a jet fountain on opening the dura and arachnoid. The taut, thickened or lipomatous filum is a linear elongated pearly, bluish or greyish white structure. It is the most posteriorly located structure and distinctly different in appearance and texture from the surrounding glistening yellow normal nerve roots. It has an accompanying anterior blood vessel always present. The filum terminale should be made bare by dissecting the nerve roots or slender rootlets by a blunt dissector. In order to minimize the possibility of inflicting a neurological deficit, the filum should be traced till S2 insertion. A blunt dissector can be inserted below the filum S2 insertion and sequentially coagulated under low bipolar power taking care to secure the accompanying artery alongwith it. It is then sequentially severed at this level. The sacral transhiatal exposure is considered as minimally invasive surgical approach for sectioning of the filum with the advantage of avoiding bone removal and requiring a smaller incision (4 cm), minimal blood loss, minimal soft tissue injury, reduced pain after surgery, and shorter hospitalization.[7,8] The distance between neural structures and sacrococcygeal membrane is less than 3 mm in sacral canal. It is also done to reduce intradural scar formation and the risk of retethering. However, intradural exposure at S2 allows inspection for aberrant nerve root, arachnoidal bands, split cord malformation causing tethering in addition to filum terminale. Sectioning of Filum terminale is also described through 1cm skin incision and interlaminar approach at L4,5 using an endoscope.[9] Filum sectioning below the conus tip and extracting the distal filum by releasing it from its intradural attachments to minimize any remnants of the filum terminale although described, can pose risk of neurological deficits.[10] The preoperative diagnosis of a tethered cord syndrome needs a well equipped radiology center with advanced magnetic resonance imaging (MRI) and an experienced radiologist. The constructive interference steady state (3D gradient-echo) advanced MRI sequence is immensely useful in diagnosis of abnormal filum terminale, aberrant roots, fibrous septa and arachnoidal bands.[11] In the postoperative period, the radiological features of relaxed conus are less reliable.[12] The recoil discontinuity, along with thickened upper and lower remnants of a sectioned filum constitute important radiological feature of a detethered filum. The proximal retraction or the recoil of the filum terminale can determine the elasticity or tenacity within the filum connective tissue fibres which indirectly reflects the pull or tension on the conus medullaris and the cauda equina. The blunt dissector over the dissected portion of the filum flings outside the operative field and the proximal end of filum retracts beyond the confines of the dural sac rostrally. The elasticity within the filum terminale can be judged by the recoil length, which is defined by the distance between the proximal and the distal end of the sectioned filum. The exact relationship between the recoil length, terminal syrinx and neurological deficits is still not understood. The relationship between the length of the filum terminale and the low disposition of the spinal cord and its effect on the recoil length is also not known.[13] It needs to be ascertained whether the recoil length can be used as a parameter for prognostication of tethered cord syndrome. The postoperative characteristics of a sectioned filum need further investigation to aid in the management of the complex spinal dysraphism. The filum interna is intradural while filum externa is extradural and the interface between the two is at the S2 level.[14,15] The interlaminar approach at L4-S1 level addresses the filum interna while transhiatal minimally invasive approach has a direct access to filum externa. Intradural exploration during filum sectioning is important to assess for additional tethering elements like aberrant nerve roots and arachnoidal bands. Although sacral transhiatal approach is likely to avoid intradural scar formation and the risk of retethering, it may fail to address the potential additional intradural tethering elements, if any. Although the filum is a fibrofatty connective tissue linear structure, active neural elements may be present within it. The role of intraoperative neuromonitoring in tethered cord syndrome is crucial and need not be underestimated.[16,17] It is especially helpful in cases of functional filum terminale, caudal lipomas or lipomyelomeningoceles. The neural fibres can be within the core of filum (Type I) or it can be coursing alongside as a separate neural filament. It may be difficult to predict or quantify the active functional neural element in the filum either by preoperative radiology or neurophysiological investigations. A direct stimulation of the entire filum can reveal the presence of accurate functional activity in the filum.[16] However, during surgery only the terminal part of the filum is exposed and hence it can be a limitation. Also in very small children, it may be difficult to separate nerve roots from filum.[18] In most cases, the filum is a nonfunctional nonneural structure. If neural function is active within the filum, it is more pronounced at the conus tip and reduces in a decremental manner towards its termination. Mild neural activity of filum at its termination, usually S2 level may be inconsequential and hence it is the ideal site for sectioning the filum. This would be helpful if intraoperative neurophysiological monitoring is unavailable or despite the availability of intraoperative monitoring, at times the surgeons chose not to use it depending on the microscopic appearance of the filum lipoma. A classical filum lipoma does not require routine use of intraoperative monitoring for an experienced surgeon but can pose a useful adjunct for a surgeon who has seen or performed lesser filum terminale surgery. But it is certainly helpful in complex surgery such as lipomyelomeningocele repair. Using voltage stimulation, a 1:100 ratio between the sufficiently separated motor roots and the filum threshold for filum identification is safe, expecting no response elicited or activation of a motor nerve root secondary to contiguity.[16,17] The first neurophysiological certification of filum terminale is no response, in which normal intensity above the threshold of the motor nerve roots is used. The second certification involves responses from all the motor nerve roots of the cauda equina. This double neurophysiological certification of filum terminale is accurate, quick, highly reproducible and cost effective for filum identification.[19] Sometimes a tailored surgical approach and individual threshold variability in motor rootlets in pediatric population can pose a surgical dilemma. In cases with secondary fatty filum terminale, filum should be diagnosed on preoperative thin-slice T1W axial MR images in the sacral region in all patients with spinal dysraphism.[15] The filum should be explored and sectioned if it is in the vicinity of the primary surgical field, especially in patients with split cord malformation inspite of no evidence on preoperative MR images. It should be deferred if a recording electrode located distally on the filum terminale shows conduction from proximal stimulation to distal filum terminale. Proper counseling of patients is crucial prior to surgery.[16,17] There is possibility of a small risk of neurological deficit in absence of conduction as well (0%–4%). Re-tethering can occur in 2.7%–8.6% of the cases and might indicate repeat surgery.[17] Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.