CHAPTER 217 Tethered Spinal Cord
Fatty Filum Terminale, Meningocele Manqué, and Dermal Sinus Tracts
Occult spinal dysraphism (OSD) refers to a collection of malformations that are thought to arise from incomplete formation of the dorsal midline structures during early embryogenesis. These pathologic entities may be found in isolation or in various combinations and are often associated with midline cutaneous changes over the back, such as focal hirsutism, subcutaneous lipoma, atretic meningocele, caudal appendage, or hemangioma. Such signs may hasten a patient’s diagnosis.1–4
Because OSD can result in neurological deficits from caudal traction on the spinal cord, many neurosurgeons believe that early operative untethering of the spinal cord prevents the development, inhibits the progression, and may reverse the neurological symptoms associated with these various tethering elements.5 Symptoms related to a congenital tethered cord occur most commonly in childhood, but in some patients the condition may not be diagnosed until later in life. Specific pathologies that make up the constellation of OSD include lipomyelomeningocele, split cord malformation, terminal syrinx, neurenteric cyst, atretic meningocele, fatty filum terminale, meningocele manqué, and dermal sinus tracts.6 This chapter focuses on the latter three of these anomalies.
Incidence and Epidemiology
The true incidence of OSD is not known, and unlike spina bifida aperta, OSD is usually diagnosed only in patients with cutaneous stigmata or symptoms or is found incidentally after investigation for unrelated problems (Fig. 217-1). Interestingly, the incidence of open neural tube defects has declined dramatically with the implementation of folate supplementation, whereas the incidence of OSD has risen steadily, probably because of the more common use of magnetic resonance imaging (MRI). There are limited data regarding risk factors for OSD, but some data suggest that both open and closed neural tube defects may be genetically related. Therefore, siblings of patients with a known neural tube defect may be at greater risk for OSD.7
Embryologic Overview
Neural tube formation is the essential process of neurulation. During this process, the neural ectoderm along the primitive streak is induced to proliferate by the underlying notochord. With differential growth, the edges begin to fold inward toward one another to form the neural groove. As the folding edges of the neuroectoderm join, they are covered by adjacent cutaneous ectoderm. Disjunction of the two ectoderm types occurs as the edges meet.8 The neural ectodermal cell collection caudal to the neural tube is called the CCM or tail bud. Canalization of the CCM begins around day 28 with the formation of vacuoles within the middle of the CCM. The vacuoles begin to coalesce to form a central canal within the CCM. This canal eventually connects with the central canal of the cephalic neural tube formed during neurulation. The distal lumbar, sacral, and coccygeal segments are now formed. The filum terminale and terminal ventricle are formed through regression of the caudal portion of the CCM. As regression occurs and differential growth of the vertebral canal and the neural tube progresses, the filum terminale is formed as the spinal cord ascends relative to its original position.9
The position of the conus medullaris ranges from the midlevel of T12 to the lower portion of L3. However, the majority of normal coni are found between L1 and the L1-2 interspace. Barson performed postmortem examinations of 252 neurologically normal infants and children.10 This report suggested that the coni of term infants lie at L2-3 but continue to ascend to the average adult level of L1-2 by 3 months of age. However, MRI and ultrasound studies seem to suggest that the conus ascends to the L1-2 interspace sooner in life—by the 40th postmenstrual week.11–13
Fatty Filum Terminale
Up to 6% of the normal population will be found to have fat within their filum terminale,14 and many of these individuals will have symptoms of a tethered spinal cord (Fig. 217-2). The term filum terminale syndrome was coined in 1953 by Garceau,15 who reported three patients with progressive spinal deformity and neurological dysfunction. Garceau theorized that tension on the distal end of the cord from a pathologic filum terminale was the origin of these problems and noted good recovery in all these patients after sectioning the filum.15 In 1956, Jones and Love reported good recovery of neurological and urologic function after sectioning the filum and subsequent retraction of the cut ends of the filum,16 thus implying that this structure had been under pathologic tension. In 1976, Hoffman and colleagues suggested the term tethered spinal cord for patients with a low-lying conus and a thickened filum.17
The filum terminale is classically believed to fixate the distal end of the cord and thereby decrease its movement within the vertebral canal. This viscoelastic band usually allows the distal part of the cord to move slightly, but if its nature is compromised by either fatty infiltration14 or abnormal thickening, undue caudal tension may result. In utero, such pathology is believed to inhibit normal ascension of the cord. However, in rare cases, caudal displacement of the conus is not necessary for symptoms to occur.18–20
In animals, Yamada and coworkers have shown that caudal traction on the cord results in impairment of oxidative metabolism and that the degree of impairment correlates with the severity of the neurological deficits.21–23 It was then postulated that such cord traction causes traction-induced hypoxia and stretching of the neuronal membrane with “loss of transmembrane ion homeostasis and electrical activity depression.”24
Symptoms
The symptoms of a spinal cord displaced caudally as a result of a fat-infiltrated filum terminale include pain and orthopedic, urologic, and neurologic problems. Common clinical findings include the presence of cutaneous signs associated with OSD, a neurogenic bladder with the development of primary or secondary incontinence or urinary tract infection, leg or foot weakness, numbness or spasticity (or both), leg or foot length discrepancy, foot deformity (pes cavus, claw toes), spinal deformities, and nondermatomal back and leg pain.25,26 Although pain is a major initial symptom in the adult tethered spinal cord population, it is less common and more difficult to identify in the pediatric population because pain is often manifested simply as irritability, especially in infants. The neurological deficits may not be reversible with surgical intervention. Commonly, the initial clinical symptom in patients with a tethered cord secondary to a fat-infiltrated filum is gradual and progressive loss of coordinated bladder activity. This may become manifested as repeated bouts of urinary tract infection or primary or secondary urinary incontinence. Urinary symptoms may be combined with evidence of spasticity of the lower extremities, which is frequently a combination of hyperactive deep tendon reflexes with upgoing toes and muscle wasting. The combination of upper and lower motoneuron disturbances in the lower extremities is the signature of this pathology.26 Rectal incontinence is usually delayed until late in the course. Nonradicular pain in the back and legs may be the primary manifestation in the adult population.
In at least 10% to 20% of patients with congenital anorectal atresia (e.g., VATER syndrome—vertebral defects, imperforate anus, tracheoesophageal fistula, and radial and renal dysplasia), a tethered spinal cord secondary to a fatty filum terminale will also be found.27–30 Moreover, many patients with a thickened filum terminale are seen to have some other form of neural tube abnormality. Additionally, patients with the Currarino triad and cloacal exstrophy are more likely to harbor such malformations.31
Suspicion of a fatty filum terminale is confirmed with MRI. In general, three criteria are necessary to confirm the clinical impression of a tethered spinal cord on imaging: caudal descent of the conus, fatty infiltration and thickening of the filum terminale, and a drawn-out appearance of the distal conus. Imaging will identify a thickened (usually >3 mm in diameter) filum, and if infiltrated with adipose tissue, it will easily be seen on T1-weighted MRI (Fig. 217-3; also see Fig. 217-2). Occasional adult patients will have many decades of symptom-free life only to come to clinical attention because of irreversible bladder dysfunction.
Treatment
There is clear consensus in the neurosurgical community that symptomatic patients with a low-lying conus (conus positioned below the L1-2 interspace) and a fatty filum should undergo surgical untethering.5 We believe that surgery is indicated for asymptomatic patients with a low-lying conus and a fatty filum to prevent future deterioration. A less common scenario for which we would occasionally offer surgery would be an asymptomatic patient with a fat-infiltrated filum and a normally positioned conus in the setting of other congenital anomalies.
Sectioning of the filum is carried out with an approach through the L5-S1 interspace with laminotomies of L5 and S1 to assist in exposure. After removal of the ligamentum flavum and opening of the dura mater, the midline filum is identified and any associated nerve fibers teased away from this structure32 (Fig. 217-4). Electrocautery is performed at two points and the filum transected at each point. The dura mater and overlying soft tissues are then closed in routine fashion.
Meningocele Manqué
Congenital dorsal bands from the spinal cord to the dorsal dura mater have been described in association with OSD.33 These bands, known as meningocele manqué (manqué, or “that which might have been but is not”; French for “missing”), are found in association with multiple other elements of spinal dysraphism and consist of aberrant nerve roots, dorsal adhesions, and dorsal fibrous bands that tether the spinal cord within the inner aspect of the dura mater.33,34 These bands may be composed of aberrant neural tissue with dorsal root ganglion cells or fibrous tissue with even hamartomatous elements. They frequently penetrate the dura of an associated bony median septum or may simply exit dorsally. However, they need not be associated with a split cord malformation and may occur distal to the site of OSD. This tethering results in neurological deficits referable to the caudal spinal cord.
We previously reported 19 patients with meningocele manqué confirmed at surgery.35
