Embryologic Considerations

Differentiating the extremes of normal from subtle pathologic states is one of the most difficult aspects of managing patients who are at risk for TCS. When TCS was first described, clinically devastated patients with extreme caudal displacement of the conus medullaris into the sacrum from a thickened and unyielding filum terminale were easily differentiated from normal.^1,2 However, the widespread use of magnetic resonance imaging (MRI) has allowed physicians to attempt to define a “normal” level of the conus medullaris in an adult. For some clinicians, below the L1–2 disc space is abnormally low, whereas for others, below the inferior border of L2 is abnormally displaced. This definition is confounded by the fact that in some cases, the conus medullaris, which normally has an anteroposterior diameter ranging from 5.0 to 8.0 mm, gently tapers into a very thick filum rather than demonstrating an obvious caudal tip.

The filum terminale and the distal conus medullaris are thought to arise through the process of secondary neurulation. Distally, the intradural filum terminale travels to fuse with the dorsal dura mater, usually in the midline, and then continues within a dural sheath as the coccygeal ligament to the dorsal coccyx. Historically, in the term infant, the inferior tip of the conus medullaris was described as being located at the L2–3 interspace in 98% of cases and at the L3 level in 1.2% of cases. Presumably, by 3 months of age the tip of the conus achieved its adult position by “ascending” to the L1–2 interspace. However, Wilson and Prince³ concluded that a conus positioned at L2–3 should be considered normal at any age. Reimann and Anson⁴ compiled both their data and those from three other large series (a total of 801 adult spinal cords) to find that the conus medullaris is found above the L2–3 disc level in approximately 94% of cases, whereas the mean conus lies at the lower third of the L1 vertebra. Saifuddin and colleagues⁵ have also found that the mean termination of the conus is at the lower third of the L1 vertebra. It would be logical then to assume that tight fixation of the cord by the filum during in utero development would result in the most caudal position of the conus but that lesser degrees of tension would allow some cephalad migration. It might even be possible that mild tension on a relatively elastic cord could result in a relatively normal position of the conus. In such a case, symptoms, possibly secondary to persistent microtrauma, might not occur until adult life.

Presentation and Physical Examination

Although they harbor similar, or identical, pathology, adults with OSD tend to present with a different array of signs and symptoms than children. In children, cutaneous stigmata such as focal hypertrichosis, pigmented nevi, subcutaneous lumbosacral masses, and skin dimples are often observed. However, in the adult OSD population, these signs are present in only 36% of cases.⁶ Similarly, foot deformities are found in only 37% of adult patients.⁶

In the adult population, there is a slight female preponderance of congenital TCS, with ratios of 1:1.3 to 1:2.6, and the mean age at presentation is approximately 37 years.^7–10 Although there is no universally accepted classification system that stratifies adults with TCS, van Leeuwen and colleagues¹⁰ proposed that the origin of tethering might reasonably correlate with surgical outcomes and should be taken into account during surgical planning. Lee and colleagues¹¹ found satisfactory concordance between their experience and this classification scheme. Van Leeuwen’s scheme included four groups of TCS: post-repair myelomeningocele, tight filum terminale or filum terminal lipoma, Conus lipoma/lipomyelomeningocele, and split cord malformation (SCM).

The most common presenting complaint in adults with OSD is pain, which is encountered in as many as 80% of patients.⁶ The most common type of pain is insidious nonradicular back pain.^6,9,12,13 A thorough history might reveal that this pain has been present since childhood. Pang and Wilberger⁹ proposed three pathophysiologic mechanisms that led to the acute presentation of symptoms in 61% of their adult population with TCS. These mechanisms were transient stretching of the conus medullaris, narrowing of the spinal canal due to degenerative disease, and direct trauma. The added ischemia or mechanical distortion associated with these events is postulated to reveal symptoms in an already injured or stressed caudal spinal cord.

Adults also present with lower extremity symptoms, most commonly pain or weakness, or both. The weakness that results from distal spinal cord fixation is classically associated with both upper and lower motor neuron signs. Less common presenting signs and symptoms include changes in bowel and bladder function (approximately 69%⁶), scoliosis (8% to 31%,^6,11 Fig. 183-1), and loss of lower extremity sensation resulting in foot ulceration (Fig. 183-2).

FIGURE 183-1 Severe atrophy of the lower extremities, notably in the gluteal musculature. This patient presented with a history of scoliosis and at operation was found to have a fatty infiltrated filum terminale.

FIGURE 183-2 A case of foot amputation in an adult with tethered cord syndrome who had a chronic history of anesthesia of the left lower extremity distal to the knee.

The physical examination findings that can be expected in a patient with TCS can be divided into cutaneous lesions, lower extremity findings, and changes in rectal tone. Although they may be encountered in the adult, the most telling findings (e.g., cutaneous stigmata and lower extremity asymmetry) are more common in the pediatric population. The cutaneous signatures of OSD include midline capillary hemangioma, subcutaneous lipoma, dermal sinus tract, caudal appendage, focal hirsutism, and atretic meningocele (Figs. 183-3 through 183-8). Cutaneous hemangiomas that may be common in other parts of the body are unusual in the lumbosacral region (see Fig. 183-3). In this area, especially in the midline, they have been associated with OSD and are often found in conjunction with other skin findings, such as focal hirsutism and subcutaneous lipomas. However, capillary hemangiomas are thought to be the least sensitive indicator of intradural pathology, with an approximately 10% incidence of associated intradural anomalies when seen in the lumbar region.¹⁴ Focal hirsuitism (see Fig. 183-4) should alert the clinician to the possibility of an underlying SCM or atretic meningocele (see Fig. 183-5).

FIGURE 183-3 Operative photograph demonstrates a flat capillary hemangioma of the midline lumbar spine. This patient was found to harbor a conus medullaris at the S1 vertebral level and a thickened fatty filum terminale.

FIGURE 183-4 Adult female patient with a history of severe back pain. Note the area of focal hirsutism, which was found to be associated with a split cord malformation.

FIGURE 183-5 A female patient with two cutaneous signatures. Note the area of hypertrichosis and a centered atretic meningocele.

FIGURE 183-6 An elderly patient with a history of leg length discrepancy, urinary dysfunction, and back pain. Note the subcutaneous mass and area of focal hirsutism. This patient was found to have a lipomyelomeningocele with dorsally tethering bands (meningocele manqué).

FIGURE 183-7 A patient found to have a subcutaneous mass and dermal sinus. At operation, both a dermal sinus tract and dorsal lipoma of the cord were found.

FIGURE 183-8 A large dermal sinus on the midline lumbar back of a 50-year-old man with back pain and increased sensory deficit of the lower extremities.

Typical lower extremity findings result from a combination of upper and lower motor neuron weakness in the legs without upper extremity symptoms. Such findings can manifest as decreased muscle bulk (Fig. 183-9) with increased or pathologic reflexes, and should alert the examiner to the possibility of TCS. Other findings include progressive arching of the feet and lower extremity size discrepancies (Fig. 183-10). The clinician should evaluate for hypalgesia of the lower extremities in patients with potential TCS. This sensory disturbance is often found in a patchy distribution.

FIGURE 183-9 Atrophy of the leg musculature in a 75-year-old woman with tethered cord syndrome. Note the forefoot amputation on the right side.

FIGURE 183-10 Foot length discrepancy in a 55-year-old adult with tethered cord syndrome.

Radiographic Assessment

Pathologic Subtypes of Occult Spinal Dysraphism