= TIGHT FILUM TERMINALE SYNDROME = LOW CONUS MEDULLARIS

= abnormally short + thickened filum terminale with position of conus medullaris below L2-3

Normal location of tip of conus medullaris: L4/5 at 16 weeks of gestation, L2/3 at birth, L1-2 >3 months of age

• RULE OF THREES: above L3 by age 3 months!

Etiology: incomplete involution of distal spinal cord with failure of ascent of conus

Pathophysiology: stretching of cord → vascular insufficiency at level of conus

Age at presentation: 5–15 years (in years of growth spurt); M÷F = 2÷3

Associated with: filar lipoma in 29–78%, filar cyst, diastematomyelia, imperforate anus

• dorsal nevus, dermal sinus tract, hair patch (50%)
• bowel + bladder dysfunction in childhood
• spastic gait with muscle stiffness; radiculopathy (adults)
• lower extremity weakness + muscle atrophy
• asymmetric hyporeflexia + fasciculations
• orthopedic anomalies: scoliosis, pes cavus, tight Achilles tendon
• hypalgesia, dysesthesia; paraplegia, paraparesis
• hyperactive deep tendon reflexes; extensor plantar responses
• anal / perineal pain (in adults)
• back pain (particularly with exertion)
• Tight filum
  • diameter of filum terminale >2 mm (normal range, 0.5–2.0 mm) at L5/S1 level (55%)
  • small fibrolipoma within thickened filum (23%)
  • small filar cyst (3%)
  • spinal cord ending in a small lipoma (13%)
• Tethered cord (100%)
  • conus medullaris below level of L3 at birth + below L2 by age 12 (86%)
  • abnormal dorsal fixation of cord adjacent to vertebral arches (in prone position)
  • reduced / absent pulsatile movement of the cord + nerve roots (on M-mode scanning)
  • widened triangular thecal sac tented posteriorly (thecal sac pulled posteriorly by filum)
  • abnormal lateral course of nerve roots (>15° angle relative to spinal cord)
• Vertebrae
  • lumbar spina bifida occulta with interpedicular widening
  • scoliosis (20%)

MR:

• prolonged T1 relaxation in center of spinal cord on T1WI in 25% (? myelomalacia / mild hydromyelia)

Rx: decompressive laminectomy / partial removal of lipoma ± freeing of cord

Dx: tip of conus medullaris below L2-3