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Lipoma of the Filum Terminale

Summary

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  • Benign fatty lesion of the filum terminale, most commonly discovered incidentally on lumbar spine imaging
  • Represents intradural lipomatous tissue along the filum, which may cause tethered cord syndrome when associated with a low-lying conus medullaris
  • MRI demonstrates characteristic T1 hyperintense fat signal within the filum terminale that suppresses on fat-saturated sequences

Pathophysiology

  • Congenital anomaly resulting from focal premature disjunction of the neural tube
  • Mesenchymal tissue enters through the neural tube defect and differentiates into adipose tissue
  • Fatty infiltration causes thickening of the filum terminale (>2mm at L5-S1 level)
  • May lead to tethered cord syndrome through:
    • Increased tension on the conus medullaris
    • Restricted ascent of the conus during growth
    • Ischaemic injury from vascular compromise
  • Can occur in isolation or as part of caudal regression syndrome

Demographics

  • Prevalence: 0.2-6% of the general population (often incidental finding)
  • No significant gender predilection
  • Age at presentation:
    • Asymptomatic cases: discovered at any age
    • Symptomatic cases: typically childhood or adolescence during growth spurts
    • Adult presentation possible with degenerative changes or trauma
  • Associated conditions:
    • Spinal dysraphism
    • VACTERL association
    • Anorectal malformations

Diagnosis

  • Clinical presentation:
    • Often asymptomatic (incidental finding)
    • Tethered cord syndrome symptoms:
    • Lower back pain
    • Lower extremity weakness or sensory changes
    • Bowel/bladder dysfunction
    • Orthopedic deformities (foot deformities, scoliosis)
    • Cutaneous stigmata (hairy patch, dimple, hemangioma)
  • Physical examination:
    • Neurological deficits in lower extremities
    • Diminished or absent reflexes
    • Positive straight leg raise test
    • Cutaneous markers of spinal dysraphism

Imaging

  • MRI (modality of choice):
    • T1: hyperintense signal within filum terminale (follows fat signal)
    • T2: hyperintense to intermediate signal (less bright than CSF)
    • T1 + fat saturation: complete signal suppression confirming fat
    • T1+C: no enhancement (distinguishes from other enhancing lesions)
    • STIR/T2 fat saturation: hypointense signal with fat suppression
    • Sagittal imaging: essential for evaluating conus position and filum thickness
    • Axial imaging: confirms intradural location and filum thickening
  • Associated findings:
    • Thickened filum terminale (>2mm at L5-S1 level)
    • Low-lying conus medullaris (below L2-L3 disc space)
    • Syringohydromyelia (in chronic tethering)
  • CT:
    • Limited role, may show fat density (-50 to -100 HU) within spinal canal
    • Useful for evaluating bony abnormalities (spina bifida, segmentation anomalies)
  • Plain radiographs:
    • May show spina bifida occulta
    • Widened interpedicular distance
    • Scoliosis or other spinal deformities
  • Ultrasound (in neonates):
    • Echogenic mass within the spinal canal
    • Limited use after ossification of posterior elements

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  • A 30-year-old patient presented with worsening left S1 sensory disturbance and claf weakness.
  • MRI showed a low lying conus associated with a large fat-containing lesion.
  • In the upper sacrum, there posterior elements was anomalous midline fusion.

Treatment

  • Conservative management:
    • Asymptomatic patients with normal conus position
    • Regular clinical and imaging follow-up
    • Patient education regarding symptoms of tethered cord
  • Surgical intervention:
    • Indications:
    • Symptomatic tethered cord syndrome
    • Progressive neurological deficits
    • Low-lying conus with filum lipoma
    • Prophylactic in select paediatric cases
    • Surgical approach:
    • Laminectomy or laminotomy
    • Sectioning of the filum terminale
    • Debulking

Differential diagnosis

Differential diagnosis Differentiating feature
Myxopapillary ependymoma Enhances with contrast; heterogeneous signal on T1/T2; often has cystic components
Paraganglioma of filum terminale Intense enhancement; serpentine flow voids; may have haemorrhage/haemosiderin cap
Schwannoma Heterogeneous enhancement; may have cystic degeneration; eccentric to nerve root
Dermoid cyst Heterogeneous signal; may contain calcification, hair, or sebaceous material; chemical shift artefact
Epidermoid cyst Restricted diffusion on DWI; follows CSF signal on most sequences; no enhancement
Intradural metastasis Enhancement with contrast; multiple lesions along nerve roots and cauda equina; no fat suppression signal
Fibrolipoma Contains both fibrous and fatty components; more heterogeneous than pure lipoma
Angiolipoma Contains vascular elements; shows enhancement; flow voids may be visible
Teratoma Complex heterogeneous mass with fat, soft tissue, and calcific components
Retained surgical fat graft History of prior spine surgery; irregular configuration; no mass effect