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Neuroradiology Case of the Week

Case 258

B. Keegan Markhardt, MD, and P-L Westesson, MD, PhD, DDS

Clinical Presentation: Patient is a 57-year-old female presenting with left-sided pelvis pain and left L5 dermatome radiculopathy exacerbated by back flexion.

Imaging Findings: On sagittal T1- (Fig. 1) and T2-weighted (not shown) sequences, there is a linear hyperintensity involving the filum terminale anterior to the caudal rootlets extending from the L1 to L4 levels, suggestive of fatty infiltration. Presence of fat in the filum is confirmed on axial images that show chemical shift artifact illustrated by a crescentic hypointensity adjacent to the hyperintense filum (Fig. 2).

Diagnosis: Lipoma of the filum terminale (fatty filum)

Discussion: Lipoma of the filum terminale is identified by having fat density on CT or fat signal intensity on MR. Small amounts of fat in the filum are usually asymptomatic in kids and may be termed "fibrolipomas." The occurrence of incidental fat within the terminal filum in the normal adult population has been estimated to be 4 to 6% [1].
     Lipoma of the filum terminale is a form of spinal dysraphism, and has an association with midline bony defects in the lumbosacral spine. The defect of spinal dysraphism occurs in the first 8.5 weeks of fetal life. The skin is separated from the ectoderm derived neural tube by the mesoderm (bony elements, meninges, and muscle). Incomplete separation of skin ectoderm from the neural tube ectoderm results in cord tethering, diastematomyelia, or a dermal sinus. Premature separation of the cutaneous ectoderm from the neural tube results in incorporation of mesenchymal elements between the neural tube and skin, which may result in the development of lipoma.
     In adults, a small percentage of fat containing filum are associated with tethered cord syndrome (TCS). The normal filum is less than 2 mm in cross-sectional diameter [2]. Significant thickening of the filum terminale may cause the conus medullaris to be dragged downward and result in TCS. New onset of TCS in the adult population is an unusual but well-described entity [3, 6]. The late onset of presentation may be related to the cumulative effects of repeated microtrauma during flexion and extension [3].
     Tethered cord syndrome features are primarily secondary to conus ischemia from traction. These include lower extremity sensory and motor defects, bladder and bowel incontinence, high-arched feet, hammer toes and abnormal gait. Development of scoliosis and exaggerated lordosis are thought to be a functional adjustment by the paraverebral muscles to minimize the intramedullary tension [4]. Position of the conus below the L2-3 is suggestive of cord tethering [5].
    Surgical untethering in the adult population may relieve pain in 80% of patients,[6] and it may improve bowel and bladder dysfunction in 38% of patients [6].

References:

1. Brown E, Matthes JC, Bazan C, Jinkins JR. Prevalence of incidental intraspinal lipoma of the lumbosacral spine as determined by MRI. Spine. 1994 Apr 1;19(7):833-6. [Medline]
2. Grossman R, Yousem D. The Requisites: Neuroradiology. 2nd ed. 2003, Mosby, Philadelphia; pg 463-4.
3. Gupta SK, Khosla VK, Sharma BS, Mathuriya SN, Pathak A, Tewari MK.: Tethered cord syndrome in adults. Surg Neurol. 1999 Oct;52(4):362-9; discussion 370. [Medline]
4. Yamada S, Won D, Yamada SM. Pathophysiology of tethered cord syndrome: correlation with symptomatology. Neurosurg Focus. 2004 Feb 15;16(2):E6. [Medline]
5. Ali Nawaz Khan AN, Turnbull I, Macdonald S, Sabih D. E-medicine. February 20,2007. http://www.emedicine.com/radio/topic643.htm
6. Pang D, Wilberger JE: Tethered cord syndrome in adults. J Neurosurg. 1982 Jul;57(1):32-47. [Medline]