Far Out Syndrome
Far-out syndrome is a radicular syndrome where there is extraforaminal compression of the L5 nerve root between the L5 transverse process and the sacral ala. It was first described by Wiltse et al in 1984, and "far-out" refers to the site of compression being far lateral.
The anatomy of the lumbar spinal canal is divided into the intraspinal canal, the foraminal zone, and the extraforaminal zone. Extraforaminal stenosis occurs when the spinal nerve passes outside the foraminal zone.
The far-out syndrome is one of the clinical manifestations of lumbosacral transitional vertebrae. Transitional anatomy is classified from Types I-IV. Type I is where there is a hypertrophied L5 transverse process which can lead to compression. In types II and IV there is a pseudoarticulation between the L5 transverse process which can develop degenerative changes leading to extraforaminal compression. There may be an associated disc herniation.
It can also occur with degenerative lumbar scoliosis or asymmetric disc degeneration at L5. This causes the 5th vertebra to tilt, causing the L5 transverse process to dip downwards to the sacral ala. Disc bulging can alter the nerve course and increase the risk of compression at the extraforaminal site. Osteophytosis can also add to the entrapment. Pre-existing transitional anatomy is another factor. A similar condition can occur higher in the lumbar spine with higher nerve roots where the two pedicles and transverse processes come together causing extraforaminal compression.
Isthmic spondylolisthesis (with at least a 20% slip) is another cause. With forward slipping, the transverse processes move anteriorly and caudally, and the transverse processes settle down against the sacrum. The forward slip is often asymmetric causing unilateral symptoms, but bilateral symptoms can also occur. Pre-existing enlarged transverse processes (but not necessarily transitional) can increase the risk. This can occur at higher levels, not necessarily only L5.
The lumbosacral ligament extends from the L5 vertebra to the sacral ala. It forms with the structures to which it is attached, an osteofibrotic tunnel. This tunnel is an extension of the intervertebral foramen. The L5 nerve root passes through the tunnel over the sacral ala and behind the lumbosacral ligament. Large osteophytes on the inferior border of L5 and thightness of the lumbosacral ligament are common causes of L5 entrapment against the sacral ala.
Extra-foraminal entrapment of the L5 nerve root causes radicular pain of the buttock and leg. Higher nerve roots can be involved.
Radiological diagnosis is difficult. With conventional imaging the area of interest is typically a "hidden zone."
With radiographs, the Ferguson view (20 degrees caudocephalic anteriorposterior) is the most useful.
With CT imaging it is important to use a wide window to include the area clear to the tip of the transverse process of L5.
A selective L5 nerve root block can provide diagnostic information.
Minimally invasive decompressive surgery can be performed. This should include decompressing arong the pseudo-articulation, and any concomitant disc hernation. Unrecognised extraforaminal stenosis is a cause of failed back surgery syndrome.
- Wiltse et al.. Alar transverse process impingement of the L5 spinal nerve: the far-out syndrome. Spine 1984. 9:31-41. PMID: 6719255. DOI.
- Nathan et al.. The lumbosacral ligament (LSL), with special emphasis on the "lumbosacral tunnel" and the entrapment of the 5th lumbar nerve. International orthopaedics 1982. 6:197-202. PMID: 7166449. DOI.
- Heo et al.. Usefulness of Oblique Lumbar Magnetic Resonance Imaging for Nerve Root Anomalies and Extraforaminal Entrapment Lesions. Asian spine journal 2018. 12:423-427. PMID: 29879768. DOI. Full Text.
- Kitamura et al.. A case of symptomatic extra-foraminal lumbosacral stenosis ("far-out syndrome") diagnosed by diffusion tensor imaging. Spine 2012. 37:E854-7. PMID: 22246537. DOI.
- Heo et al.. Endoscopic Treatment of Extraforaminal Entrapment of L5 Nerve Root (Far Out Syndrome) by Unilateral Biportal Endoscopic Approach: Technical Report and Preliminary Clinical Results. Neurospine 2019. 16:130-137. PMID: 30943715. DOI. Full Text.
- Takeuchi et al.. Lumbar extraforaminal entrapment: performance characteristics of detecting the foraminal spinal angle using oblique coronal MRI. A multicenter study. The spine journal : official journal of the North American Spine Society 2015. 15:895-900. PMID: 25681229. DOI.
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