Lumbar foraminal stenosis

Last revised by Francis Deng on 12 Nov 2023

Lumbar foraminal stenosis or lumbar neuroforaminal stenosis is described as narrowing of the neural exit foramina. The patency of the neural exit foramina is assessed as part of the routine evaluation of lumbar MRI studies to determine what impact, if any, the surrounding structures have on the exiting nerve root. 

Lumbar foraminal stenosis has a prevalence of ~10% (range 8-11%) 4.

Lumbar foraminal stenosis is a common cause of low back pain (including buttock pain) and lower limb pain (radiculopathy), which is worsened by sitting or lying supine. Motor weakness may also occur 4. On examination, pain may be exacerbated by lumbar extension (Kemp's sign) 4.

Foraminal stenosis is more common in the lower lumbar spine 5. Lumbar foraminal stenosis develops in-step with disc and facet degeneration 4,5:

  • vertical (craniocaudal) stenosis from vertebral osteophytes and/or bulging or herniated disc results in nerve root compression against the superior pedicle; posteriorly the foramen is patent and perineural fat is generally preserved

  • transverse (anteroposterior) stenosis from anterosuperior facet subluxation +/- spurs and ligamentum flavum thickening results in nerve root compression between the superior articular facet and the posterior vertebral body

  • circumferential stenosis is a combination of both vertical and transverse stenoses

Most commonly, lumbar neural foraminal stenosis is graded as mild, moderate, severe, or normal (no stenosis) based on Lee et al. 1 as a modification of the Kunogi and Hasue classification 2:

  • grade 0 (normal): fat surrounds the nerve root circumferentially 

  • grade 1 (mild): absence of perineural fat visualization in one dimension, either vertically or horizontally

    • anteroposterior narrowing from ligamentum flavum thickening/buckling

    • superoinferior narrowing from the disc and/or osteophyte encroachment or disc height loss

  • grade 2 (moderate): absence of perineural fat visualization circumferentially but without compression of the nerve root

    • multidirectional encroachment from degenerative facet hypertrophy, ligamentum flavum thickening/buckling, disc and/or osteophyte encroachment, and/or disc height loss

  • grade 3 (severe): absence of perineural fat visualization with compression (morphologic change) of the nerve root

While not being able to visualize the nerve root, lumbar foraminal stenosis may be indicated by a posterior disc height ≤4 mm and/or foraminal height ≤15 mm 4,5.

Neural exit foramen are best evaluated on sagittal sequences, with findings of lumbar foraminal stenosis mainly reduced foraminal area and reduced perineural fat signal 4.

First-line treatment for symptomatic lumbar foraminal stenosis includes pain medication and rehabilitation. Refractory or severe pain can initially be treated with transforminal epidural steroid injection or selective nerve root block and/or surgery 4.

Surgical options include direct decompression (e.g. laminotomy/ectomy +/- foraminotomy/ectomy) +/- posterior fusion (e.g. posterolateral fusion, transpendicular instrumented posterior fusion) 4,5. Lumbar interbody fusion allows for indirect decompression by restoring intervertebral and thus foraminal height 4.

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