Cauda equina syndrome

Last revised by Daniel J Bell on 23 Mar 2023

Cauda equina syndrome is considered an incomplete cord syndrome, even though it occurs below the conus. Cauda equina syndrome refers to a collection of symptoms and signs that result from severe compression of the descending lumbar and sacral nerve roots. It is most commonly caused by an acutely extruded lumbar disc and is considered a diagnostic and surgical emergency. 

Cauda equina syndrome is rare with prevalence estimated at approximately 1 in 65,000 (range 33,000 to 100,000) 1. It has been estimated to occur in ~1% (range 0.1-2%) of herniated lumbar discs 2,3

Cauda equina syndrome can present either acutely or chronically and requires two sets of symptoms/signs 1-3:

  1. perianal and "saddle" paresthesia

  2. bowel, bladder and/or sexual dysfunction

There is a host of associated symptoms and signs, which may be unilateral or bilateral and have a variable presence 1-3,6,10:

  • low back pain

  • radiculopathy/sciatica (unilateral or bilateral)

  • paresthesia of lower limbs and perianal/saddle region (variable)

  • weakness of lower limbs in a lower motor neuron pattern (variable)

  • reduction/absence of lower limb reflexes

Additionally, cauda equina syndrome can be classified as incomplete or complete based on the presence of bowel and bladder symptoms 1,2,10:

  • incomplete

    • may have loss of urgency or decreased urinary sensation without incontinence or retention

    • accounts for ~40% (range 30-50%) of presentations 6

  • complete

    • urinary and/or bowel retention or incontinence

    • accounts for ~60% (range 50-70%) 6

There is a long list of conditions that can cause cauda equina syndrome (some of these are very rare) 1-3:

  • limited value; may demonstrate gross degenerative or traumatic bony disease 2

  • useful in patients in whom MRI is contraindicated or not available

  • partial or complete blockage of contrast

    • may demonstrate an "hourglass" shape to the contrast-filled thecal sac incomplete blockage 2

  • imaging modality of choice 2,3

  • sagittal and axial T1 and T2 sequences are usually sufficient 4

  • post-contrast and STIR sequences may be required if infective causes are suspected 3,4

Cauda equina syndrome is considered a diagnostic and surgical emergency, although there is some debate about the timing of surgery, which is also dependent on whether the pathology is acute or chronic. Surgical decompression within 24 hours seems to have the best outcome 1,3,6.

Patients with complete cauda equina syndrome have a poorer outcome 3. Approximately 20% of patients will have a poor outcome in terms of urological and/or sexual function, as well as lower limb paresthesia and weakness 6

Clinically the main differential is that of conus medullaris syndrome

It is worth remembering that cauda equina syndrome is a clinical diagnosis and thus the term should not be used in a radiology report unless the appropriate symptoms and signs are known. In the absence of corroborating history, a better phrasing is "compression of the cauda equina" which should then be correlated clinically. This is an important distinction as many elderly patients may have marked canal stenosis with compression of the cauda equina but not present acutely with cauda equina syndrome. 

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