Conus Medullaris Syndrome vs Cauda Equina Syndrome : Anatomical basis and Mnemonic

Definitions

ConditionVertebral level of injuryNeurological level of injuryISNCI level of injury
Conus Medullaris Syndrome (CMS)T12-L2T12-S5T11
Cauda Equina Syndrome (CES)L3-L5L3-S5L2
conus medullaris and cauda equina

Anatomy

The spinal cord ends as a tapered structure called the conus medullaris at the level of L2–L3 disc in the neonate and the L1–L2 disc or cephalad at 1 year and older. It consists of the sacral (S2-S5) and coccygeal spinal cord segments.

The rest of the caudad spinal canal only houses nerve roots that are collectively known as cauda equina. The nerves in the cauda equina region include lower lumbar and all of the sacral nerve roots.

The pelvic splanchnic nerves carry preganglionic parasympathetic fibers from S2-S4 to innervate the detrusor muscle of the urinary bladder. Conversely, somatic lower motor neurons from S2-S4 innervate the voluntary muscles of the external anal sphincter and the urethral sphincter via the inferior rectal and the perineal branches of the pudendal nerve, respectively.

Pathophysiology

Some important points to note:

1. Conus medullaris comprises of a spinal cord and is in proximity to the nerve roots. Hence, conus medullaris syndrome is a combination of upper motor neuron (UMN) and lower motor neuron (LMN) lesion.

2. Cauda equina comprises of nerve roots only. Hence, cauda equina syndrome is a lower motor neuron (LMN) lesion.

3. Nerve roots in cauda equina have poorly developed epineurium making it susceptible to injury.

4. Not only the magnitude but also the length and the speed of obstruction are important in damaging the cauda equina region.

5. With fractures located above the L2 level – the conus medullaris and spinal cord occupy the spinal canal in this location, and these neural elements are more prone to neurologic injury than the nerve roots of the cauda equina.

Conus medullaris vs Cauda equina syndrome

Conus medullaris syndromeCauda equina syndrome
PresentationSudden and bilateral (predominantly symmetric)Gradual and unilateral (often asymmetric)
ReflexesKnee jerks preserved but ankle jerks affectedBoth ankle and knee jerks affected
Radicular painLess severeMore severe
Low back painMoreLess
Sensory symptoms and signsMore localized to perianal region; sensory dissociation occursMore localized to saddle area; no sensory disscoiation
Motor strengthHyper-reflexic distal paresisAreflexic paraplegia
ImpotenceFrequentLess frequent
Sphincter dysfunctionEarly; both urinary and fecal incontinenceLate; urinary incontinence

Mnemonic: Conus rhymes with ANUS
1. periAnal localized
2. Nimble (fast and early involvement)
3. Upper motor neuron involvement
4. Symmetric involvement


Write your Viewpoint 💬

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.