Why does cauda equina syndrome (CES) present purely with lower motor neuron signs?

Why Cauda Equina Syndrome Presents with Lower Motor Neuron Signs

Cauda equina syndrome (CES) presents purely with lower motor neuron signs because it involves compression of the nerve roots (cauda equina) below the level of the conus medullaris, affecting peripheral nerves rather than the spinal cord itself. 1, 2

Anatomical Basis

• The cauda equina consists of lumbar, sacral, and coccygeal nerve roots that extend beyond the terminus of the spinal cord (conus medullaris), resembling a "horse's tail" within the vertebral canal 2
• These nerve roots are peripheral nerves, not part of the central nervous system, so their compression results in lower motor neuron signs rather than upper motor neuron signs 1
• Most commonly, CES is caused by massive midline disc herniation at L4-L5 and L5-S1 levels, directly compressing these peripheral nerve roots 3, 2

Clinical Manifestations of Lower Motor Neuron Involvement

• Bilateral radiculopathy (bilateral radicular pain, sensory disturbance, or motor weakness) represents lower motor neuron dysfunction 4, 3
• Progressive neurological deficits in the legs manifest as flaccid weakness rather than spasticity 4
• Impaired perineal sensation occurs due to compression of sacral sensory nerve roots 4, 3
• Reduced anal tone results from compression of motor nerve roots supplying the external anal sphincter 4
• Urinary disturbances (retention, hesitancy, poor stream) occur due to compression of sacral nerve roots controlling bladder function 4, 3

Diagnostic Considerations

• The diagnosis of CES is primarily clinical, corroborated by radiological findings of cauda equina compression 5
• MRI is the gold standard imaging modality to confirm compression of the cauda equina nerve roots 1, 2
• No single symptom or sign has high positive predictive value in isolation, making diagnosis challenging 3

Atypical Presentations

• Rarely, atypical presentations of CES may include upper motor neuron signs such as clonus, which can confuse the diagnosis 5
• In such cases, if other clinical and radiological findings strongly suggest CES, the presence of upper motor neuron signs should not delay treatment 5
• Additional imaging of the brain and cervicothoracic cord may be warranted to rule out concurrent pathologies if upper motor neuron signs are present 5

Clinical Stages and Progression

• CES can be classified into stages: suspected CES (CESS), incomplete CES (CESI), and complete CES with retention (CESR) 4, 2
• Early recognition of "red flag" symptoms (bilateral radiculopathy, progressive neurological deficits) is crucial before progression to irreversible "white flag" symptoms (urinary retention, fecal incontinence, perineal anesthesia) 4, 1
• Outcomes are significantly better when surgical intervention occurs before complete loss of bladder function 2

Management Implications

• Emergency MRI and neurosurgical consultation for potential decompressive surgery are required for CES 1
• Surgical decompression is recommended within 48 hours, preferably within 24 hours, with better outcomes associated with surgery within 12-72 hours of symptom onset 1, 2
• Patients treated at the CESI stage typically have better outcomes with normal or socially normal bladder and bowel control 2

Understanding the lower motor neuron presentation of CES is critical for timely diagnosis and intervention to prevent permanent neurological damage and optimize patient outcomes.