Formation of the Spinal Nerves
The spinal nerves are mixed nerves that originate from the spinal cord, forming the peripheral nervous system.
Each spinal nerve begins as an anterior (motor) and a posterior (sensory) nerve root. These roots arise from the spinal cord, and unite at the intervertebral foramina, forming a single spinal nerve.
The spinal nerve then leaves the vertebral canal via the intervertebral foramina, and then divides into two:
- Posterior rami – supplies nerve fibres to the synovial joints of the vertebral column, deep muscles of the back, and the overlying skin.
- Anterior rami – supplies nerve fibres to much of the remaining area of the body, both motor and sensory.
The nerve roots L2-S5 arise from the distal end of the spinal cord, forming a bundle of nerves known as the cauda equina.
Clinical Relevance: Cauda Equina Syndrome
The cauda equina is a bundle of spinal nerves that arise from the distal end of the spinal cord. They run in the subarachnoid space, before exiting at their appropriate vertebral level.
Compression of these nerves produces a range of signs and symptoms collectively termed cauda equina syndrome. There are many causes of compression, including intervertebral disc prolapse, extrinsic or primary cord tumours, spinal stenosis, trauma and abscess formation.
Suspected cauda equina patients should be assessed with a full lower limb neurological assessment. The main signs to assess for are:
- Saddle-area anaesthesia.
- Incontinence / retention of urine or faeces.
- Reduced anal tone.
- Paralysis ± sensory loss.
If sufficient clinical evidence exists, an MRI is required immediately for diagnosis. Any confirmed case must be sent for surgery within 36 hours of first presentation of the symptoms for surgical decompression.