2.12 - Disorders of the PNS

Question 1

Spinal cord labels (from image)

Answer 1

• A - dorsal root (afferent)
• B - dorsal root ganglion
• C - mixed spinal nerve
• D - ventral root (efferent)

Question 2

What would damage to the dorsal root do? (V)

Answer 2

• leads to loss of sensation in dermatome supplied by the corresponding spinal nerve
• probably would not be detectable if only one root affected as there is considerable overlap of dermatome innervation by adjacent spinal nerves

Question 3

What would damage to the ventral root do? (W)

Answer 3

• leads to weakness of muscles supplied by the corresponding spinal nerve
• most limb muscles are innervated by two or more spinal nerves therefore paralysis is unlikely unless all spinal roots are damaged

Question 4

What would damage to the mixed spinal nerve do? (X)

Answer 4

• damage to spinal nerve leads to combined effects of V (dorsal root) and W (ventral root)

Question 5

What would damage to a sensory nerve do? (Y)

Answer 5

• damage to a sensory nerve (e.g. in skin) leads to loss of sensation in the area of distribution of that peripheral nerve

Question 6

What would damage to a muscle (motor) nerve do? (Z)

Answer 6

• leads to weakness/paralysis of the muscle supplied by that peripheral nerve

Question 7

What would be the effect if lesions Y and Z were more proximal to peripheral nerves?

Answer 7

• currently affect nerve branches close to their targets so have purely either sensory/motor effect
• more proximal lesions to peripheral nerves would affect both sensory and motor function

Question 8

What does the plexus do?

Answer 8

• contrasting effects of V/W and Y/Z demonstrates effect of the plexus which redistributes axons from spinal nerves into the peripheral nerves

Question 9

What common medical scenarios could result in lesions occurring at each point?

Answer 9

• spinal root and spinal nerve damage is most often a consequence of strain injuries to the spine e.g. prolapsed or herniated intervertebral disc
• peripheral nerves may be affected by trauma or disease (peripheral neuropathy)
• brachial plexus may be affected by trauma to the shoulder joint
• lumbosacral plexus is much better protected so unlikely to be injured

Question 10

Case

Answer 10

A 50-year old patient has pain in his right leg. He reports that it extends from his buttock, down his thigh, calf and into his toes and that it gets worse when he moves. He describes the pain as stabbing, burning or shooting and has pins and needles in his right leg. He says that he feels as though his right leg is weaker than the left.

Question 11

Case - which structures are involved to produce the symptoms?

Answer 11

• anterior and posterior nerve roots of lumbar spinal nerves L5 and S1
• their dermatomes cover the buttock, back of thigh, front and back of calf, big toe and side of foot

Question 12

Case - what is the name of the condition?

Answer 12

• lumbar spinal radiculopathy
• aka sciatica

Question 13

Case - what could cause the condition?

Answer 13

• compression of spinal nerves can arise from: herniated/slipped disc, narrowing of intervertebral foramina where nerves emerge (foraminal stenosis), slippage of vertebrae with respect to one another (spondylolisthesis)
• non-disc causes include: malignancy (e.g. metastatic bone disease)
• arthritis
• bone growths
• piriformis syndrome - where nerves are compressed by the contraction of the piriformis muscle in buttocks

Question 14

What treatment options are available?

Answer 14

• painkillers (nonsteroidal anti-inflammatory drugs e.g. ibuprofen)
• exercise and stretching
• epidural injections into the space around the dura mater - can be delivered into caudal part of spine (sacral hiatus) rather than site commonly used for pain relief during childbirth (L3/4 space)
• decompression surgery/discectomy
• the straight leg rise (SLR) test can help confirm diagnosis as it stretches the sciatic nerve and recreates pain felt by patient, imaging used if necessary