Self-assessment Questions

17 Peripheral nerve injuries

• 1. Can tracts in the central nervous system regenerate once divided?

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  Correct answer:

  No.

• 2. What are the three degrees of peripheral nerve injury?

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  Correct answer:

  (1) Neurapraxia.
  (2) Axonotmesis.
  (3) Neurotmesis.
  

• 3. How can peripheral nerves be injured in general?

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  Correct answer:

  Laceration, stretching (traction) or compression.

• 4. What is neurapraxia?

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  Correct answer:

  Damage to the nerve fibres without loss of continuity of the axis cylinder; this is analogous to concussion within the central nervous system. The conduction along the fibres is interrupted for only a short period of time. Recovery usually commences within a few days and is complete in 6-8 weeks.

• 5. What is axonotmesis?

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  Correct answer:

  Injury to the axon and myelin sheath without disruption of the continuity of its perineural sheath. The axon distal to the lesion degenerates (Wallerian degeneration) and regrowth of the axon occurs from the node of Ranvier proximal to the injury. As the sheath is intact, the correct axon will grow into its original nerve ending. The rate of regeneration is approximately 1 mm/day; therefore, the time to recovery depends upon the distance between the injury and the end organ.

• 6. What is Wallerian degeneration?

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  Correct answer:

  In axonotmesis the axon distal to the lesion degenerates and regrowth of the axon occurs from the node of Ranvier.

• 7. What does the time to recovery following an axonotmesis depend on?

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  Correct answer:

  The nerve will regenerate at a rate of 1 mm/day, so the time to recovery depends on the distance between the injury and the end organ.

• 8. What is neurotmesis?

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  Correct answer:

  Actual physical disruption of the peripheral nerve. Regeneration will take place provided the two nerve ends are not too far apart, but functional recovery will never be complete. Following the complete disruption of neurotmesis, the distal part of the severed nerve undergoes Wallerian degeneration. The medullary sheath is depleted of myelin and the axon cylinders vanish; the empty endoneural sheaths remain as tubules composed of proliferating neurilemmal cells. The proximal end of the nerve degenerates up to the first uninjured node of Ranvier. New axis cylinders proliferate from this point and grow into the empty neurilemmal tubules. However, there is no selection of tubules for the appropriate axon; the distal growth is governed solely by the position of the nerve fibres. Thus, with most mixed nerves, there is likely to be considerable wastage owing to regenerating fibres growing into endings which are functionless, i.e. motor nerve endings growing into sensory nerve endings and vice versa. Even when a motor nerve grows into a motor nerve ending it may not supply the original muscle and the patient will have to relearn the affected movement.

• 9. Can different severities of nerve injury occur within the same nerve?

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  Correct answer:

  Because a peripheral nerve contains a large number of individual fibres it is quite possible in a nerve injury for some fibres to suffer from neurapraxia, others axonotmesis and others neurotmesis. However, a distinction between the first two and the last may be quite clear in that, if the nerve is found to be severed at surgical exploration, neurotmesis must have occurred.

• 10. What are the commonest causes of a partial nerve injury?

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  Correct answer:

  Partial nerve injury may occur as the result of pressure or friction, for instance from a crutch, a tightly applied plaster cast or a tourniquet, as well as from closed injuries or open wounds.

• 11. What special investigation would you use to investigate peripheral nerve injury?

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  Correct answer:

  Electromyography (EMG) plays an important part in the diagnosis and assessment of nerve injuries. Serial studies are useful in demonstrating the amount and rate of regeneration. EMG is also useful in the diagnosis of nerve compression syndromes.

• 12. How would you treat neurapraxia and axonotmesis?

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  Correct answer:

  Those joints whose muscles have been paralysed are splinted in the position of function to avoid contractures. They are put through passive movements several times a day so that, when recovery of the nerve lesion occurs, the joint will be fully mobile.

• 13. How would you treat neurotmesis? What is the prognosis?

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  Correct answer:

  Operative repair using an operating microscope is usually required. If a section of the nerve has been lost such that approximation is not possible, the nerve is freed proximally, or even moved from its original position to a new anatomical plane where more length will be available. For example, the ulnar nerve can be transposed from the posterior to the anterior aspect of the elbow joint to allow compensation for a distal loss of nerve substance. After nerve suture, recovery cannot be expected to take place until the time for regeneration has been allowed for, at the rate of 1 mm/day. Eventual recovery will seldom be full.