Introduction

• Cause: trauma - usually RTA (HBC).
• Most common cause of thoracic limb paralysis.
• Signs: sudden onset neurological deficits in thoracic limb following trauma.
• Diagnosis: thoracic limb paralysis.
• Treatment: local wound treatment, physiotherapy/amputation.
• Prognosis: guarded, very poor to fair.

Pathogenesis

Etiology

• Trauma, eg road traffic accident → traumatic traction of nerve roots at their origin inside dura mata.

Predisposing factors

• Entire male.
• (Pro) estrus female.
• Dog lives on busy road or in city.
• Dog allowed to exercise unsupervised.
• Old or faulty extending lead.
• Dog exercised off the lead.

Pathophysiology

• Trauma can occur to a single nerve (misplaced injection into the sciatic nerve) or multiple nerves (brachial plexus avulsion).
• The severity of involvement clinically will determine prognosis.
• A neuropraxia (least severe injury) is an interruption in function and conduction in the nerve, usually associated with a lesion of the myelin without severe axonal involvement.
• Axonotmesis suggests separation and damage of axons, where neurotmesis (most severe injury) is complete severance of all structures of the nerve.
• The likelihood of regeneration is less with neurotmesis as compared to neuropraxia.
• Brachial plexus avulsion occurs as a result of a trauma.
• Clinical signs include LMN paresis/plegia in the affected thoracic limb.
• Ipsilateral loss of the cutaneous trunci reflex (due to damage to the lateral thoracic nerve that exits the spinal cord at C8-T1 area) and/or Horner's syndrome (sympathetic nerves exit the spinal cord at T1-3) may be associated signs.
• It is the nerve roots that are actually avulsed off of the spinal cord.
• The innervation of regional nerves from the plexus varies in animals.
• When regional nerves are formed by more cranial spinal rootlets than is usually seen, the plexus is said to be prefixed.
• When the nerves originate from more caudal spinal cord segments than normal, the plexus is said to be post-fixed.
• The relationship between prefixed, median, and post-fixed plexus types in the dog is 1:3:1.
• Allam (1952) studied the nerves that form the brachial plexus in dogs and found the following percentages of dogs had the brachial plexus derived from the associated spinal segments:
  • 58.6 % were formed by C 6, 7, 8, and T1.
  • 20.7 % were formed by C 5, 6, 7, 8, and T1.
  • 17.24% were formed by C 6, 7, 8, T1, and T2.
  • 3.4% were formed by C 5, 6, 7, 8, T1, and T2.
• Brachial plexus avulsion most commonly occurs in a single thoracic limb following automobile trauma.
• (a) Pathologically, the nerve roots are contused or separated from the spinal cord.
• (b) Diagnosis is based upon history and appropriate clinical signs.
• (c) There is no currently available treatment.
• (d) Prognosis depends upon the severity of nerve injury. Loss of pain sensation in the limb is a worse prognostic sign.
• Limb forcibly abducted or rotated at its attachment to the body → traumatic traction on the spinal nerve roots at their origin inside the dura mater → disruption of the neural elements → interruption of spinal reflexes.
• Skin desensitization and decreased muscle tone → neurogenic muscle atrophy → decreasing area of skin desensitization when nerves grow in from surrounding innervated areas.
• Initial assessments should attempt to determine the extent of injury to the nerve:
  • Neuropraxia: a functional rather than anatomical interruption in peripheral nerve function.
  • Axonotmesis: more severe than neuropraxia. Actual axons within the nerve are separated, however, the nerve itself remains intact.
  • Neurotmesis: a complete severance of the nerve with anatomical separation of all axons.

Timecourse

• Neurological deficits apparent immediately after injury.
• Neurogenic muscle atrophy from 7-10 days after injury.

Diagnosis

Treatment

Prevention

Outcomes