Traumatic injuries causing avulsion of the nerve roots of the brachial plexus are the most common cause of acute thoracic limb monoparesis or monoplegia in small animals. They are usually caused by road traffic accidents or falls from a height, as a result of abduction and simultaneous caudal displacement of the thoracic limbs.
The site of root avulsion is usually intradural where the nerve roots arise from the spinal cord. At this point, nerve roots lack a well defined perineurium and constitute the weakest structure between the spinal cord and the peripheral nervous system. Ifthe avulsion is severe enough, it may place traction over the spinal cord and damage spinal cord pathways, causing ipsilateral pelvic limb neurological deficits. Both dorsal and ventral nerve roots can be affected, but the motor roots appear to be more susceptible to this type of trauma ()
Clinical signs: Signs are peracute in onset following the traumatic incident. Depending on which nerve roots are affected, avulsions are divided into three types:
- Cranial avulsions (C6-C7 nerve roots)
- Caudal avulsions (C8-T2 nerve roots)
- Complete avulsions (C6-T2 nerve roots).
Cranial avulsions are rare and result in few clinical signs. The elbow extensor muscles are not affected, so the animal can bear weight on the affected limb. There is loss of shoulder movement and elbow flexion, and atrophy of supraspinatus and infraspinatus muscles usually develops.
Caudal and complete avulsions are more common and cause more severe clinical signs (). They both cause paralysis of the triceps brachii muscle, so the animal cannot extend the elbow or bear weight on the affected limb. Affected animals drag the limb knuckled over (). Thoracic limb muscles are hypotonic and severe neurogenic atrophy starts about one week after the injury. Spinal reflexes and postural reactions are lost. If the elbow flexor muscles are spared (caudal avulsions) the animal can carry the limb flexed at this level, avoiding contact with the floor.
Sensory signs are also common. The pattern of decreased or absent sensation in the affected limb allows better determination of the type of avulsion (). Cutaneous sensation should be checked in the entire limb, but particular attention should be paid to deep pain sensation in the denial (radial and musculo-cutaneous) and lateral (radial and ulnar) digits, since it is essential in determining a prognosis.
A high percentage of patients with brachial plexus avulsions show partial Horner’s syndrome and/or loss of cutaneous trunci reflex ipsilateral to the side of the avulsion (). Avulsion of the T1 ventral nerve root causes injury to the pre-ganglionic sympathetic nerve fibres to the eye, causing ipsilateral miosis (partial Horner’s syndrome). Loss of the cutaneous trunci reflex is caused by damage to the C8-T1 ventral spinal nerve roots that form the lateral thoracic nerve and innervate the cutaneous trunci muscle. The contralateral reflex is usually present after ipsilateral stimulation.
Diagnosis: A history of thoracic limb monoparesis after a traumatic incident should raise a high suspicion of brachial plexus avulsion. Every animal unable to use one thoracic limb after trauma should be examined carefully to detect orthopaedic as well as neurological abnormalities.
MRI of the affected plexus may provide information on the degree of nerve and associated soft tissue trauma.
Electromyography allows detection of spontaneous electrical activity in the denervated muscles 7-10 days after the injury (). Nerve conduction velocity studies of the radial and ulnar nerves allow determination of the degree of injury (). Since the radial nerve is commonly injured in brachial plexus avulsions, serial electrodiagnostic evaluations of this nerve may provide useful diagnostic and prognostic information. Early decreased radial nerve conduction velocity indicates a poor prognosis ().
Treatment: Unfortunately, there is no routinely effective treatment for this type of injury. The degree of recovery depends only on the severity of the nerve lesion at the time of injury.
- If deep pain is present in the medial and lateral digits, prognosis for recovery is good and aggressive physiotherapy should be recommended to the owner ().
- If deep pain sensation is absent, prognosis will depend on the severity of axonal injury, being good for neurapraxic lesions, but guarded to poor for axonotmetic and neurotmetic lesions. Pure axonotmesis occurs rarely, so potential for recovery, although present, is low and prognosis poor.
When the proximal branches of the radial and musculocutaneous nerves are spared ― so that the elbow flexor and extensor muscles are not denervated ― corrective surgery can be used to provide carpal extension and prevent the distal part of the limb from collapsing (). Tendon transplantation or carpal arthrodesis procedures can be performed.
- Muscle tendon transplantation can be successful in partial avulsions.
- Carpal fusion can be useful in animals with adequate triceps function that walk knuckling over on their carpus ().
Before performing any type of surgery, it is essential to perform EMG to determine that elbow extensors or the muscles to be transplanted have normal function and are not denervated. However, amputation maybe necessary if sensation is lost and excoriations and self-mutilations secondary to paresthesias develop. The reader is referred to standard surgical texts for further information on these surgical procedures.
Finally, experimental studies in dogs demonstrate that ventral root reimplantation can promote successful reinnervation (). Supportive treatment is essential during the recovery time to prevent contractures and excoriations from dragging the limb. Keeping the limb clean and dry, treating any wounds that may develop, covering the foot with boots or bandages are all important therapeutic measures. At the same time, performing physical therapy (flexion and extension movements) several times a day is crucial to lessen muscle atrophy as well as to prevent muscle contractures and joint fusions ().
Prognosis: Outcome is often good for animals with cranial plexus avulsions that are able to bear weight on the affected limb and maintain normal sensation over the distal part of the limb.
Animals with caudal and complete brachial plexus avulsions have a poor to guarded prognosis if neurotmesis has occurred. Only those with neurapraxic injuries show improvement and recover completely. However, most animals do not improve but go on to show severe limb atrophy, eventually developing serious complications such as: trophic ulcers; joint con-fractures or self-mutilations from paraesthesias; or abnormal sensation in the affected areas produced by regeneration of sensory nerves (Sharp, 1995). In these cases, amputation of the limb is necessary.
Decreased radial nerve conduction velocity at the beginning of clinical signs is a poor prognostic indicator (). If the situation remains unchanged after 4 weeks, with concurrent severe EMG changes in the triceps, there is virtually no chance of spontaneous recovery. However, the presence of giant motor unit potentials in any of the affected muscles is an indicator of reinnervation () and may represent a favourable prognosticsign.
The best predictor of complete recovery seems to be pain perception (). Preservation of pain sensation is an indicator of a milder type of injury and should prompt the clinician to recommend supportive therapy while waiting for motor function to recover. However, if no improvement is seen during the first 2 months, recovery is unlikely to occur.