45 LMN paresis and paralysis
Brachial plexus avulsion
Failure of nerve transmission due to a functional disturbance from compression, ischaemia, or blunt trauma is called neurapraxia. The larger motor and proprioceptive axons are affected preferentially with preservation of pain perception. Structural integrity of the nerve’s Schwann cells and endoneurial connective tissue persist, giving a good prognosis for recovery. Neurotmesis is the complete or partial transection of the nerve’s axon, myelin sheath and endoneurial connective tissue. All axon types are affected and pain perception is lost. The destruction of the connective tissue and Schwann cell scaffolding prevents effective nerve regeneration. Electrodiagnostics and repeat clinical examination monitor clinical improvement.
Non-weight-bearing left-fore paresis began suddenly after being hit by a car 8 months prior to referral. The dog occasionally licked the left forepaw. No improvement had been noticed.
The dog was alert and ambulatory with a non-weight-bearing left forelimb. Severe generalized atrophy of the left fore was present (Fig. 45.1). The left elbow was held flexed. Muscle contracture prevented extension of the right carpus, elbow and shoulder. The left cutaneous trunci reflex was absent when either side was stimulated. The left pupil failed to dilate fully in darkness. Pain perception was absent in the radial and ulnar nerve distributions but was present in the distribution of the musculocutaneous nerve.
Left brachial plexus, including the ventral nerve roots supplying the lateral thoracic nerve (C8–T1) to account for the cutaneous trunci deficit, and those supplying oculosympathetic innervation to the eye (T1–3) to explain the left miosis in darkness.