Trauma, eg road traffic accident traumatic traction of nerve roots at their origin inside dura mata.
Predisposing factors General
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 (incubation, duration)
Neurological deficits apparent immediately after injury.
Neurogenic muscle atrophy from 7-10 days after injury.
55 % cases have partial Horner's syndrome on affected side:
Ipsilateral miosis.
Slight ptosis on same side as affected limb.
Neurological deficits:
Panniculus deficit on affected side.
Absence of panniculus reflex on the affected side of the body.
Sensory deficits variable.
Variable loss of sensation in distal thoracic limb.
Affected limb does not bear weight:
Limb carried with elbow and shoulder flexed.
Complete avulsion
Affected limb does not bear weight:
Dragging of dorsal paw, causing severe skin excoriation.
Complete absence of limb function.
Dropped elbow with carpal flexion.
Affected limb looks longer.
Occasionally brachium advanced when dog is walking.
Neourological deficits:
No spinal reflexes in affected limb.
Complete absence of sensation in distal limb.
Partial Horner's syndrome in 55% of cases.
Loss of panniculus reflex.
Cranial avulsion
Pain sensation intact despite severe motor dysfunction.
Weightbearing lameness:
Lack of elbow flexion.
Decreased protraction of the limb.
Atrophy of supra- and infraspinatus muscles.
Diagnostic investigation
Radiography
Lateral and dorsoventral views of humerus and thorax .
Rule out concurrent orthopedic injuries, which are rare.
Other
Electromyography (EMG) is helpful in determining evidence of denervation in muscle.
Electromyography will show evidence of denervation in muscles of the limb suggesting a neurogenic rather than orthopedic cause for the dysfunction.
EMG changes, however, may not be present for up to 5-7 days after the injury.
Nerve conduction velocities (NCV) assess the speed of impulse transmission.
Late potentials (H waves; F waves) are used to assess the integrity of the proximal peripheral nerve and nerve root.
Confirmation of diagnosis Discriminatory diagnostic features
History.
Signs.
Definitive diagnostic features
Neurological signs.
EMG.
Gross autopsy findings
Hemorrhage is often noted in the intradural area around the nerve roots.
Histopathology findings
Severence of a nerve may result in Wallerian degeneration:
The neuronal cell enlarges, and nucleus become eccentrically positioned, and the Nissl substance usually comes together peripherally (central chromatolysis).
The distal segment of nerve below the severence will degenerate.
The proximal stump will enlarge and form collateral sprouts.
Differential diagnosis
Causes of partial paralysis of thoracic limb
Brachial plexus neoplasia .
Brachial plexus neuritis .
Brachial plexus neuritis is an idiopathic inflammation primarily involving the nerves of the brachial plexus.This uncommon disease may result in signs similar to a brachial plexus tumor or injury. In one dog, this disease was suspected to be due to an allergic reaction to ingestion of a horse meat diet.
Braund K G (1991) Nerve and muscle biopsy techniques.Prog Vet Neurol2 , 35-56.
Kline D G (1990) Surgical repair of peripheral nerve injury.Muscle and Nerve13 , 843-852.
Gibson K L & Daniloff J K (1989) Peripheral nerve repair.Comp Cont Educ Pract Vet11 , 938-944.
Allam M W, Lee D G, Nulsen F E & Fortune EA (1952) The anatomy of the brachial plexus of the dog.Anat Rec114 , 173-180.
Other sources of information
Rodkey W G (1993) Peripheral Nerve Surgery. In: Textbook of Small Animal Surgery. 2nd edn. Slatter D(ed). Philadelphia: W B Saunders. pp 1135-1141.
Vetstream contributor(s)
H Scott BVSc CertSAD CertSAO MRCVS , 207 Daventry Road, Cheylesmore, Coventry, West Midlands CV3 5HH, UK.
Dr Rod Bagley DVM DipACVIM , Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Washington State University, WA 99164-6610, USA.
traumatic traction of nerve roots at their origin inside dura mata.