The Art and Science of Equine Neurology - Peripheral Nerves & Stringhalt

Peripheral Nerve Injury  

Mechanical injuries to peripheral nerves occur because of compression, entrapment, transection,  laceration, ischemia, crushing, stretching, or chemical or burn damage.^1,2 Neurapractic lesions are characterized by a failure of conduction of the action potential across the injured axonal  segment.³ Axonotmesis is axonal interruption, while neurotmesis reflects disruption of endoneurium, perineurium, and/or epineurium. Recovery by axonal regrowth is unlikely after  neurotmesis.⁴ After axonotmesis of motor nerves, muscle reinnervation occurs by 2 separate  mechanisms: collateral sprouting and axonal regrowth.¹ If there is incomplete loss of axons, reinnervation of muscle units by sprouting occurs in days to weeks. Reinnervation by axonal  regrowth occurs at a rate of 1 mm per day (approximately 1 inch/month). Reinnervation may not be possible after more than 12 months although this remains a controversial issue. Peripheral nerve injuries are characterized by weakness of the innervated muscle accompanied within 2-4  weeks by appreciable atrophy. When the injured nerve supplies important extensor muscles of  the limbs (e.g., radial, femoral, sciatic, peroneal), there is obvious alteration of gait. Areas of  cutaneous anesthesia occasionally accompany peripheral nerve injuries. Over the neck and trunk, cutaneous sensory innervation occurs in defined bands associated with segmental dermatomes. Damage to a peripheral spinal nerve or dorsal nerve root results in cutaneous  anesthesia/hypalgesia over the supplied dermatome. With damage to the pudendal nerve or its  sacral nerve roots, there is anesthesia/hypalgesia of the perineal area. In contrast, relatively small  autonomous zones have been defined for sensory components of the ulnar, musculocutaneous, median, femoral, tibial and peroneal nerves.^5,6 Sympathetic fibers are distributed with peripheral  nerves, so denervated skin also may be evident as circumscribed spontaneous sweating. The  most common and important syndromes of mechanical injury to peripheral nerves of the limbs are described below.^6-8 

Suprascapular Nerve  

Arises from C6 and C7. Injury occurs most commonly when a horse’s shoulder is impacted at speed such that the nerve is injured as it curls around the front of the neck of the scapula. There  immediately is laxity and lateral instability of the shoulder joint, which bows out or “pops” as the  affected limb bears weight. Within 2-4 weeks of injury, there is obvious atrophy of the supraspinatus and infraspinatus muscles (sweeney). Successful reinnervation is evident as recovery of muscle bulk in the ventral part of the supraspinatus muscle beginning within 3  months of injury. Maximal recovery takes an additional 3-12 months. Regrowth of the  suprascapular nerve can be facilitated by surgery to resect scar tissue (external neurolysis) and  reduce tension on the nerve as it crosses the front of the scapula (often performed 3-6 months  after injury)

Radial Nerve  

The radial nerve innervates a flexor of the shoulder and the extensors of the elbow, carpal, and  digital joints. It arises from T1. The nerve root may be lacerated by fractures of the C7 or T1  vertebrae or first rib. The nerve is commonly damaged as a result of humeral fracture⁹ and can be  injured at the brachial plexus by trauma to the shoulder region. Ischemic damage may occur in horses anesthetized in lateral recumbencyT.¹⁰ The lower part of the nerve may be damaged by  dislocation or fractures that involve the elbow.  

Horses with complete radial paralysis stand with the shoulder extended, the elbow “dropped” and  the dorsum of the hoof rest on the ground. When forced to walk, the horse may partially protract the limb by exaggerated extension of the shoulder; however, the toe drags and the horse  collapses on the limb during the weight-bearing phase of the stride. If the site of damage is distal, the shoulder and elbow are normal. Although the radial nerve has numerous cutaneous sensory  branches, injury to this nerve does not result in any consistent area of cutaneous anesthesia. 

Musculocutaneous Nerve  

Arises from C7 and C8 and supplies flexors of the elbow. Injury is uncommon and causes only  transient toe-dragging. The shoulder may be held in a flexed position and the elbow in an extended position. There is hypalgesia/anesthesia over the dorsomedial aspect of the knee and  proximal metacarpus and atrophy of the biceps and brachialis muscles  

Median and Ulnar Nerves  

The median nerve arises from C8 and T1 and the ulnar nerve from T1 and T2. Injury to either can be caused by injury at the brachial plexus or along the medial aspect of the upper limb. There is a “tin soldier” gait, with decreased flexion and dragging of the toe and hypalgesia/analgesia of the skin of the caudal forearm, lateral metacarpus, and medial pastern areas and atrophy of the carpal and digital flexors.  

Femoral Nerve  

Arises from L3 to L5. The femoral nerve innervates muscles that flex the hip and extend the  stifle. The nerve can be damaged by ilial, femoral or vertebral fractures. Ischemic injury caused by prolonged stretch or increased tissue pressure during anesthesia in dorsal recumbency or after severe prolonged dystocia.¹¹ With unilateral paralysis the pelvic limb is abnormally flexed usually with the foot flat on the ground and buckles when the limb bears weight. In the case of  bilateral involvement, the horse is either unable to rise or stands uncomfortably in a crouched  position. If the nerve injury occurs proximal to the saphenous branch, there is anesthesia/hypalgesia of the skin over the medial surface of the thigh and atrophy of the  quadriceps muscle. 

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