Radial-Nerve Paralysis Associated With Humeral Shaft Fractures
In the horse, the humerus creates a right angle with the scapula and does not have as steep of a slope as in smaller animals. The greater tubercle and lesser tubercle are well developed and approximately the same size compared to other species in which they differ in size. Each tubercle is separated into cranial and caudal areas which are divided by an intertubercular groove which is broken up by an intermediate tubercle. Five processes enclose the head on every aspect except the caudal aspect. The greater tubercle is easily felt on the horse and its cranial separation is the ‘point of the shoulder’. Distally the deltoid tuberosity lies, which is also easily palpated (Dyce et al., 2010) [Figure 1].
A fractured humerus can occur in any type of horse, however, foals less than a year old, jumpers, racing thoroughbreds and steeple chasers are more susceptible to a humeral fracture (Kraus et al., 2005; Pasquini et al., 2005; Stashak, 2002). Causes can include, falls, impact trauma, postoperative recovery and racing on hard surfaces. Fractures to the deltoid tuberosity and greater tubercle alongside a humeral fracture often indicates a trauma was the cause of injury (Bordelon et al., 2008; Levi et al., 2011; Stashak, 2002). There are two types of humerus fractures: stress fractures and complete fractures. Stress fractures affect the proximal caudal lateral cortex and distal cranial medial cortex. It is common to find a remodelled stress fracture upon necropsy of a horse with a complete humeral fracture. Complete fractures which are most common in the middle third of the bone. These can be oblique or spiral, frequently comminuted or the distal segment displaced caudally (Kraus et al., 2005; Pasquini et al., 2005). Complete fractures, however, are rare as the humerus is a thick, short bone covered by muscles (Pasquini et al., 2005; Stashak, 2002). Complete fractures can affect the deltoid tuberosity, greater tubercle or the physis of the humeral head or distal humeral condyle of foals. Radial nerve damage is the greatest problem associated with a humeral fracture as the radial nerve travels in the brachial groove. Trauma can occur and sever the nerve (Pasquini et al., 2005).
Prognosis for adult horses is very poor and for foals is guarded. A prognosis cannot be given until the fracture heals because of complications such as rotation of P3, radial nerve paralysis, angular limb deformities, flexural deformities or failure of plates and pins. Prognosis for a stress fracture is good if the horse is rested. Prognosis is also good for a fracture of the deltoid and greater tubercle fracture (Pasquini et al., 2005; Stashak, 2002).
Figure 3. Illustration depicting the nervous system of the horse. The radial nerve is shown branching off the brachial plexus and travelling down the forelimb (Thomas et al., 2016).
References
Bordelon, K.L., Mathis, S. C. and Munroe, J. L. 2008, 'What Is Your Diagnosis?', Journal of The American Veterinary Medical Association, vol. 233, no. 6, pp. 873-874.
BRADLEY, J. 2013, Radial nerve damage, YouTube, 23/8/13, viewed 28/7/17, https://www.youtube.com/watch?v=vVMAsjAs7i4
Dyce, K. M., Sack, W. O. and Wensing, C. J. G. 2010, Textbook of Veterinary Anatomy, 4th ed., Missouri, Saunders Elsevier.
Lacroix, J. 2005, The Project Gutenberg ebook, Lamness of the Horse, American Journal of Veterinary Medicine, Chicargo, viewed 26/7/17.
Levi, O., Vaughan, B., Puchalski, S. M. and Macdonald, M. H. 2011, 'What is your diagnosis? Complete communited sagittal fracture of the medial aspect of the humeral head and lesser tubercle', Journal of The American Veterinary Medical Association, vol. 283, no. 4, pp. 431-432.
Lopez, M. J., Nordberd, C. and Trostle, S. 1997, ‘Fracture of the 7th cervical and 1st thoracic vertebrae presenting as radial nerve paralysis in a horse’, The Canadian Veterinary Journal, vol. 38, no. 2, pp. 112.
Kraus, B. M., Ross, M. W. and Boswell, R. P. 2005, 'Stress Remodeling and Stress Fracture of the Humerus in Four Standardbred Racehorses', Verinary Radiology and Ultrasound, vol. 46, no. 6, pp. 524-528.
Murdoch University 2013, 'Equine Humerus', Murdoch University Virtual Anatomy Museum, Murdoch University, Western Australia.
Pasquini, C., Jann, H., Pasquini, S. & Bahr, R. 2005, Guide to Equine Clinics Lameness, 2nd ed., Texas, Sudz Publishing.
Rooney, J.R. 1998, The Lame Horse, The Russell Meerdink Company Ltd., Wisconsin.
Stashak, T. S. 2002, Adam’s Lameness in Horses, 5th ed., Maryland, Lippincott Williams & Wilkins
Thomas, W. B., Bedenice, D. Braund, K. G., Chrisman, C. L., Hahn, C. N., Hendrix, C. M. Mackay, R. J., Munana, K. R., Traub-Dargatz, J. L. and White, S. L. 2016, Parts of the Nervous System Merck Veterinary Manual, viewed 26/7/17 2017.
Wagner, A. E. 2008, 'Complications in Equine Anesthesia ', Veterinary Clinics of North America: Equine Practice, vol. 24, no. 3, pp. 735-752.
Figure 1. Labelled equine humerus, detailing the points of the humerus (Murdoch University, 2013).
- Treatment for stress fractures is stable confinement until the lameness is no longer evident (Pasquini et al., 2005; Stashak, 2002).
- For treatment of complete fractures in foals less than 6 months of age with minimal displacement, stable rest for six weeks should be undertaken. A PVC splint to prevent contracture and for support can be used. Alternatively, heavy bandages applied contralateral to the limb can prevent breakdown of flexor tendons (Pasquini et al., 2005; Stashak, 2002).
- Foals can also undergo surgery as treatment which involves either multiple intermedullary pins, ASIF tibial nail which ream out the medullary cavity or bone plates in the brachial groove under the brachialis muscle and radial nerve (Pasquini et al., 2005; Stashak, 2002).
- In the adult horse plating and stack pinning is not strong enough to hold the bone together. The horse would need to be confined in a sling for six to eight weeks with extensive care. Non-union is common. A PVC splint on the opposite limb should be utilised to prevent flexural contracture. Laminitis can also frequently occur in the opposite limb in three to six weeks (Pasquini et al., 2005).
- For fracture of the deltoid tuberosity lag screws in foals or tension bands in adults can be applied or small pieces removed (Pasquini et al., 2005; Stashak, 2002).
- Greater tubercle fractures which are small and nonarticular are treated conservatively with stable rest. Large or articular fractures are removed (Pasquini et al., 2005).
- Physeal fractures are treated conservatively unless displaced in which the fracture is treated with a PVC splint from elbow to foot and a support wrap on the opposite limb (Pasquini et al., 2005).
- Euthanasia (Levi et al., 2011; Pasquini et al., 2005; Stashak, 2002).
Clinical Signs & Diagnosis
Treatment & Prognosis
- For a stress fracture the horse presents with mild to non-weight bearing and lameness which can improve over 1-2 days (Pasquini et al., 2005; Stashak, 2002).
- For a complete fracture, the horse displays acute three-legged lameness with a swollen shortened limb and a ‘dropped elbow’ [Figure 2.] (Pasquini et al., 2005).
- Diagnosis is via manipulation of the limb for instability or crepitation. Radiographs must be taken for a definitive diagnosis as well as to determine the conformation of the fracture and detect any secondary problems (Bordelon et al., 2008; Levi et al., 2011; Pasquini et al., 2005; Stashak, 2002).
- Scintigraphy should also be used to diagnose a stress fracture (Kraus et al., 2005; Pasquini et al., 2005).
- Radiographs should be taken slightly oblique from both a medial and lateral view with the limb in extension. Many views may be required to identify fractures to the deltoid tuberosity and greater tubercle however the horse may need to be anesthetised to obtain cranial and caudal views (Stashak, 2002).
Fractured Humerus
Figure 2. Photograph of a horse displaying the ‘dropped elbow’ and swollen shortened limb associated with a humeral fracture (Stashak, 2002).
Radial Nerve Paralysis
Treatment & Prognosis
- Conservative treatment involves stable confinement and rest to see if nerve function returns to the limb (Pasquini et al., 2005; Rooney, 1997; Stashak, 2002). To reduce inflammation dexamethasone can be given for 3-5 days as well as intravenous phenylbutazone and ice packs for the first 24 hours (Pasquini et al., 2005; Stashak, 2002).
- Corticosteroids in a low dose can also be administered in addition to non-steroidal anti-inflammatories in acute cases (Stashak, 2002).
- Topical cold therapy as well as dimethyl sulphide can be utilised to reduce swelling (Stashak, 2002).
- For pain Benamine, phenylbutazone and Demerol can be administered. Slings and deep bedding can also aid in reducing pain (Pasquini et al., 2005; Stashak, 2002).
- Recumbent animals must be turned frequently to prevent ulcers (Pasquini et al., 2005).
- A bandage or light cast can also be employed to prevent contracture of flexors and to protect the dorsal fetlock (Pasquini et al., 2005; Stashak, 2002).
- If laminitis in the supporting limbs is a concern deep sand covered with straw can be used as bedding or a PVC bandage splint can be employed. This splint should extend from the caudal aspect of the injured limb to the proximal antebrachium to the fetlock and keep the limb extended. This allows the horse to bear weight on the limb until it can do so unsupported (Stashak, 2002).
- Surgery can be undertaken to fix humeral fractures, take out bone chips and free the nerve 6-10 weeks after the fracture. End to end anastomoses of the severed nerve should be conducted also (Pasquini et al., 2005).
- As nerve function returns exercise can recommence based upon the level of function. Electromyographic studies can be undertaken at 4 to 6-week intervals to detect nerve function and the capabilities of nerve and muscle stimulation (Stashak, 2002).
Clinical Signs & Diagnosis
Figure 4. Horse with radial nerve paralysis standing depicting the characteristic ‘dropped elbow’ of the condition (Lacroix, 2005).
The radial nerve transverses the axilla and travels into the triceps brachii muscle and passes the lateral side of the arm superficially under the lateral head of triceps brachii (Pasquini et al., 2005). It travels distally within the forelimb, caudal to the brachial artery prior to separating into the long and medial triceps brachii heads to travel down the spiral groove of the humerus and down the craniolateral aspect of the forelimb [Figure 3.] (Dyce et al., 2010). This is a frequent site of injury to the forelimb however paralysis of the radial nerve is uncommon (Pasquini et al., 2005; Stashak, 2002). The radial nerve innervates every extensor muscle of the forelimb as well as the flexor ulnaris lateralis (Dyce et al., 2010; Pasquini et al., 2005; Rooney, 1998; Stashak, 2002). Therefore, paralysis of the radial nerve results in paralysis of these muscles as well as reduced sensation upon the craniolateral aspect of the forelimb (Stashak, 2002). The radial nerve originates predominantly from the C8 and T1 nerve roots, with the C7 nerve root also playing a role in some horses (Dyce et al., 2010; Stashak, 2002).
Radial nerve Paralysis is caused by either hyperextension of the limb, trauma or great abduction of the shoulder. This is mainly the result of direct trauma from a laceration, kick or humeral fracture (Pasquini et al., 2005; Stashak, 2002). Another cause of radial paralysis is being placed in lateral recumbency for an extended period of time during surgery without sufficient padding on the shoulder and lateral side of the forelimb resulting in compression of the vascular supply. Another danger of surgery is overstretching the nerve by hyperextension of the limb during recovery (Pasquini et al., 2005; Rooney, 1998; Stashak, 2002; Wagner, 2009). Abscesses, tumours and enlarged axillary lymph nodes along the pathway of the nerve can also result in radial nerve paralysis (Stashak, 2002).
Prognosis is guarded to mild in some cases. Poor in severed nerve cases and if dysfunction is occurring after more than 2 weeks however partial or complete recovery is possible after an entrapment or compression injury (Pasquini et al., 2005; Stashak, 2002).
- For a ‘high’ radial nerve paralysis injury has occurred above the nerves entrance into the triceps. This is evident by a ‘dropped elbow’ [Figure 4]; where the limb is unable to bear weight as the horse is unable to extend the phalangeal, carpal and elbow joints and neurogenic atrophy of the extensor muscles of the limb occurs. In the standing position, the shoulder is in extension and the elbow appears to be ‘dropped’ and also extended while the carpus and digits are in flexion (Lopez et al., 1997; Pasquini et al., 2005; Rooney, 1998; Stashak, 2002).
- For a ‘lower’ radial nerve paralysis the forelimb appears longer than the opposite forelimb. The horse also displays ‘knuckling over’ involving the dragging of the foot in the case of dorsal fetlock trauma. The horse however can still bear weight on the limb however has a loss of sensation to the craniolateral forearm dorsal to the fetlock (Pasquini et al., 2005).
- Diagnosis is via the horse’s history and clinical signs of a dropped elbow or knuckling over. Palpation should be undertaken to feel for any fractures to the humerus and radiographs should be taken for a definitive diagnosis of a humeral fracture (Lopez et al., 1997; Pasquini et al., 2005; Stashak, 2002).
- Electromyography of the extensor muscles 7 days after injury should also be undertaken to determine selective denervation of these muscles. As it takes around 7 days for axons distal to the injured area to deteriorate electromyography needs to be undertaken after this time (Pasquini et al., 2005; Stashak, 2002).
- Lab analysis of muscle enzymes as well as ultrasounds can aid in a definitive diagnosis (Stashak, 2002).
(Bradley, 2013)