Myopathy:

• Myopathy denotes non-inflammatory degeneration of skeletal muscle characterized clinically by muscle weakness and pathologically by hyaline degeneration of muscle fibres.
• Serum activities of some muscle enzymes are elevated and myoglobinuria is common accompainment.

Etiology:

• Most important myopathies in farm animals is due to nutritional deficiency of vitamin E and selenium, and effects of unusual exercise.
• Skeletal myopathies can be classified into two; primary and secondary myopathies.
• Enzootic nutritional muscular dystrophy:
  • Deficiency of vitamin E and/or selenium
  • Usually occurs in young calves, lambs, foals and piglets.
  • Factors enhancing disease includes rapid growth, highly unsaturated fatty acid in diet and unusual exercise.
• Exertional or post-exercise rhabdomyolysis:
  • Occurs as tying up syndrome (Azoturia or equine paralytic myoglobinuria) after unusual exercise or insufficient training
  • It also occurs in cattle after running wildly for several minutes, in sheep chased by dogs and in wildlife during capture of wild animals.
• Equine atypical myopathy:
  • Hypoglycin A, found in seeds of maple trees.
  • Affected horse has sudden onset of clinical signs
  • It was originally known as atypical myoglobinuria but was renamed to atypical myopathy.
• Equine polysaccharide storage myopathy:
  • It is metabolic disease recognized in many breed of horse; quarter horse, appaloosa, paint-related horse
  • Mutation in glycogen synthase-I (GYS I) gene affects carbohydrate metabolism
• Congenital or inherited myopathies:
  • Porcine stress syndrome in some breed of pig
  • Congenital myopathy in Brown-Swiss calves
  • Doubling-muscling in cattle
  • Splay legs of newborn pigs
  • Xanthosis in skeletal muscles and cardiac muscle of cattle
• Toxic agents:
  • Poisonous plants; Cassia occidentalis, Karwinskia humboldtiana, Ixioloena sps, Geigenia spp
  • Solanum malacoxylon, Tricetum spp, Cetrum spp causes enzootic calcinosis of all tissue
• Ischemia:
  • Ischemic myonecrosis occurs in thigh muscle of cattle, recumbent for approximately 48 hours
  • Iliac thrombosis is important cause of ischemic myopathy in horse
• Neurogenic:
  • Traumatic injury to nerve or subsequent degeneration or complete severance of nerve supply to skeletal muscle
  • Suprascapular nerve paralysis (sweeny) in horse results in traumatic neuropathy
• Neoplasm:
  • Rhabdomyosarcomas are reported in horse
  • Neoplasm of striated muscle are uncommon in animals.

Clinical Findings:

Primary myopathy:

• Sudden onset of weakness
• Pseudoparalysis of affected muscle
• Respiratory and circulatory insufficiency
• Animals are usually bright and alert and appear to be in pain
• Temperature are usually normal but may be elevated slightly in severe case
• Tachycardia, myoglobinuria in adult horse, yearling cattle
• Affected muscle in acute cases may feel swollen, hard and rubbery
• Animals with acute case may die within 24 hours after onset of signs

Acute nutritional myopathy:

• Occurs most commonly in foals from birth to 7 months of age
• Muscle stiffness and pain
• Myoglobinuria, edema of head and neck
• Recumbency, death within few days

Myopathy associated with excessive exercise or running at pasture:

• Dejection, stiffness
• Disinclination to move
• Stiffness of gait
• Pronounced swelling and firmness of subcutaneous tissue at base of mane and over gluteal muscle
• Excessive salivation
• Desquamation of lingual epithelium
• Board-like firmness of masseter muscle
• Foals are unable to suck milk because of inability to bend their necks

Tying up:

• Sudden onset of muscle soreness; 10-20 minutes following exercise
• Profuse sweating
• Degree of soreness varies from mild to severe
• In mild case; horse move with short, shuffling gait
• In acute case; great disinclination to move at all
• In severe case; horse unable to move their hindlegs, swelling and rigidity of croup muscle
• Myoglobinuria is common.

Post-anesthetic myositis:

• Horse may experience difficulty during recovery from anesthesia
• Prolonged recovery, when initial attempts made to stand, there is lumbar rigidity, pain, reluctance to bear weight
• Limbs are rigid, and muscles are firm on palpation
• In severe cases, temperature rises
• Other signs include; anxiety, tachycardia, profuse sweating, myoglobinuria, tachypnea
• Death may occur in 6-12 hours

Exertional rhabdomyolysis:

• Recurrent stiff gait after exercise in young horse
• Short-stepping gait and poor performance
• Horse are reluctant to move when placed in stall, apphrensive and anorexia
• Frequent shifting of weight
• Hard and painful muscle, profuse sweating, refuse to walk, tachycardia and tachypnea in severely affected horse
• Consistent abdominal pain
• Deep-red or coffee-coloured urine

Equine polysaccharide storage myopathy:

• Pain, weakness
• Segmental fibre necrosis
• Stiffness, spasm, atrophy of muscle
• Most commonly affected muscle are powerful rump, thigh and back muscles, including gluteals, semimembranosus, semitendinosus and longissimus.

Exertional rhabdomyolysis:

• In sheep chased by dogs, affected animals are recumbent, cannot stand and appear exhausted.
• Myoglobinuria is common
• Death usually follows

Hyperkalemic periodic paralysis:

• Initially, there is brief period of myotonia with prolapse of third eyelid
• In severe case, horses become recumbent
• Myotonia is replaced by flaccidity
• Generalized muscle fasciculations with large group of muscle fibre contracting simultaneously at random
• Animals remain bright and alert and responds to noise and painful stimuli
• In mild case; affected horse remain standing and generalized muscle fasciculations are prominent over neck, shoulder and flank
• Tendency to stand base-wide
• Horse are unable to lift its head, usually will not eat and may yawn repeatedly early in course of disease.

Secondary myopathy from ischemia:

• Affected animals are unable to rise.
• Affected hindlegs are directed behind cow in frog-leg attitude.
• Appetite and mental attitude are usually normal.
• In calves with aortic and iliac artery thrombosis, there is acute onset of paresis or flaccid paralysis of one or both pelvic limb.
• Affected limbs are hypothermic and have diminished spinal reflexes and arterial pulse pressure.

Neurogenic atrophy:

• Marked loss of total muscle mass
• Flaccid paralysis
• Loss of tendon reflexes
• Failure of regeneration
• Animal are unable to bear weight on affected leg, if large muscle mass is affected.

Dystrophy of diaphragmatic muscle:

• Loss of appetite
• Decreased rumination, eructation
• Recurrent bloat
• Increased respiratory rate with forced abdominal respiration, forced movement of nostril
• Death from asphyxia in few weeks

Severe diaphragmatic necrosis:

• Affected horse may have severe respiratory distress and respiratory acidosis
• Animal donot respond to supportive therapy

Diagnosis:

• Physical examination of affected muscles
• Clinical pathology:
  • Increased level of serum CK
  • Increased level of AST. It may remain elevated for several days following acute myopathy due to longer half-life
  • CK has short half-life of about 4-6 hours, so enzyme level may return to normal within 3-4 days, if no further degeneration
  • Marked drop in CK activity and slow decline in serum AST suggest that no further degeneration is occurring whereas constant elevation of CK suggest active degeneration.
  • Serum CK values will increase from <100 IU/L to 1000-5000 IU/L or higher
  • In calves, it will increase from 50 IU/L to 5000 IU/L.
• Muscle biopsy: Open biopsy is recommended to obtain strips of muscle. Semimembranosus, semitendinosus muscle at site between base of tail and tuber ischium provides adequate sample.
• Myoglobinuria: Level > 40mg/dl of urine produces dark coffee colored urine. Urine also test positive for presence of protein
• Electromyography: For evaluation of degree of neurogenic atrophy
• Necropsy Findings:
  • Affected areas of skeletal muscle have white, waxy, swollen appearance like fish flesh
  • Hyaline degeneration to severe myonecrosis histologically
  • Calcification of affected tissue

Treatment:

• Vitamin E and selenium supplementation should be made.
• In acidosis, sodium bicarbonate should be administered.
• Animal should be provided rest and thick bedding with hay straws, removal of solid floors to softer ground, frequent turning from side to side
• Animal should be provided fluid therapy to prevent myoglobinuric nephrosis
• Analgesic drugs are recommended in case of pain.
• Dantrolene sodium at 4 mg/kg body weight (BW) given orally immediately upon recognition of clinical signs is efficacious.
• Large quantities of intravenous polyionic balanced electrolyte fluids (50 to 100 L) must be given over a 24-hour period.
• Prevention of porcine stress syndrome will depend on careful handling and transportation techniques combined with genetic selection of resistant pigs.
• The prevention of exertional myopathy in the horse depends on a progressive training program and avoidance of sudden unaccustomed exercise in animals that are in good body condition and have been inactive.