In general, muscle disorders fall into one of four categories:
I. Those that involve muscle fasciculations – either physiologic in basis, or resulting from abnormal electrical conduction
within the muscle
1. Pain, fear
2. Electrolyte abnormalities (hypocalcemic tetany)
3. Hyperkalemic periodic paralysis
4.
Otobius Megnini ear tick infestation
5. Myotonia
II. Those that involve muscle atrophy
a. Myogenic atrophy
• Disuse
• Pituitary dysfunction
• Immune-mediated myositis
b. Neurogenic atrophy
• Equine Protozoal Myelitis
• Local nerve trauma
• Equine Motor Neuron Disease
III. Those that involve muscle necrosis (rhabdomyolysis)
A. Non-exercise associated rhabdomyolysis:
a. Inflammatory myopathies
• Clostridial myositis
• Equine Influenza and herpesvirus 1
•
Sarcocystis myositis
•
Streptococcus equi myopathy
• Infarctive purpura hemorrhagica
• Immune mediated myopathy with atrophy
b. Nutritional myopathy
• Vitamin E or selenium deficiency
c. Toxic myopathy
• Ionophore toxicity
• White snake root or rayless goldenrod
• Vitamin D simulating plants (Casea sp.)
• Pasture Myopothy
d. Traumatic myopathy
• Anesthetic myopathy
• Trauma
• Compartment syndrome
• Capture myopathy
e. Metabolic myopathy
• Glycogen branching enzyme deficiency (Quarter Horses)
• Polysaccharide storage myopathy
• Malignant hyperthermia
B. Exertional Rhabdomyolysis (increased CK, associated with exercise)
a. Sporadic Tying-up (historically first episode, normal AST)
• Dietary imbalances in vitamins, minerals, electrolytes
• Excess carbohydrate
• Low vitamin E/Se
• EHV-1
• Overexertion
• Sustained endurance exercise
b. Chronic Tying-up
• Polysaccharide storage myopathy (genetic test/biopsy diagnosis)
• Recurrent exertional rhabdomyolysis (biopsy diagnosis)
• Idiopathic chronic exertional rhabdomyolsis
IV. Those that involve muscle dysfunction with normal CK
a. Mitochondrial myopathy (persistent lactic acidosis with light exercise)
b. Hyperkalemic periodic paralysis
