Myopathies Flashcards
Types of muscle fibre
Type 1 : slow twitch —> relatively high levels of oxidative enzymes and lipid, low glycogen content
Type 2 : fast twitch —> high level of glycogen, less amount of oxidative enzymes and lipid.
Muscle dystrophy (XLMD)
Group of inherited myopathies with specific defect in the skeletal muscle protein
Most common form is associated with the proteins dystrophin (important structural role for myofibers) —> X-linked inherited defect (male affected, female carriers)
Presentation
— Dogs : progressive atrophy (> trucal and temporalis), often inefficient suckers, bunny-hopping gait and partial trismus apparent by 6 month of age, plantigrade stance, possible excessive drooling, esophageal dysfunction, lumbar kyphosis (progresses to lordosis). Spinal reflexes are normal initially.
— Cats : hypertrophy. Stiff gait
Diagnosis
- CK elevation, marked CK elevation (often >10 000) within the first few weeks of life is often a hallmark of the disease. Females carrier often show elevation in their serum CK without significant clinical disease.
- EMG : high frequency discharges
- Biopsy results : myofiber degeneration/regeneration with out without mineralization
- Immunohistochemistry demonstrated dystrophin deficiency
Treatement : none, minor improvement noted with glucocorticoid, do not survive past 6 months.
Other causes of degenerative myopathy
Myotonia congenita : inherited autosomal recessive trait in ChowChow and Schnauzers. Sustained muscle contraction after cessation of voluntary movement due to abnormal sodium conductance. Clinical signs are usually appreciated when affected puppies and kittens begin to ambulate. Worsen with rest. Generalized muscle hypertrophy, pelvic limbs are more severely affected than the thoracic limbs.
Fibrotic myopathy (gracilis/semitendinous myopathy) : predominantly in adult male German Shepherd. Replacement of muscle tissue with dense fibrous connective tissue.
Crycopahryngeal achalsia : failure to relax the cricopharyngeus muscle during the oropharyngeal phase of swallowing.
Myokymia and neuromyotonia : intermittent and excessive muscle contractions. Hyperexcitability is thought to be due to abnormalities of voltage fated potassium channels, which is often due to an auto-immune process or heritable disorder.
Labrador retriever myopathy
Autosomal recessive patter of inheritance
Abnormal resting potential affecting type 2 muscle fiber.
Clinical signs : variable age of onset (usually 3-4 months), short strides gait, bunny hopping, ventroflexion and abnormal joint posture (carpal hyperextension and valves), splaying of the digits. Reduced to absent tendon reflex, possible megaesophagus.
Hypokaliemia myopathy
Cause : low extracellular potassium concentration (<3.5 ), more common in cat. Inherited disorder in Burmese cats. Hypokaliemia result in hyperpolarization of the resting membrane potential in the muscle cell. Muscle cells therefore become refractory to depolarization which prevents muscle contraction.
Clinical signs :
- ventroflexion of the neck
- myalgia
- LMN tetraparesis
- Normal cranial nerve examination.
- Muscle atrophy,
- Rhabdomyolysis and respiratory paralysis in severe cases.
Other causes of metabolic myopathies
HYPP - reported in one dog only, flaccid weakness that lead to transient paralysis. Due to excessive potassium release in the myofiber.
Hyperadrenocorticoid myopathy : increased in circulation of glucocorticoid levels, may interfere with mitochondrial function and possible decrease Na, K ATPase concentration. Affect predominantly type II myofibers.
Hypothyroid myopathy : pathogenesis is unknown, type II muscle atrophy predominates on biopsy. Usually associated with non specific signs of weakness.
Malignant hyperthermia : autosomal dominant, abnormal calcium release, often trigger by stress or anesthetic event (volatil anesthesia agent).
Exertional myopathy (exertional rhabdomyolysis) : more common in racing Greyhound, Pathophysiology unknown. Exercice will induce muscle swelling and necrosis. Can develop myoblobinuria.
Other : lipid storage mitochondrial myopathy and glycogen myopathy.
Masticatory myosistis
Auto-immune disease with antibody formation against muscle of mastication (temporalis, masseter and pterygoid muscles) which contains IIM fibers. Multiple dog breed can be affected and both sexes are affected.
Clinical signs :
- Usually acute onset - pain and swelling of the masticatory muscles, pain upon opening of the jaw, trismus. Eventually will lead to atrophy of the masticatory muscles and fibrosis. Exophtalmos and fever may be present.
Other causes of inflammatory myosistis
Auto-immune polymyositis : Affect predominantly the appendicular muscles. Clinical signs may be acute or chronic and can include generalized weakness that is often worsen with exercice, hyperesthesia upon muscle palpation, regurgitation, dysphagie, depression, fever, muscle swelling in the acute phase, muscle atrophy in the chronic phase, shifting leg lameness and voice change.
Dermatomyositis : best described in colllie, inflammatory disorder of the skin and lesser extend muscles, believed to be auto-immune and heritable as an autosomal dominant trait. Skin lesions predominated on the nose, lips and tip of the ears. Clinical signs of myositis are typically mild or innaparent and develop after the skin lesions are noted.
Feline hyperesthesia syndrome : inclusion body myositis (degenerative myopathy) with secondary immune-mediated inflammatory response. Clinical signs include rippling of the skin over the dorsum, muscle spasm in the thoracolumbar epaxial region, violent licking and biting at the back, agitated demeanor etc.
Tetanus
Dogs and cats are generally resistant to tetanus — > no profilactic vaccination.
Resulting effects of the toxin production : interferes with release of inhibitory neurotransmitters from inhibitory interneurons in spinal cord and brain —> induce contraction of the skeletal muscle.
Will induce autonomic effects such as tachycardia/bradycardia, arrythmias and hyper/hypertension.
Clinical signs : progressive muscle weakness —> localized and generalized.
- Lock jaw, rises sardonicus, narrowed palpebral fissure, whrinkled forehead, muscle spasm, worsen with stimuli. Possible hyperthermia due to muscle spasm.
Traitement : penicillin G and metronidazole
Administration of antitoxin is contreversial due to risk of anaphylaxis.
Administration of muscle relaxant such as methocarbamol
