Skeletal Muscle

Question 1

Changes in bloodwork with muscle damage

Answer 1

Elevations in:
Creatinine kinase (CK)
Lactate dehydrogenase (LDH)
Aspartate aminotransferase (AST)
Alanine aminotransferase (ALT)

Question 2

Changes in urine with muscle damage

Answer 2

Myoglobinuria (myoglobin released into bloodstream + filtered by kidney into urine)

Question 3

Muscle atrophy

Answer 3

Loss of myofilaments (actin + myosin) —> decrease in myofiber size

Question 4

Causes of muscle atrophy

Answer 4

Denervation
Disuse
Malnutrition

Question 5

Histology of denervation atrophy

Answer 5

Patchwork myofiber atrophy (due to motor unit organization)

Question 6

Causes of denervation atrophy

Answer 6

Wallerian degeneration (most common) - secondary to trauma to peripheral nerve

(Rarely) Axonal degeneration or demyelination

Question 7

Example of denervation muscle atrophy

Answer 7

Laryngeal hemiplegia

Question 8

Reinnervation

Answer 8

Schwann cells proliferate at motor end plate of denervated fiber

Reinnervatioin by collateral sprouting or axon regrowth —> hypertrophy (regeneration of muscle)

Question 9

Histology of disuse muscle atrophy

Answer 9

Diffuse myofiber atrophy

Cells uniformly decreased in size (due to low mechanical load)

Question 10

Causes of hypertrophy

Answer 10

Exercise conditioning

Compensatory hypertrophy (compensation for loss of other myofibers)

Question 11

Histology of hypertrophy

Answer 11

Diffuse enlargement of muscle cells

Question 12

Histology of muscle degeneration

Answer 12

Multifocal (or regional) random pattern
Hydropic swelling, fatty change

Question 13

Causes of muscle degeneration

Answer 13

Peracute/acute nutritional/toxic causes (i.e. selenium deficiency)

Question 14

Histology of muscle necrosis (myonecrosis, rhabdomyolysis)

Answer 14

Segmental (portion of myfiber)
Loss of striations + hyper eosinophilic cytoplasm
Within 24-48 h —> macrophages/neutrophils infiltrate

Question 15

Sequelae of necrosis

Answer 15

Regeneration (if basal lamina intact)

Fibrosis (if basal lamina destroyed)

Question 16

Process of muscle regeneration

Answer 16

Removal of debris by macrophages —> sealing off injured areas —> satellite cell proliferation in basal lamina —> fusion of satellite cells into myofiber

Question 17

Histo/gross appearance of muscle mineralization

Answer 17

Chalky white areas of gross specimen

Blue-purple granular material on histo

Question 18

Types of mineralization

Answer 18

Dystrophic (necrosis, primary myopathy)

Metastatic (hypercalcemia)

Question 19

Myositis

Answer 19

Inflammation of muscle

Follows necrosis

Associated with trauma, infection, infarction, immune-mediated

Inflammatory infiltrate gives clues to cause

Question 20

X-linked muscular dystrophy

Answer 20

Mutation of dystrophin gene in young male dogs/cats

Dystrophin helps anchor muscle cell membranes to ECM —> integrity of muscle cell membrane affected

Polyphasic muscle necrosis —> macrophage infiltration / mineralization —> fibrosis

**Golden retrievers, Rottweilers

Question 21

Clinical presentation of muscular dystrophy

Answer 21

Diffuse muscle atrophy
Splaying of limbs
Weakness

Question 22

Equine Exertional Rhabdomyolysis

Answer 22

Aka: chronic intermittent rhabdomyolysis, exertional myopathy, Monday morning disease, tying up

Acute myofiber necrosis triggered by exercise

Potential cause: metabolic abnormality leading to oxidative damage

Monophonic or polyphasic

Question 23

Clinical pathology of exertional rhabdomyolysis

Answer 23

Elevation of CK / AST
Myoglobinuria
(May lead to acute renal failure)

Question 24

Clinical signs of exertional rhabdomyolysis

Answer 24

Sudden onset of stiff gait/reluctance to move/pain

+/- swelling of affected muscles (gluteal, femoral, lumbar groups)

Question 25

Equine polysaccharide storage myopathy

Answer 25

Carbohydrate metabolism defect Abnormal aggregates of polysaccharide, variable degress of myofiber necrosis Related to equine exertional rhabdomyolysis

Question 26

Masticatory myositis

Answer 26

Loss of any of muscles of mastication (Masseter, temporal, Pterygoids) Multifocal, polyphasic necrosis with inflammation (eosinophilic, + lymphocytic) AUTO-IMMUNE —> autoantibodies to Type 2M myosin isotope and myositigen (myosin-binding protein) Young large breed dogs predisposed (G. Shepard, Rott)

Question 27

Polymyositis

Answer 27

AUTO-IMMUNE - but don’t know protein target of antibodies Multifocal, polyphasic necrosis with inflammation Lymphocytic inflammation Variable clinical signs by muscle affected (weakness, stiff gate, muscle atrophy, regurgitation, etc) Adult dogs - Boxers, Newfies

Question 28

Extraocular muscle myositis

Answer 28

Myositis of extraocular rectus and oblique muscles RARE, not painful Multifocal polyphasic necrosis with lymphocytic inflammation Young female dogs (Goldens) Immune mediated

Question 29

Immune mediated myositis of horses

Answer 29

Associated with Strep equi —> immune complexes of antibodies to strep M protein Hemorrhagic necrosis of muscle due to vascular injury Increase in CK/AST, muscle weakness, pain Vasculitis + fibrinoid necrosis of vascular walls, hemorrhage/infarcts in muscle; inflamm infiltration not seen in muscle

Question 30

Infectious causes of muscle disease

Answer 30

Clostridium myositis —> black leg, pseudo-black leg Leishmania

Question 31

Nutritional myopathies

Answer 31

Selenium / Vit E deficiency

Question 32

Toxic myopathies

Answer 32

Ionophores Plants

Question 33

Vascular myopathies

Answer 33

Vascular occlusive syndrome Postanesthetic myopathy in horses Downer syndrome (LA) Compartment syndrome

Question 34

Neoplasias

Answer 34

Rhabdomyoma/rhabdomyosarcoma Infiltrative lipoma Metastatic neoplasms (lymphosarcoma, hemangiosarcoma, malignant melanoma, carcinomas, sarcomas)

Question 35

Rhabdomyoma

Answer 35

Benign tumor Most common in larynx of young dogs —> respiratory distress (difficult to remove, need wide margins)

Question 36

Rhabdomyosarcoma

Answer 36

Malignant, locally invasive; Rare Histologic subtypes: Embryonal, botryoid, alveolar (young dogs) Pleomorphic (adult dogs)

Question 37

Common sites of rhabdomyosarcoma

Answer 37

Urinary bladder, face skill, tongue

Question 38

Infiltrative lipoma

Answer 38

Benign (doesn’t metastasize), but infiltrates muscle Wide excision is curative but difficult