Musculoskeletal - Diagnostics

Question 1

creatinine kinase

Answer 1

enzyme present in cytoplasm of skeletal and cardiac muscle
increased levels in the blood indicate muscle injury
short half-life (~ 2hours) so single insult will cause very rapid increase and rapid decline, persistent elevation indicates ongoing muscle injury
magnitude of increase is proportional to the degree of msucle damage

Question 2

analytical concepts with CK

Answer 2

CK activity 2.5 times higher in serum samples compared to plasma
puppies and young dogs have higher CK than adult dogs
CK activity stable for 1 week refrigerated, 1 month frozen
in vitro haemolysis causes falsely increased CK activity

Question 3

interpreting CK results

Answer 3

mild increase in CK = minor muscle damage e.g. IM injections, exercised animals, prolonged recumbency / anaesthesia in horses and cattle
moderate to severe increase in CK = severe myonecrosis
very severe increase - = rhabdomyolysis (snake bite)

Question 4

aspartate transaminase (AST)

Answer 4

not specific for muscle damage - also expressed by hepatocytes
stable 1 day at room temp
in vitro haemolysis causes falsely increased AST activity

Question 5

interpreting AST and CK together

Answer 5

increased CK and normal AST = myonecrosis recently occurred
increased both = myonecrosis continues to be active
increased AST and normal CK = myonecrosis no longer active

Question 6

alanine aminotransferase (ALT)

Answer 6

not specific to musscle damage - also expressed by hepatocytes
increased activity due to skeletal muscle damage, but mild increased relative to CK changes
AST & CK are better indicators

Question 7

lactate dehydrogenase (LDH)

Answer 7

increased due to muscle damage (mostly skeletal, rarely cardiac)
not organ-specific = not particularly useful on its own

Question 8

myoglobin

Answer 8

heme protein that stores and transports oxygen to myofibers
myoglobinuria is associated with acute myopathies caused by trauma, excessive exertion or exertional rhabdomyolysis
if not completely reabsorbed by the renal tubules, myoglobin is excreted in the urine
renal tubular necrosis -> acute renal failure
(overwhelming level of myoglobin to reabsorb damages epithelial cells of renal tubules)

Question 9

synovial fluid evaluation

Answer 9

colour - normally lightly yellow, red = haemorrhage, yellow/orange = previous haemorrhage or haemoglobin breakdown, very yellow = inflammatory

transparency - should be clear, increased turbidity is caused by suspended particulates e.g. RBCs, WBCs, fibrin)

protein

viscosity - should be very high, will be reduced with disease

nucleated cell count - increased RBCs for hemarthrosis, 1000-10,000 for degenerative arthropathy, 5000-100,000 for inflammatory arthropathy

differential cell count and cytology - erythrophagia for haemorrhage, >90% mononuclear cells for degenerative, >10 - 100% neutrophils for inflammatory

should not clot - does tend to form a gel, inflamed joints may form fibrin precipitates/clots (clotting occurs if there is significant blood contamination)

Question 10

describe the synovial fluid analysis for a non-inflammatory arthritis sample - DJD, joint trauma, hemarthrosis

Answer 10

colour normal or blood-tinged if haemorrhage present
viscosity normal to decreased
mucin clot test good to poor
protein unchanged to slightly increased
total nucleated cell count normal to moderately increased (<5000cells/uL)
cytology = mononuclear cells predominant (usually macrophages), neutrophils <10%, RBCs and erythrophagia if haemorrhage present

Question 11

describe the synovial fluid analysis for an inflammatory arthritis sample

Answer 11

colour yellow to cream
transparency cloudy/opaque
viscosity markedly decreased
mucin clot test poor
protein consistently increased due to increased synovial permeability
total nucleated cell count markedly increased (almost always >5000, may exceed 100,000)
cytology = neutrophils predominant, concurrent haemorrhage may be seen

Question 12

lymphocytic synovitis

Answer 12

synovial hyperplasia with nodular aggregates of lymphoid cells
reported after cruciate rupture
usually in fetlock joints in horses, associated with nodule formation on the dorsal aspect of the joint
associated with trauma and mainly occurs in young racehorses

total nucleated cell count 5-20,000/uL, dominated by small lymphocytes
(increased but not as high as inflammatory synovitis)