Page 44 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Musculoskeletal system: 1.1 A pproach to the lame horse 19

VetBooks.ir 1.41 1.42

Fig. 1.41 MRI scan of the foot in a transverse plane Fig. 1.42 MRI scan in a transverse plane showing
demonstrating an injury of the collateral ligament of a lesion in the deep digital flexor tendon (arrow).
the distal interphalangeal joint (arrow). (Photo courtesy Alex Font)

for air to +1,000 for bone, therefore a large grey scale MRI, the availability of CT as part of a lameness
of gradient is available for depiction of tissues within investigation continues to improve.
the region of interest. Cross-sectional images are
obtained as required. Depending on the type of Laboratory tests
machine, general anaesthesia may be required to Muscle damage may be indicated from the results of
allow acquisition, but new CT scanners are now measuring certain serum enzymes from a heparin-
available that allow examination of the head and ised blood or serum sample.
neck routinely in standing horses (see Fig. 4.21).
Robotic CT machines are also being developed to Creatinine phosphokinase
allow limbs (up to distal radius or tibia) to be scanned Creatinine phosphokinase (CPK) is found in skeletal
in the standing horse. As with MRI, knowledge of and cardiac muscle and in nervous tissue. It usually
normal anatomy and CT images is essential for peaks at 4–6 hours post muscle damage and is spe-
interpreting abnormalities. In general, MRI is most cific for this. Normal ranges for CPK are labora-
useful for soft tissue and CT for bone pathology. tory specific, but in the first author’s practice they
CT has shown that radiography can underestimate are 110–250 IU/l. Mild elevations are seen in horses
the degree of bone damage present. Navicular bone in training (<1,000 IU/l) and after heavy competi-
lesions imaged include prominent synovial invagi- tion such as the cross-country phase in 3-day event-
nations along the distal border, cyst formation and ing (<5,000 IU/l). A rapid return to normal is seen
significant erosion of the flexor surface. ‘Bone- in these situations. Elevations of 7,000 to 100,000+
bruising’ (maladaptive bone disease or stress-related can be seen with rhabdomyolysis. Pathological ele-
bone injury) affecting young Thoroughbred and vations are also seen in post-anaesthetic myopathies
Standardbred horses can be imaged before radio- and recumbent horses.
graphic changes are seen. Cervical spinal cord CT
scans can reveal sites of spinal cord and nerve com- Aspartate aminotransferase
pression. Examination by CT (as an adjunct to pre- Aspartate aminotransferase (AST) elevation indicates
operative surgical planning) of joints for cartilage high activity in skeletal and cardiac muscle, liver and
damage in osteoarthritis (OA) (with arthrography), RBCs. Unlike CPK, AST elevation is not specific for
subchondral bone anomalies, osseous cysts and skeletal muscle damage. It peaks at 12–24 hours after
complex fractures has proven useful to date. As with muscle damage. CPK must be measured with AST.