History
A 5-year-old Morgan gelding used for western pleasure riding was referred to the Colorado State University Veterinary Teaching Hospital for evaluation of right forelimb lameness. There was no known history of trauma, and the lameness had persisted without change for approximately 8 weeks.
Physical examination performed at the time of referral did not reveal any palpable abnormalities or responses to the hoof testers. The horse had a grade 1/5 lameness in the right forelimb and hind limb when examined while trotting on asphalt. The horse responded mildly to full-limb flexion of the right hind limb and flexion of the right metacarpophalangeal joint. Results of all other manipulative tests were considered to be within normal limits.
Because of the nebulous nature of the lameness, minimal physical examination findings, and multiple limb involvement, nuclear scintigraphy of all 4 limbs was performed (Figure 1). Radiographic evaluation of areas of increased radiopharmaceutical uptake was also performed (Figure 2).
Lateral scintigraphic views of the distal portions of the right radius (A) and left tibia (B) of a 5-year-old Morgan gelding with an 8-week history of right forelimb lameness.
Citation: Journal of the American Veterinary Medical Association 231, 5; 10.2460/javma.231.5.695
Lateral and craniocaudal radiographic views of the right radius (A and B, respectively) and left tibia (C and D, respectively) of the horse in Figure 1.
Citation: Journal of the American Veterinary Medical Association 231, 5; 10.2460/javma.231.5.695
Determine whether additional imaging studies are required, or make your diagnosis from Figures 1 and 2—then turn the page →
Diagnostic Imaging Findings and Interpretation
Nuclear scintigraphy revealed intense, symmetric radioisotope uptake within the distal diaphysis of the right radius and left tibia. Similar radioisotope uptake was also evident in the contralateral radius and tibia. Mildly increased uptake was also seen in the distal tarsal region bilaterally.
Radiography revealed areas of lucency within the medullary cavity of the distal portion of the diaphyses of the right radius and left tibia (Figure 3). Similar lucencies are visible on the contralateral radius and tibia. Increased bone opacity (endosteal proliferation) and sclerosis surrounding the medullary lucencies are evident in both radii and tibiae and periosteal new bone is evident on the caudal portion of the right radius. Differential diagnoses included primary or metastatic neoplasia, multifocal fungal or bacterial osteomyelitis or bone abscess formation, and unicameral or aneurismal bone cysts.
Same radiographic views as Figure 2. Notice areas of medullary lucency surrounded by areas of increased bone opacity (white arrows) that are evident in the distal portion of the diaphyses of the right radius and left tibia. Caudally, an area of periosteal proliferation is evident on the lateral radiographic view of the right radius (small arrowhead). Additionally, endosteal proliferation can be seen on the lateral view of the right radius and the craniocaudal view of the left tarsus (large arrowheads).
Citation: Journal of the American Veterinary Medical Association 231, 5; 10.2460/javma.231.5.695
Comments
A CBC, serum biochemical analyses, and plasma protein electrophoresis did not reveal any abnormalities. The lesion within the right radius was confirmed with fluoroscopy and a biopsy specimen was obtained, by use of a Jamshidi bone biopsy needle, after the horse was anesthetized. Fungal culturing of the specimen failed to yield any growth. Bacteriologic culturing yielded a mild growth of Clostridium sordelli, but it was considered to be a contaminant. Histologic examination did not reveal any evidence of neoplasia. Multiple sections contained fewer than normal bone marrow elements within the intratrabecular spaces and abundant fibrous tissue. The diagnosis was nonneoplastic myelofibrosis of the medullary cavity. The lack of compromised bone marrow function in this horse was attributed to the localized nature of the lesions. No other diagnostic tests were performed.
No treatment was given for the myelofibrosis. The horse was discharged with instructions that it be confined for 30 days and given phenylbutazone (2.2 mg/kg [1 mg/lb], PO) once daily for 6 days. One month after discharge, radiography of the right radius revealed minimal change in the lesion. The horse was rested for an additional 2 to 3 months and then started back into work. The lameness had resolved, and the owner did not return for repeat radiography for 2 years. At that time, the horse had become mildly lame in the right hind limb. The referring veterinarian diagnosed mild osteoarthritis of the distal portion of the tarsus on the basis of clinical and radiographic evaluations. The most distal aspects of the medullary lucencies within both tibias could still be identified on the radiographic views and did not appear to have changed from the original lesions. The horse was treated by the referring veterinarian with intra-articular injection of medication within the distal tarsal joints. The lameness subsequently resolved.
To our knowledge, myelofibrosis has been documented as a recognized disease process in 1 horse but has not been previously associated with bone destruction. In companion animal species, myelofibrosis is associated with anemia, often with complications of variable leukemias and is most commonly referred to as a type of myeloid neoplasia.1 Diagnosis must be supported by histologic analysis of bone marrow specimens; however, these can be difficult to obtain because of an increase in connective tissue deposits within the marrow, as well as marrow necrosis.
One case of myelofibrosis has been reported in a pony.2 The pony developed colic and anemia, and a diagnosis of myelophthisic pancytopenia was made on the basis of pancytopenia identified on a CBC in conjunction with hypocellularity of multiple successive bone marrow aspirates. No radiographic findings were reported in that case.
