JPC SYSTEMIC PATHOLOGY
CARDIOVASCULAR SYSTEM
February 2022
M-M01 (NP)

Signalment (JPC #2314368):  3-day-old foal 

 

History:  This foal died of unknown causes.  This section is from a 2 cm raised nodule on the shaft of the radius.  Similar nodules were present on the ribs and other long bones.

 

HISTOPATHOLOGIC Description:  Bone, radius (per contributor):  Elevating the periosteum, extending from the periosteum into the underlying more normal cancellous bone, are multiple 1 to 5 mm diameter nodules composed of an outer rim of hyaline cartilage undergoing variably organized endochondral ossification with thin zones of chondrocyte proliferation, hypertrophy, and calcification with formation of primary and secondary spongiosa progressing into bony trabeculae that blend into the more normal underlying cancellous bone.  The underlying cancellous bone trabeculae contain variably sized islands of cartilage (retained cartilage cores) and are moderately thickened.  The marrow cavities contain sparse or absent hematopoietic cells. 

 

Morphologic Diagnosis:  Bone, radius:  Osteochondromas, multiple (cartilaginous exostosis), breed unspecified, equine.

 

Condition:  Multiple cartilaginous exostoses (MCE)

 

SYNONYMS:  Osteochondroma (if single), osteochondromatosis (if multiple), hereditary multiple exostoses, osteocartilaginous exostoses, diaphyseal aclasis, metaphyseal aclasis, dyschondroplasia, hereditary deforming chondrodysplasia

 

General DISCUSSION:

• Benign, cartilage-capped tumor-like exostosis arising from the metaphysis of any bone that undergoes endochondral ossification
• May be single (monostotic) or multifocal (polyostotic)

 

Pathogenesis:

• In dogs and horses
  • Polyostotic form is inherited, autosomal dominant disorder
  • Genetic defect is unknown in animals; in humans, the EXT1 and 2 genes has been associated with multiple exostoses due to decreased heparan sulfate and excessive chondrogenesis at the perichondrial ring
  • Tend to be bilaterally symmetrical, and grow corresponding to period active bone growth
  • Malignant transformation to chondrosarcoma or osteosarcoma may occur in dogs; not reported in horses
• In cats
  • Different condition; it occurs primarily in mature animals within flat bones that develop from intramembranous ossification; behaves more like an actual neoplasm
  • Cause is undetermined, but feline leukemia virus and transmissible feline sarcoma implicated
  • Undergo malignant transformation

 

TYPICAL Clinical FINDINGS:

• Occurs in young animals (dogs, horses) and humans, and mature cats
  • Dogs: Arise in young animals and continue to grow until the skeleton matures, bilaterally symmetrical
  • Horses: Same as dog, lesions are usually present at birth; caudal distal radius is the most common monostotic site in Thoroughbreds
  • Cats: usually occurs in mature animals
• Often animals are asymptomatic or presented for cosmetic reasons
• Clinical signs often follow impingement of the lesion on adjacent tissue
  • Impinge nerves, tendons, muscles, or blood vessels
  • Vertebral exostoses may protrude into the spinal canal and compress the spinal cord, leading to neurologic signs
• Radiographically, the majority of lesions appear as cancellous, bony nodules with radiolucent cartilaginous caps

 

TYPICAL Gross FINDINGS:

• May be solitary or multiple
• In dogs and horses
  • Typically occur at metaphyses of long bones, the pelvis, ribs, scapula, and vertebrae
  • Affected bone may be bowed or attenuated
• In cats
  • Tends to involve flat bones

 

TYPICAL LIGHT Microscopic FINDINGS:

• In dogs and horses
  • During expansion, consist of an outer cap of hyaline cartilage resembling a poorly organized growth plate which undergoes endochondral ossification from its deep surface
  • Trabecular bone and bone marrow within the mass are continuous with the marrow cavity of the parent bone
  • Once growth ceases, the cartilage cap is replaced by bone
  • Trabecular bone is not remodeled, and mineralized cartilage cores persist
• In cats
  • Multifocal areas of progressively enlarging areas of osteocartilaginous hyperplasia
  • Not continuous with marrow cavity

 

Differential Diagnosis:

• Osteosarcoma or chondrosarcoma: Malignant tumors with anaplastic cells
• Ecchondroma with extensive endochondral ossification: Absence of cartilaginous cap and medullary communication with marrow cavity

 

COMPARATIVE PATHOLOGY:

• Dogs:
  • Younger than 6 months; often with lameness or weakness caused by spinal cord impingement
  • Older dogs rarely have pulmonary metastases of malignantly transformed lesions
• White-tailed deer: Cranial osteochondromas have been reported
• Koalas: have craniofacial osteochondromas that often extend into the nasal and paranasal sinuses; they do not appear to be malignant but can be quite expansile
• Pigs: Case report of osteochondromatosis

 

 

REFERENCES:

1. Craig LE, Dittmer KE, Thompson KG. Bones and joints. In: Maxie MG, ed. Jubb, Kennedy and Palmer’s Pathology of Domestic Animals. Vol 1, 6th ed. St. Louis, MO: Elsevier; 2016:54, 116-117.
2. Olson EJ and Carlson CS. Bones, Joints, Tendons, and Ligaments. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6^th, St. Louis, MO: Elsevier; 2017:989-990.
3. Higgins D, Rose K, Spratt D. Monotremes and Marsupials. In: Terio KA, McAloose D, St. Leger JA, eds.  Pathology of Wildlife and Zoo Animals. San Diego, CA: Academic Press. 2018: 463. 

 

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