Osteochondromatosis

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Osteochondromatosis is a rare neurological disease of dogs[1].

The etiology of canine osteochondromas is uncertain. They may arise directly from growth plate cartilage as a result of a defect in the perichondrial ring, from physical stresses causing proliferative responses at the margin of the physis, or from some form of periosteal disturbance that induces perichondrial growth.

Pathology

The cartilage-cap portion of osteochondromas undergo endochondral ossification with subsequent replacement of much of their central mass by bone, so that eventually the cortical surfaces of the parent bone and the developing bony stalk are continuous and have confluent marrow spaces[2]. Although canine osteochondromas are a developmental, chondrodysplastic anomaly, for purposes of differential diagnosis they are also included as one of several primary skeletal tumors.

Any bone of endochondral origin may be affected; however, in decreasing frequency, vertebrae (especially spinous processes, but also body and arch), ribs, long bones, feet and pelvis are most often involved in dogs. Bones of intramembranous origin (i.e., calvarium and facial bones) are not affected in dogs. Growth of osteochondromas in dogs typically ceases at the time of skeletal maturation, although occasionally, some may progress after skeletal maturity[3]. While a familial or genetic etiology is suspected[4], there is no apparent breed or sex predisposition, although several reports involve Alaskan Malamute, and in one study, seven of the eight affected dogs had mixed Terrier breeding[5]. Osteochondromatosis is frequently a subclinical condition diagnosed as an incidental radiographic finding. However, neurological signs occasionally occur in animals associated with spinal cord compression secondary to vertebral osteochondromas in any region of the spinal column, but most commonly cervical and/or thoracic areas[6][7].

Clinical signs

Signs observed will depend on the location of the masses (e.g., cervical syndrome, cervicothoracic syndrome, and thoracolumbar syndrome). There may be variable signs of pain on palpation of the thoracic or cervical spine. Onset of neurological signs typically occurs prior to 1 year of age, although osteochondromatosis may be first diagnosed in older dogs (see also, malignant transformation, below). Osteochondromatosis may involve other tissues such as synovial joints, and tracheal rings. Concurrent skeletal and tracheal osteochondromatosis has been observed in a young Alaskan Malamute. Diagnosis may be made using survey radiography but evidence of cord compression will require myelography and/or imaging[8]. Radiographically, osteochondromas usually appear as large, smoothly contoured cystic bony masses, with irregular or well-delineated borders, sometimes with mottled patterns of radiolucency and radiodensity[9]. Fusion of vertebrae at articular facets in the presence of normal intervertebral disks, may be observed[10]. Microscopic examination of a biopsy specimen, which includes the cartilage cap and bony stalk covered by a membrane continuous with the periosteum, will confirm the diagnosis. During active growth, the cartilage resembles a physis with typical endochondral ossification present. The cartilage cap may be incomplete or absent in mature lesions. Osteochondromas may also be characterized using special imaging techniques, such as CT[11].

Treatment

Surgical excision (including removal of the perichondrial membrane on the surface of the cartilage cap), spinal cord decompression, and perhaps vertebral stabilization, are necessary in animals with clinical evidence of spinal cord attenuation. While post-operative vertebral fracture has been reported, there are several reports of successful surgical outcomes[12]. Surgical removal may be easier when osteochondromas are less well developed, at which time they are softer and poorly vascular, since within a few months, the cancellous bone becomes harder and much more vascular[13]. Recurrences may occur.

Prognosis is guarded, especially in young animals with osteochondromas involving multiple vertebral sites where subclinical masses may assume importance as they grow until the skeleton matures. Early surgical removal may eliminate development of clinical complications. Furthermore, there is evidence that osteochondromas may undergo malignant transformation to chondrosarcoma and osteosarcoma in older dogs, frequently between 7 and 10 years of age, with potential for metastasis[14]. Thus, early removal will also remove this latent threat of malignancy. Breeding of affected dogs should be discouraged because of the occurrence of osteochondromas in 2 dogs from a litter of 5 sired by a dog that also had the condition. Recently, malignant transformation of solitary spinal osteochondroma to an osteosarcoma was reported in 2 mature dogs[15].

There are sporadic reports of a focal cartilaginous lesion, termed solitary cartilaginous exostosis, resulting in spinal cord compression[16]. Young and mature, large-breed dogs (4 month old Rottweiler, 5 month old Bernese Mountain Dog, 3.5 month old St. Bernard, and a 3.5 year old Bernese Mountain dog) were affected and the mass in each dog occurred between the dorsal arch of the atlas and the spinous process of the axis. Radiographically, the masses were partially calcified, seemed to arise from the dorsal arch of the atlas or from the dorsoatlantoaxial ligaments and extended into the vertebral canal. In some cases, the dorsal arch of the atlas was irregular or thickened with erosion and shortening of the pedicles and spinous process. Histologically, the masses were composed of a fibrocartilaginous matrix, but without bone formation. Surgical removal of the mass in one dog resulted in a complete recovery.

References

  1. Vite, Ch (2004) Developmental disorders. In: Braund's Clinical Neurology in Small Animals: Localization, Diagnosis and Treatment. IVIS, Ithaca, New York, USA
  2. Pool RR. (1993) Osteochondromatosis. In: Bojrab MJ, Smeak DD, Bloomberg MS, ed. Disease mechanisms in small animal surgery. 2nd ed. Philadelphia: Lea & Febiger, pp:821-833
  3. Jacobson LS, Kirberger RM. (1996) Canine multiple cartilaginous exostoses: unusual manifestations and a review of the literature. J Am Anim Hosp Assoc 32:45-51
  4. Chester DK. (1971) Multiple cartilaginous exostoses in two generations of dogs. J Am Vet Med Assoc 159:895-897
  5. Doige CE. (1987) Multiple cartilaginous exostoses in dogs. Vet Pathol 24:276-278
  6. Beck JA, Simpson DJ, Tisdall PL. (1999) Surgical management of osteochondromatosis affecting the vertebrae and trachea in an Alaskan Malamute. Aust Vet J 77:21-23
  7. Caporn TM, Read RA. (1996) Osteochondromatosis of the cervical spine causing compressive myelopathy in a dog. J Small Anim Pract 37:133-137
  8. Silver GM, Bagley RS, Gavin PR, et al (2001) Radiographic diagnosis: cartilaginous exostoses in a dog. Vet Radiol Ultrasound 42:231-234
  9. Pool RR. (1993) Osteochondromatosis. In: Bojrab MJ, Smeak DD, Bloomberg MS, ed. Disease mechanisms in small animal surgery. 2nd ed. Philadelphia: Lea & Febiger, pp:821-833
  10. Jacobson LS, Kirberger RM. (1996) Canine multiple cartilaginous exostoses: unusual manifestations and a review of the literature. J Am Anim Hosp Assoc 32:45-51
  11. Caporn TM, Read RA. (1996) Osteochondromatosis of the cervical spine causing compressive myelopathy in a dog. J Small Anim Pract 37:133-137
  12. Ness MG. (1993) Osteochondroma causing progressive posterior paresis in a lakeland terrier puppy. Vet Rec 132:608-609
  13. Beck JA, Simpson DJ, Tisdall PL. (1999) Surgical management of osteochondromatosis affecting the vertebrae and trachea in an Alaskan Malamute. Aust Vet J 77:21-23
  14. Banks WC, Bridges CH. (1956) Multiple cartilaginous exostosis in a dog. J Am Vet Med Assoc 129:131-135
  15. Green EM, Adams WM, Steinberg H. (1999) Malignant transformation of solitary spinal osteochondroma in two mature dogs. Vet Radiol Ultrasound 40:634-637
  16. Bichsel P, Lang J, Vandevelde M, et al (1985) Solitary cartilaginous exostoses associated with spinal cord compression in three large-breed dogs. J Am Anim Hosp Assoc 21:619-622