DEFINITION/OVERVIEW

• A nonneoplastic, noninflammatory proliferative disease of the bones of the head
  
• Primary bones affected: mandibular rami; occipital and parietal; tympanic bullae; zygomatic portion of the temporal
  
• Bilateral symmetric involvement most common
  
• Affects musculoskeletal system

ETIOLOGY/PATHOPHYSIOLOGY

• Believed to be hereditary: occurs in certain breeds and families
  
• West Highland white terriers: autosomal recessive trait
  
• Scottish terriers: possible predisposition
  
• Possible link to infection: pyrexia; histological evidence of inflammation only at the periphery of the lesion
  
• Young terrier with periosteal long bone disease: monitor for disease

SIGNALMENT/HISTORY

• Scottish, cairn, and West Highland white terrier breeds: most common
  
• Labrador retrievers, Great Danes, Boston terriers, Doberman pinschers, Irish setters, English bulldogs, and boxers: may be affected
  
• Usually growing puppies 4–8 months of age
  
• No gender predilection
  
• Neutering may increase incidence
  
• Usually relate to pain around the mouth and difficulty eating
  
• Angular processes of the mandible affected: jaw movement progressively restricted
  
• Difficulty in prehension, mastication, and swallowing: may lead to starvation
  
• Lameness or limb swelling: may precede cranial involvement

CLINICAL FEATURES

• Temporal and masseter muscle atrophy: common
  
• Palpable irregular thickening of the mandibular rami and/or temporomandibular joint (TMJ) region
  
• Inability to fully open jaw, even under general anesthesia
  
• Intermittent pyrexia: 40°C
  
• Bilateral exophthalmos

DIFFERENTIAL DIAGNOSIS

• Osteomyelitis: bones not symmetrically affected; generally not as extensive; lysis; lack of breed predilection; history of penetrating wound
  
• Traumatic periostitis: bones not symmetrically affected; generally not as extensive; history of trauma
  
• Neoplasia: mature patient; not symmetrically affected; more lytic bone reaction; metastatic disease
  
• Calvarial hyperostosis: young patient—frontal, parietal, and occipital bones; does not involve mandible; may have long bone involvement

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