A Rare Case of Myositis in a Cavalier King Charles Spaniel
Unraveling the Mystery: A Cavalier's Battle with Myositis
Early Signs and Initial Suspicions: The Jaw's Restriction
Rigby, a four-month-old Cavalier King Charles Spaniel, was initially brought to veterinary attention due to a limited range of motion in his jaw, causing him discomfort when attempting to open his mouth. A specialist examination revealed a vertical jaw opening of merely 1 cm.
Advanced Imaging and Evolving Diagnoses: Beyond the Initial Findings
A CT scan, analyzed by a certified radiologist, indicated bilateral temporomandibular joint (TMJ) abnormalities, including subluxation and widening of the right joint, along with thickening of the rostral tympanic bullae walls. These findings initially suggested early craniomandibular osteopathy (CMO). Following a successful recovery from Parvovirus, Rigby was referred for further evaluation of his jaw issues, which had slightly improved to a 1.3 cm opening.
A New Perspective: Identifying Masticatory Muscle Myositis (MMM)
Upon subsequent examination, Rigby's ocular movements were normal, but he showed moderate atrophy of the temporal and masseter muscles, with an improved jaw opening of 3.8 cm, indicating less likelihood of TMJ dysplasia. This led to suspicion of Masticatory Muscle Myositis (MMM), prompting a specific antibody test. The positive result for the 2M antibody confirmed the MMM diagnosis, which was then managed with a course of prednisolone, resulting in significant improvement and a normalized jaw range of motion within three months.
The Emergence of a New Challenge: Extraocular Muscle Myositis
Two years later, Rigby developed bilateral exophthalmos, a new condition without jaw pain. While MMM can cause this, other possibilities included zygomatic sialadenitis and extraocular muscle myositis. An MRI, reviewed by both the radiologist and the author, revealed normal masticatory muscles and salivary glands but suggested the extraocular muscles were the most probable cause. Rapid and dramatic improvement followed the initiation of prednisolone therapy, with Rigby now maintained on a tapering dose.
Understanding Masticatory Muscle Myositis (MMM)
MMM is an autoimmune inflammatory condition affecting only the masticatory muscles, which contain unique type 2M fibers. Dogs with MMM produce autoantibodies targeting these fibers, leading to muscle necrosis and fibrosis. The 2M antibody test is a highly sensitive and specific diagnostic tool. Corticosteroid treatment is the standard approach, with muscle biopsies offering an alternative diagnostic method.
Insights into Atypical and Extraocular Myositis
Literature describes atypical forms of MMM, particularly in Cavalier King Charles Spaniels, highlighting the breed's predisposition. Extraocular muscle myositis, though rare, has been documented in various breeds, typically affecting only the extraocular muscles. Eosinophilic myositis is another rare form, with Cavaliers being prone to eosinophilic diseases, adding another layer of complexity to Rigby's case.
A Remarkable Recovery: Rigby's Ongoing Journey
Rigby's case exemplifies the challenges and successes in managing complex autoimmune myositis conditions in dogs. His journey underscores the importance of thorough diagnostics and tailored treatment plans for these rare diseases, showcasing a positive outcome for a beloved pet.