Case report: Atypical and chronic masticatory muscle myositis in a 5-month old Cavalier King Charles Spaniel. Clinical and diagnostic findings, treatment and successful outcome

Abstract

Masticatory muscle myositis (MMM) is the second most common inflammatory myopathy diagnosed in dogs, but it is rarely described in puppies. The disease is characterized by the production of autoantibodies against 2M myofibers contained in masticatory muscle, although the cause of this production is still unclear. The aim of the present case report was to describe the clinical presentation, diagnostic findings, treatment, and follow-up of an atypical case of chronic masticatory muscle myositis in a very young dog. A 5-month old Cavalier king Charles Spaniel (CKCS) was presented to the AniCura Istituto Veterinario Novara with a two weeks, progressive history of lethargy and difficulty in food prehension. Neurological examination revealed bilateral masticatory muscle atrophy, mandibular ptosis with the jaw kept open, inability to close the mouth without manual assistance, jaw pain, and bilateral reduction of palpebral reflex and menace reaction; vision was maintained. A myopathy was suspected. Computed tomography (CT), magnetic resonance imaging (MRI), enzyme-linked immunosorbent assay test for 2M antibodies, and histopathological examination of masticatory muscle biopsy confirmed the diagnosis of MMM. Glucocorticoids treatment was started and clinical signs promptly improved. To the authors' knowledge, this is the first case describing mandibular ptosis in a dog affected by chronic MMM, successfully managed with medical treatment and the first report describing the CT and MRI findings in a young CKCS affected by MMM.

Keywords: CKCS; CT; MRI; dog; masticatory muscle myositis.

Figure 1

Computed tomography pre-contrast (A) and post-contrast (B) images of the head of a 5-month old Cavalier King Charles Spaniel affected by masticatory muscle myositis. Note the severe, diffuse, and bilateral atrophy of temporalis, masseter, and pterygoid muscles and contrast enhancement of the masticatory muscle in post-contrast image (* temporalis muscle, ^#masseter muscle,^†pterygoid muscle, ^||digastricus muscle).

Figure 2

Magnetic resonance imaging study of the head of a 5-month old Cavalier King Charles Spaniel affected by masticatory muscle myositis; pre-contrast T1-W image (A), post-contrast T1-W image (B), fluid attenuated inversion recovery (FLAIR) (C) and short tau inversion recovery (STIR) (D) images. Note the severe, diffuse, and bilateral atrophy of temporalis, masseter, and pterygoid muscles that appear isointense on T1-W images pre-contrast (A) and showing a strong contrast enhancement (B); these muscles appear hyperintense on FLAIR (C) and STIR (D) images, compared to the signal in normal muscles (* temporalis muscle, ^#masseter muscle,^†pterygoid muscle, ^||digastricus muscle).

Figure 3

Magnetic resonance imaging study of the head of a 5-month old Cavalier King Charles Spaniel affected by masticatory muscle myositis; pre-contrast T1-W image (A), post-contrast T1-W subtraction image (B), pre contrast 3D T1-W image (C). Note the absence of extraocular muscle swelling and contrast enhancement; no evidence of exophthalmos.

Figure 4

Photomicrograph of histological, hematoxylin and eosin stained, section of a temporalis muscle of a 5-month old Cavalier King Charles Spaniel affected by masticatory muscle myositis. Note the inflammatory infiltration and the interstitial and nodular fibrosis of the temporalis muscle.