Inflammatory myopathies can be debilitating autoimmune diseases in both dogs and humans characterized by focal or generalized muscle pain, sensitivity, and weakness– the inflammation can be generalized, throughout most of the striated muscles of the body, or they can be more localized and specific, such as with masticatory myositis. Initial diagnoses are often made on the basis of history, clinical signs, serum biochemical profiles and an anti-type 2M myofiber antibody test. Blood work is often normal or relatively non-specific, with serum creatinine kinase levels occasionally increased (more so with generalized myopathies) and an inflammatory leukogram on complete blood count. The gold standard for diagnosing inflammatory myopathies is a muscle biopsy, although this is an invasive procedure, and with the oftentimes patchy distribution of inflammation throughout the muscles, it does not always result in a representative sample. EMG, or electromyography, is highly sensitive for diseases compromising muscle function but lacks the specificity to distinguish amongst inflammatory, degenerative, neoplastic and/or infectious etiologies. Cross-sectional imaging in the form of both magnetic resonance imaging and computed tomography has been found to be both sensitive and specific for inflammatory myopathies, albeit that they cannot always differentiate between infectious and inflammatory causes of muscle disease. The following two cases detail the clinical and imaging findings in two patients with masticatory myositis, one of whom had an MRI performed and the other a CT scan.
“Brutus” Miller, a 6yr male castrated Rottweiler, presented with a several week history of progressive dysphagia. The initial physical exam revealed a normal range of motion of the jaw and normal temporal and masseter muscle mass through discomfort on palpation of those muscles. Routine bloodwork and radiographs of the skull were unremarkable. Initial differentials included inflammatory myositis, infectious myositis, or neoplasia. An anti-type 2M myofiber antibody test was sent out and later returned a positive result of 1:1000 (normal <1:100).
An MRI of the patient’s head was performed, with routine sequences including T1, T2, T2*, FLAIR and post-contrast T1-weighted Fat Sat images. The scan revealed ill-defined T2-hyperintensities throughout the temporal and masseter muscles primarily along the right side of the head. These areas were hyperintense on FLAIR images and iso- to mildly hyperintense on T1-weighted images. After administration of gadolinium IV, ill-defined, locally-extensive regions of contrast enhancement were observed throughout the temporal, masseter and medial pterygoid muscles on both sides of the head. The digastricus muscles were unaffected. After the MRI scan, an incisional biopsy was obtained from the right temporal muscle, using the MR images as a guide to obtain the most representative sample. The histopathology was consistent with masticatory myositis.
“Hextall” Holloway, a 7yr male castrated Rottweiler, presented with a history of difficulty chewing and swelling around his right eye. The patient’s previous history had included an eosinophilic and lymphoplasmacytic gastritis diagnosed with endoscopy a year prior and successfully treated with prednisone and azathioprine. The initial physical exam showed mild exophthalmos of the right eye and pain on palpation of the temporal and masseter regions on both sides of the head. Routine bloodwork showed a mild increase in alkaline phosphatase of 337 (normal <212) and mild anemia of 36.3% (normal <37.3). A serum titer for 2M antibodies was negative at <1:100, although the patient had been on prednisone for seven days prior to the test being run.
A CT scan was performed on the patient’s head using routine series, including 1.25mm helical scan using a bone algorithm and a 1.25mm post-contrast helical scan using a detailed algorithm after administration of Iohexol IV. The pre-contrast scan demonstrated asymmetry of the masticatory muscles, with regional atrophy of the left temporal muscle, as well as ill-defined focal regions of hypoattenuating tissues within the temporal and masseter muscles on either side of the head. After administration of contrast, patchy and irregular areas of contrast enhancement were present throughout the temporal, masseter and medial pterygoid muscles bilaterally, although to a much greater extent throughout the left side. The degree of asymmetry of the contrast enhancement was unusual for this patient though in general it is not an atypical finding with masticatory myositis. The hypoattenuating areas on pre-contrast images likely corresponded to areas of necrosis and/or inflammatory infiltrate within the muscles, although this is not specifically backed up by the corresponding histopathology. The pattern and distribution of the patchy areas of contrast enhancement represent a classic appearance for masticatory myositis.
The two described cases demonstrate the utility of cross-sectional imaging in the diagnosis of masticatory myositis. The magnetic resonance characteristics of inflammatory myopathies, in general, are reliably consistent and have been well-described in the literature for both human and veterinary medicine. For masticatory myositis, in particular, it is often at the time of diagnosis– with the patients exhibiting signs ranging from stiffness and trouble walking to dropping food and trouble eating– that the MR characteristics are most apparent, paralleling the inflammatory infiltrates in the affected musculature. The imaging findings cannot necessarily distinguish an inflammatory process from neoplastic infiltration of the muscle– however, neoplastic lesions affecting the muscle tend to be more focal and better defined, with more of a random distribution. Similarly, infectious causes of muscle inflammation are typically more diffuse and do not have the characteristic distribution of masticatory myositis, in which only the muscles of mastication are involved.
The computed tomographic characteristics of masticatory myositis are well described, and they are just as sensitive and consistent, with almost identical patterns of contrast enhancement and variable muscle swelling and atrophy. Furthermore, computed tomography is typically less expensive and is a much faster imaging modality, resulting in less of a need for and expense due to anesthesia. Taking these factors into account, computed tomography may be a better imaging modality to pursue when there is a sufficient index of suspicion for masticatory myositis.
Submitted by Hadley Bagshaw VMD, DACVR (Radiology)
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