Rare canine overlap syndrome links myositis and thyroiditis
Bottom line
A new Frontiers in Veterinary Science case report describes a 4-year-old spayed female mixed-breed dog with chronic upper airway noise and dysphagia that was ultimately diagnosed with a rare overlap syndrome: concurrent masticatory myositis, generalized polymyositis, and lymphocytic thyroiditis. The workup included markedly elevated creatine kinase, CT evidence of perivertebral and perilaryngeal muscle thickening, abnormal electrodiagnostics, inflammatory changes on both masticatory and pelvic limb muscle biopsies, a positive 2M antibody titer, and a thyroid panel supporting immune-mediated thyroiditis rather than euthyroid sick syndrome. Immunosuppressive prednisone improved, but did not fully resolve, respiratory noise and dysphagia over 763 days of follow-up. (frontiersin.org)
Why it matters: For veterinary professionals, this case is a reminder that dogs with dysphagia, stridor, trismus, elevated CK, or mixed focal and generalized muscle signs may not fit neatly into a single inflammatory myopathy diagnosis. The authors note that only a few canine overlap syndrome cases have been identified previously, including three dogs in a 200-case inflammatory myopathy review, and this report adds thyroid autoimmunity to that picture. It also underscores the value of a full thyroid panel when chronic immune-mediated disease muddies interpretation of low thyroid hormone results, because management may need to address both inflammatory myopathy and true lymphocytic thyroiditis. (frontiersin.org)
What to watch: Watch for whether additional reports clarify how often thyroid autoimmunity accompanies canine inflammatory myopathies, and whether more standardized diagnostic algorithms emerge for dogs with overlapping neuromuscular and endocrine findings. (frontiersin.org)
Key facts
- Article type
- Case report
- Journal
- Frontiers in Veterinary Science
- Patient
- 4-year-old spayed female mixed-breed dog
- Presenting signs
- Chronic inspiratory stridor, expiratory stertor, and dysphagia
- Diagnosis
- Concurrent masticatory myositis, generalized polymyositis, and lymphocytic thyroiditis
- Creatine kinase
- 3,296 IU/L
- Imaging
- CT showed perivertebral and perilaryngeal muscle thickening with contrast enhancement
- Treatment and follow-up
- Prednisone improved signs, but did not fully resolve them over 763 days
A newly published case report in Frontiers in Veterinary Science details a rare canine overlap syndrome involving three immune-mediated conditions at once: masticatory myositis, polymyositis, and lymphocytic thyroiditis. The patient, a 4-year-old spayed female mixed-breed dog, presented with chronic inspiratory stridor, expiratory stertor, and dysphagia, then remained clinically improved but not fully normal after more than two years of follow-up on immunosuppressive therapy. (frontiersin.org)
The report matters because overlap syndromes are well described in human medicine, but they remain rarely documented in dogs. In the authors’ review of prior veterinary literature, only a handful of canine cases have been reported in which dogs showed features of both masticatory myositis and generalized polymyositis. A 2004 review of 200 canine inflammatory myopathy biopsy cases identified just three dogs classified as overlap syndrome, highlighting how uncommon, or underrecognized, this presentation may be in practice. (frontiersin.org)
In this case, the diagnostic picture was unusually broad. Serum creatine kinase was 3,296 IU/L, and CT showed perivertebral and perilaryngeal muscle thickening with contrast enhancement. Electrodiagnostics found diffuse spontaneous activity with normal motor nerve conduction velocity, while biopsies from both masticatory and pelvic limb muscles confirmed inflammatory myopathy. A positive serum 2M antibody titer supported masticatory myositis, and infectious PCR and serology testing were negative, helping narrow the case toward immune-mediated polymyositis plus masticatory myositis rather than infectious or paraneoplastic causes. (frontiersin.org)
The thyroid findings added another layer. Initial in-house thyroxine was below the reference range, but the authors pursued a comprehensive thyroid profile rather than assuming non-thyroidal illness alone. That panel supported lymphocytic thyroiditis, which the paper argues was important for distinguishing true thyroid disease from euthyroid sick syndrome in a dog with chronic inflammatory illness. The authors conclude that thyroxine supplementation, alongside prednisone, likely contributed to the dog’s improved quality of life. (frontiersin.org)
Broader clinical context supports that message. Masticatory myositis is classically associated with pain on opening the mouth, jaw restriction, and type 2M fiber autoantibodies, while polymyositis tends to produce more generalized weakness, dysphagia, gait changes, and higher CK activity. Veterinary reference and review sources note that the two conditions can be difficult to distinguish clinically, especially when masticatory signs appear early or dominate the presentation. A 2019 summary of 22 canine masticatory myositis cases reported that all dogs had reduced ability to open the mouth, and most improved with prompt immunosuppressive prednisone, though relapse was not uncommon. (veterinarypartner.vin.com)
Why it matters: For veterinarians, the practical lesson is to widen the differential when upper airway noise, dysphagia, trismus, elevated muscle enzymes, and endocrine abnormalities appear together. This case suggests that a dog can have both focal and generalized inflammatory myopathy at the same time, and that low thyroid values in that setting shouldn’t automatically be written off as illness-related suppression. A more complete workup, including 2M antibody testing, electrodiagnostics, targeted muscle biopsies, infectious disease exclusion, and a full thyroid panel, may change both the diagnosis and the treatment plan. (frontiersin.org)
It also raises a prognosis point. In the older retrospective series, the few reported overlap syndrome dogs had poor or unknown outcomes, whereas this dog was still doing better 763 days after presentation, even though signs were not fully resolved. That doesn’t prove a different prognosis category, but it does suggest some dogs may do reasonably well with long-term immunosuppression and concurrent endocrine management. That’s an inference from a single case, not a settled conclusion. (frontiersin.org)
What to watch: The next step for the field will be determining whether this was a true rarity or a sign that overlap inflammatory syndromes are being missed in dogs with mixed neuromuscular presentations, especially when thyroid testing is incomplete or clinical signs are attributed to only one myopathy subtype. (frontiersin.org)