12. When is inflammatory myositis not inflammatory myositis?
Authors: McShane Annmarie , Wray Maria , Burns James , Sin Jerry , Glass Patrick , Beirne Christopher , Clinton Aoibhinn , Herron Brian 
Published: September 2019

Abstract

Introduction
A 52-year-old man presented with extensive bilateral inflammatory myositis of the quadriceps on a background of pyrexia of unknown origin (PUO). He had markedly elevated acute phase reactants but had a normal CK and did not develop muscle weakness. This was a transient phenomenon which resolved spontaneously over 10 weeks. There was not an infective process identified and he was not treated with glucocorticoids or other immunosuppressants. This case highlights the importance of careful consideration of steroid treatment in atypical presentations without a clear diagnosis and the value of multidisciplinary involvement in patient care.

Case description
The patient presented with a 2-week history of acute onset bilateral thigh pain, stiffness and persistent fevers following an episode of alcohol intake although symptoms may have preceded this. Myalgia was a prominent feature however he maintained full power of the hip flexors and all other proximal and distal muscle groups. No skin changes, swallow or respiratory symptoms.
CK was normal at presentation and remained low. Acute phase reactants were elevated including CRP 405mg/L, ESR 110mm/hr. Platelets 1000x109/L, WCC 17.7x109/L. Urinalysis and renal function normal. ALT raised at 136U/L. AST normal, other liver parameters raised in keeping with acute phase response. No previous episodes, no preceding travel, infection, bites, illicit drugs or family history of muscle disorders. He was taking a statin which was discontinued.
PUO work up including cultures, viral titres, echocardiogram and CT imaging did not identify a source. Empiric antibiotics were trialled without improvement.
ANA, ANCA and myositis panels were negative. MRI of thighs reported extensive inflammatory myositis involving the quadriceps bilaterally with mild involvement of biceps femoris. There were no features of an infected collection or rhabdomyolysis. Muscle biopsy showed a lymphocytic myositis with neurogenic atrophy. Differential included idiopathic inflammatory myositis (IIM) spectrum with viral myositis felt less likely. Clinically he remained sore but improving and mobilising. Daily pyrexias persisted but he remained fully strong with a low normal CK and a downward trend in inflammatory markers. Investigations for occult malignancy including CTPET and bone marrow biopsy were negative.
Steroid treatment was not instigated. A watchful waiting approach was taken and his symptoms and blood investigations gradually improved. He embarked on a physiotherapist guided exercise programme. Follow up MRI at 10 weeks showed complete resolution of the inflammatory features.

Discussion
This is a case of an acute onset myositis with systemic inflammatory features which resolved spontaneously. Differential diagnoses including inherited, metabolic and toxic myopathies were considered and discounted. Although alcohol had been taken and he had been taking a statin prior to onset, the clinical picture, normal CK and biopsy findings did not support these as likely culprits. Extensive viral and serology infective tests were negative and the exact cause was not identified.
The clinical features strongly mimicked an idiopathic inflammatory myositis (IIM) however atypical features including normal CK and lack of objective weakness led to continued doubt of an autoimmune aetiology. Additionally there were no upper limb or neck flexor symptoms and no extra-muscular manifestations. Applying the ACR/EULAR 2017 criteria for IIM to this case the probability of this diagnosis is 5% although this does not take into account the radiological findings.
With evidence of inflammatory myositis and a dramatic acute phase response including persistent pyrexia there was a strong temptation to begin steroid therapy. His case was discussed with radiology, pathology and neurology and ultimately the decision to withhold was justified.
In this case improvement would have occurred with steroid which would likely have led to an incorrect diagnosis of IIM and the patient would have been unnecessarily treated with prolonged high dose steroid and disease modifying drugs with the associated side effects and long term sequelae.

Key learning points
This case highlights the importance of confirming a diagnosis prior to initiating steroid therapy particularly if there are atypical features. Glucocorticoids would undoubtedly have led to a clinical improvement however this would have given rise to a misdiagnosis and unnecessary long term immunosuppression.
There were many positive test results for inflammation including blood, imaging and muscle tissue but ultimately the clinical examination findings, particularly the absence of weakness, were consistent and the basis of the rationale to withhold steroid treatment. As clinical improvement progressed with supportive management this decision was justified. This demonstrates the importance of patient centred care and the value of the clinical examination.
This was an unusual case therefore input and advice was sought from colleagues in neurology, neuropathology, rheumatology and radiology. This was vital to clinical care decisions and emphasizes the importance of a multidisciplinary approach in complex cases.
Despite extensive investigations no cause was identified for this transient inflammatory myositis. This highlights the unknown elements within this spectrum of disease and the need for ongoing reporting of atypical cases.

Conflicts of interest
The authors have declared no conflicts of interest.