26th February 2025

Inflammatory myopathies

Overview of inflammatory myopathies

This is a group of disorders characterised by muscle inflammation and breakdown. The most common forms are polymyositis (PM) and dermatomyositis (DM). These are more common in women than men.

Clinical features of inflammatory myopathies

Symmetrical proximal muscle weakness is the most common finding in both conditions, usually starting in the lower limbs. Muscle pain is less marked than in other conditions such as polymyalgia rheumatica; however, does occur in up to half of cases.

In addition, dermatomyositis has characteristic cutaneous features. Onset is often weeks to months prior to muscle weakness. Classical cutaneous features include:

• Gottron’s papules: violaceous plaques over the metacarpophalangeal and proximal interphalangeal joints
• Heliotrope rash: a purple-coloured rash around the eyes often associated with periorbital oedema
• Shawl sign: a macular eruption across the back
• V sign: a macular eruption across the anterior chest under the neck

Depending on the antibody, other organ systems may be involved. The most important thing to be aware of is respiratory involvement. ILD is common, and in some autoantibody subtypes can be rapidly progressive. There can also be cardiovascular involvement, and this significantly increases the risk of mortality.

Investigations in inflammatory myopathies

Muscle enzymes are classically elevated. Creatine kinase is the most commonly used; however, the transaminases and LDH can be elevated. Although cardiac involvement can occur, elevated troponin T values may be due to cross-reactivity with the creatine kinase assay, and in such situations, troponin I should be used to investigate further.

Inflammatory markers such as ESR and CRP do not correlate well with disease activity.

Multiple different autoantibodies are associated with inflammatory myopathies and tend to predict the disease course. ANA is the most common antibody. In exams, it is often stated that anti-Mi2 is most associated with dermatomyositis and anti-Jo-1 with polymyositis; however, there are many other antibodies.

MRI of muscles can show oedema in active myositis and can also be used to identify an appropriate site for biopsy. Muscle biopsy is gold standard; however, is not always required if the patient has classical features, raised muscle enzymes and a suggestive autoantibody.

EMG can be used to identify a myopathic process and will be abnormal in 90% of patients.

And so, when patients are diagnosed, it is common to screen for malignancy with thorough examination and sometimes further investigations. Certain antibodies are more associated with this, such as TIF1y.

Management of inflammatory myopathies

Initial treatment is generally with glucocorticoids, usually prednisolone 1-2 mg/kg. DMARDs such as methotrexate can be used as steroid-sparing agents, and there is increasing evidence for the use of biologic agents such as rituximab. IVIG has also been demonstrated to be effective.

Anti-synthetase syndrome

This is a subtype of myositis with its own classical features. Skin on the hands can be thickened and cracked, particularly at the tips of the fingers; this is known as ‘mechanic’s hands’. Raynaud’s phenomenon, arthritis and fevers are other common features. Interstitial lung disease is also commonly associated. Anti-Jo-1 antibodies are the most commonly seen; however, Anti-PL7 and PL12 may also be present and are more associated with ILD.

Inclusion-body myositis

This is a rarer form of inflammatory myositis, accounting for around 20% of cases. It is more common in middle-aged men. Compared to PM or DM as it presents with more predominant distal weakness, such as reduced grip strength, along with proximal muscle weakness. Dysphagia is also commonly seen, although dysarthria is not. An open biopsy is required for diagnosis.

Patients are not generally steroid responsive; however, these are usually given as initial treatment with subsequent DMARD therapy, such as methotrexate.

References

• Witt LJ, Curran JJ, Strek ME. The Diagnosis and Treatment of Antisynthetase Syndrome. Clinical pulmonary medicine [Internet]. 2016 Sep 1 [cited 2022 Feb 20];23(5):218–26. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5006392/
• Clunie GPR, Wilkinson N, Nikiphorou E, Deepak Jadon. Oxford handbook of rheumatology. Oxford: Oxford University Press; 2018
• Christopher-Stine L, Vleugels RA, Amato AA. Clinical Manifestations of Dermatomyositis and Polymyositis and Adults [Internet]. Targoff IN, Shefner JN, Callen J, Ramirez Curtis M, Ofori AO, editors. UpToDate. 2022 [cited 2022 Jan 12]. Available from: https://www.uptodate.com/contents/clinical-manifestations-of-dermatomyositis-and-polymyositis-in-adult
• Turner MR, Talbot K. Mimics and chameleons in motor neurone disease. Practical Neurology [Internet]. 2013 Jun 1 [cited 2022 Jan 14];13(3):153–64. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3664389/