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PH-IM14

Polymyositis, dermatomyositis and Sjögren syndrome

How autoimmune attack on muscle causes symmetrical proximal weakness in polymyositis and dermatomyositis, and how lymphocytes invading the salivary and tear glands cause the dry mouth and eyes of Sjögren syndrome.

By the end, you can

  • Describe the normal organisation of skeletal muscle and of the salivary and tear glands.
  • Contrast the immune attack in dermatomyositis (capillaries and the fascicle edge) with polymyositis (T cells invading muscle fibres).
  • Explain why inflammatory myopathy causes proximal weakness and a raised creatine kinase, and how antibodies mark lung disease and cancer risk.
  • Explain how lymphocytic infiltration and an activated gland epithelium cause dry eyes and dry mouth in Sjögren syndrome.
  • Explain the systemic features of Sjögren syndrome, including its links with lymphoma and congenital heart block.
  • Recognise the biopsy patterns, classifications and tests of the inflammatory myopathies and Sjögren syndrome, and explain how immunosuppressants, immunoglobulin and secretagogues act on the mechanism.

What it is

The idiopathic inflammatory myopathies are autoimmune diseases in which the immune system attacks skeletal muscle, causing weakness. Dermatomyositis injures small vessels of muscle and skin and has a characteristic rash; polymyositis is T-cell attack on muscle fibres without a rash, now diagnosed less often as other forms are recognised. Sjögren syndrome, increasingly called Sjögren’s disease, is an autoimmune disease in which lymphocytes infiltrate the exocrine glands, mainly the salivary and tear glands, causing dry mouth and dry eyes; it occurs alone or with other autoimmune diseases.

4 types to tell apart
Dermatomyositis
Weakness with a heliotrope rash and Gottron papules; injury centred on capillaries and the edge of fascicles; in adults, a link with cancer.
Polymyositis
Weakness without a rash; cytotoxic T cells invade muscle fibres. Many patients once given this label are now classed under other myopathies.
Other inflammatory myopathies
Antisynthetase syndrome (anti-Jo-1, lung disease, "mechanic’s hands"), immune-mediated necrotising myopathy (anti-SRP or anti-HMGCR, the latter sometimes after statin use) and inclusion body myositis, a slowly progressive disease of older adults.
Primary and associated Sjögren syndrome
Primary when it occurs alone; associated (secondary) when it accompanies rheumatoid arthritis, lupus or systemic sclerosis.

US spelling: fibres → fibers, fibre → fiber.

Normal function

Muscle fibres and secretory glands

A skeletal muscle is bundled in connective tissue: the wraps the whole muscle, the wraps each , and the surrounds each . Larger vessels run in the perimysium, and a dense network of capillaries in the endomysium supplies each fibre.

Each fibre is a long cell with many nuclei, packed with contractile filaments. Its membrane keeps enzymes such as creatine kinase inside. Healthy fibres show very little MHC class I on their surface, so cytotoxic T cells largely ignore them.

The salivary and tear (lacrimal) glands are exocrine glands: clusters of secretory cells (acini) drain through ducts. Parasympathetic nerves release acetylcholine onto M3 receptors, driving secretion of watery saliva, which moistens food, buffers acid and protects the teeth from microbes, and of the tear film that keeps the cornea moist and clear.

Point to it on the model