M. de Visser, M.D., Ph.D., Professor of Neuromuscular Diseases
J.E. Hoogendijk, M.D., Ph.D.
J. van de Vlekkert, M.D.
Clinical features
Patients with PM or DM present with subacute (weeks to months) progressive, symmetrical, proximal muscle weakness of arms and legs, and neck flexor weakness. They often complaint about pain and tenderness in the affected muscles. Dysphagia, caused by both pharyngeal weakness and hypomotility of the lower oesophagus, has been reported in one third of patients. General symptoms like fatigue, weight loss, fever and arthralgias are often present. In severe cases of both types of myositis, respiratory muscles can be involved (up to 10%). Interstitial lung disease is often seen in patients with autoantibodies to tRNA synthetases or a mucinlike glycoprotein: anti-synthetase syndrome. Read more
Natural course and prognosis
The natural course of polymyositis and dermatomyositis has never been evaluated, but spontaneous remission has been described. It is difficult to analyse the outcomes of patients after treatment, because studies often use various outcome measures, many studies use inadequate treatment and inclusion body myositis (unresponsive to treatment) is often not sufficiently excluded. After treatment, about 25% is markedly disabled or dead and 30% is in a remission. 5-year survival is about 95% and the 10-year survival about 84%.
Epidemiology
The exact frequencies of dermatomyositis and polymyositis are unknown, because most studies used the Bohan and Peter criteria which can not differentiate PM from IBM. The incidence (in the total population of children and adults) of IIM without distinguishing between DM, PM or sIBM is 5.5 (95% CI 4.7-6. Read more
Differential diagnosis
Muscle weakness with a subacute onset of limb-girdle muscle weakness can have many causes like electrolyte disturbances (hypokalemia, hypercalcemia), metabolic diseases (periodic paralyses), enzyme deficiencies (carnitine-palmitoyl transferase deficiency), other immune-mediated diseases (myasthenia gravis, Guillain-Barré Syndrome), endocrine diseases (hyperthyroidism, hypothyroidism, hyperparathyroidism, Cushing syndrome, Addison’s disease), drugs, toxins (ethanol, corticosteroids, colchicine, D-penicillamine, lipid-lowering drugs, zidovudine) and infections (influenza, toxoplasmosis, human immunodeficiency virus).
Even muscular dystrophies can have a seemingly subacute onset although careful history taking usually shows that signs and symptoms have started much earlier.Ancillary investigations
The diagnosis of myositis is based upon the history, clinical examination, assessment of serum creatine (CK) activity and skin or muscle biopsy findings.In patients with characteristic skin lesions (Gottron’s sign or papules and heliotrope edema) no additional investigation has to be performed since these features are pathognomonic for dermatomyositis. If less charcteristic skin abnormalities are found or if the dermatologist is in doubt about the presence of Gottron’s sign or papules and heliotrope edema a biopsy of affected skin can be performed. In all other cases of subacute myositis, a muscle biopsy should be taken from an affected (weak) muscle. Read more
Genetics
Genetic factors may have a role in polymyositis and dermatomyositis, as suggested by rare familial occurrences and association with certain HLA genes.
In addition to sporadic IBM, a hereditary form has been described. These patients present very little muscle inflammation, although other pathological changes are similar to those observed in sporadic inclusion body myositis.Therapy
Dermatomyositis and polymyositis are considered to be responsive to immunosuppressive and immunomodulating therapies, in contrast to IBM, which is refractory to all treatment. In polymyositis and (juvenile and adult) dermatomyositis, first line treatment with prednisone (1-1. Read more
