Mixed Connective Tissue Disease

Mixed connective tissue disease is an overlap syndrome characterised by clinical features of systemic lupus erythematosus, systemic sclerosis and polymyositis occurring together in the same patient.

High titre anti U1 ribonucleoprotein (anti RNP) antibody is required for diagnosis and distinguishes this from a non specific overlap presentation.

Pathophysiology and natural history

The precise mechanism linking the characteristic serology to the specific combination of clinical features is incompletely understood. The shared autoantibody target across a clinically heterogeneous group of patients supports mixed connective tissue disease as a distinct entity rather than simply coincident lupus, scleroderma and myositis.

The disease course is variable. A proportion of patients evolve predominantly toward a scleroderma like phenotype over years, while others remain with a milder, more lupus like course.

Raynaud phenomenon is almost universal and often the presenting feature, frequently preceding other manifestations by months to years.

Puffy, swollen fingers (an early and characteristic feature distinct from the later sclerodactyly of established scleroderma) are typical, sometimes progressing to frank sclerodactyly over time.

Arthralgia and inflammatory arthritis, sometimes with an erosive pattern resembling rheumatoid arthritis, are common presenting features.

Myositis, ranging from mild myalgia to overt proximal muscle weakness with elevated creatine kinase, reflects the polymyositis component of the overlap.

Serositis (pleuritis, pericarditis) and mild renal involvement occur, generally less severe than typical lupus nephritis, though significant renal disease can occur in a subset.

Pulmonary involvement, particularly interstitial lung disease and pulmonary arterial hypertension, represents the most serious long term threat and the principal driver of mortality, mirroring the pulmonary risks of systemic sclerosis.

Oesophageal dysmotility, sicca symptoms, and cutaneous features (malar rash, discoid lesions) reflecting the lupus and sclerosis components may all be present in varying combination and severity.

Diagnosis requires the combination of overlapping clinical features (from the lupus, scleroderma and myositis spectrum) together with a high titre positive anti U1 RNP antibody.

It is generally made in the absence of high titre anti double stranded DNA or anti Smith antibody, which would favour a lupus predominant overlap or classification as lupus itself.

Antinuclear antibody testing typically shows a high titre speckled pattern, with confirmation via specific anti RNP antibody testing.

Creatine kinase and, where myositis is suspected clinically, electromyography and muscle biopsy assess the myopathic component.

Nailfold capillaroscopy frequently shows changes similar to systemic sclerosis, useful for identifying patients at higher risk of evolving toward a scleroderma predominant phenotype.

Baseline and regular surveillance pulmonary function testing, high resolution computed tomography of the chest and echocardiography with estimated pulmonary artery pressure are essential given the substantial pulmonary morbidity and mortality risk, mirroring surveillance protocols used in systemic sclerosis.

Renal function and urinalysis monitoring given the potential, albeit generally milder, renal involvement.

Management is directed at the predominant clinical features present in the individual patient, drawing on treatment approaches from each of the constituent diseases.

Hydroxychloroquine is commonly used as background therapy for mild disease, arthralgia and cutaneous manifestations.

Corticosteroids and steroid sparing immunosuppression (methotrexate, azathioprine, mycophenolate) are used for active arthritis, myositis, serositis or moderate organ involvement, with intensity guided by severity in the same manner as for lupus and myositis.

Interstitial lung disease and pulmonary arterial hypertension are managed following the protocols established for systemic sclerosis, given the shared pulmonary vulnerability and the fact that pulmonary disease drives long term prognosis in this condition.

Raynaud phenomenon and digital vasculopathy are managed with calcium channel blockers and, for more severe disease, the vasoactive therapies used in systemic sclerosis.

Regular, structured, lifelong surveillance for pulmonary involvement, renal function and evolving disease phenotype is essential, since the specific pattern of organ involvement and disease trajectory can shift over years, requiring ongoing reassessment rather than a fixed treatment plan established at diagnosis.