|Home | About | Journals | Submit | Contact Us | Français|
Vasculitis is a condition in which ventilation-perfusion scans can be falsely positive, with important clinical implications.
A man of 55 sought advice because of haemoptysis and dyspnoea. For several months he had been feeling unwell. He became severely hypoxic and a ventilation-perfusion (VQ) scan suggested a high probability of pulmonary embolism (Figure 1). Intravenous heparin was started. Microscopic haematuria and proteinuria were found on urine dipstick testing; renal function was abnormal (creatinine 320 μmol/L) and declined rapidly. The haemoptysis continued. Renal biopsy (with the patient off heparin) showed pauci-immune rapidly progressive glomerulonephritis. Serum was positive for anti-MPO-ANCA. A high-resolution spiral CT scan of the chest revealed patchy ground-glass opacification and consolidation consistent with pulmonary haemorrhage (Figure 1b) but not suggestive of pulmonary infarction. Small-vessel vasculitis involving the kidneys and lungs was diagnosed and he was started on methylprednisolone and cyclophosphamide together with plasmapheresis. Pulmonary symptoms and signs improved rapidly but he became dialysis dependent. 2 years later, with negative vasculitis markers, he had a successful renal transplant.
Pulmonary embolism is a common event that demands prompt treatment with anticoagulants.1,2 V/Q lung scanning is the most frequently used aid to diagnosis, with high negative predictive value and sensitivity; its specificity, however, is less satisfactory (false-positive rate in the PIOPED study 14%).3
Dyspnoea and haemoptysis are the usual presenting features of pulmonary embolism,3 but in the present case the abnormal urine findings and deteriorating renal function raised the possibility of an alternative diagnosis. The combination of acute glomerulonephritis (haematuria, red cell casts, renal insufficiency) with pulmonary haemorrhage (as manifested by haemopytsis or pulmonary infiltrates) is characteristic of vasculitis.4 In one report of 97 such patients, 48 were ANCA-positive, and most of them (like ours) had a pauci-immune glomerulonephritis with pulmonary capillaritis. 7 had both ANCA and anti-GBM antibodies; 6 had anti-GBM antibody disease alone; the remaining patients had various disorders including pulmonary embolism.
Causes of false-positives for pulmonary embolism in high-probability V/Q scans include previous embolism, infection, tumour and airways obstruction (producing localized hypoxic vasoconstriction and perfusion defects).3 Vasculitis is a rare cause that demands special consideration. The clinical presentation and V/Q results can be very similar to those of pulmonary ebolism, especially in cases of pulmonary capillaritis without renal involvement.5 The distinction can be made by CT pulmonary angiography; in pulmonary vasculitis intraluminal filling defects are usually absent.
Differentiation of these conditions is clinically very important because of the opposite implications for anti-coagulation—near-mandatory in pulmonary embolism, contraindicated in vasculitis for fear of worsening pulmonary haemorrhage.