Chronic type toxicity reported to occur in about 1 in 750 patients receiving long-term suppressive treatment. Chronic nitrofurantoin toxicity , as in this case, occurs most often in the elderly on long-term suppressive therapy for recurrent urinary tract infections. Cough, dyspnea, and cyanosis are often much worse than that seen in the acute form. Low titers of ANA and rheumatoid factor, along with elevated gama-globulin levels may be found. Chest xray shows diffuse interstitial infiltrates, but pleural effusions are rare.
Lung biopsy shows interstitial and alveolar fibrosis and inflammation.
Prognosis in the chronic type is much worse than in the acute type. High dose corticosteroid therapy is often used, but there is no clear evidence that it helps. Antibiotics can also have an adverse effect on respiratory muscle function. Nitrofurantoin, and others, may cause peripheral motor neuropathies . Amphotericin B may cause a direct myopathic effect on muscles, including the diaphragm and accessory respiratory muscles.