An atypical cause of respiratory failure

A 70 year old woman was admitted to the general medical ward (from the emergency department) with a three week history of progressive shortness of breath, which was worse on exertion, and a non-productive cough. She took amlodipine 5 mg for hypertension and lansoprazole 30 mg for gastro-oesophageal reflux disease. She had also been taking immunosuppressive therapy with mycophenolate mofetil 1.5 g twice daily for nine months to treat her lichen planus. She was allergic to tramadol, codeine, and co-trimoxazole. She had received influenza vaccine within the previous 12 months. She was normally active and went on regular five mile walks.

On examination she was haemodynamically stable and her respiratory rate was 34 breaths/min, oxygen saturation was 92% on air (arterial oxygen pressure 8.1) although it de-saturated on minimal exertion, and her temperature was 38^oC. Auscultation of her chest was unremarkable, with equal air entry bilaterally. The figure⇓ shows her admission chest radiograph. Her haemoglobin was 117 g/L (reference range 115-160), white blood cell count was 8.5×10⁹/L (4-12), neutrophils were 7.4×10⁹/L (2-7.5), and lymphocytes were 0.6×10⁹/L (1.5-4.0). Her estimated glomerular filtration rate was normal (>60 mL/min/1.73m²), as were her liver function tests. Her electrocardiograph showed sinus rhythm with no acute changes. She was treated empirically for community acquired pneumonia (CURB-65 score 1) with benzylpenicillin and clarithromycin.

Her admission atypical screen included urine antigen testing for Legionella pneumophila and serum particle agglutination for Mycoplasma pneumonia, both which were negative; a throat swab for influenza was also negative. Blood cultures were negative after five days. During admission her respiratory functions deteriorated, with worsening tachypnoea and hypoxia; she eventually needed intubation and ventilation on the intensive care unit.