Swine-influenza (H1N1 influenza) mimicking a case of high altitude pulmonary oedema (HAPO)

Discussion

HAPO generally occurs above 2500 m (8000 feet) and the risk depends on individual susceptibility, altitude attained, rate of ascent and time spent at altitude. At approximately 4500 m (14 800 feet) the incidence of HAPO ranges from 0.2% to 6%.2 HAPO is the abnormal accumulation of plasma and some red cells in the lung due to a breakdown in the pulmonary blood-gas barrier, triggered by hypobaric hypoxia. This breakdown develops from a number of maladaptive responses to the hypoxia encountered at higher altitudes, including poor ventilatory response, increased sympathetic tone, exaggerated and uneven pulmonary vasoconstriction (pulmonary hypertension), inadequate production of endothelial nitric oxide and overproduction of endothelin, many of which are genetically determined.3 The end result is a patchy accumulation of extravascular fluid in the alveolar spaces that impairs respiration and can, in severe cases, prove fatal. HAPO generally begins with a subtle, non-productive cough, shortness of breath with exertion and difficulty walking uphill. Such non-specific symptoms are easily mistaken for a benign upper respiratory tract infection or attributed to normal breathlessness at altitude or exhaustion.1 On physical examination, tachycardia, tachypnoea and low-grade fever (up to 38°C) are common. Inspiratory crackles may be more prominent in the right middle lobe initially, but become bilateral and diffuse as HAPO progresses. Chest radiography usually reveals characteristic patchy alveolar infiltrates, predominantly in the right central haemithorax, which become more confluent and bilateral as the illness progresses.4 Supplemental oxygen is first-line therapy for HAPO and should be provided in all treatment settings when available. ILI is defined as fever (temperature of 100°F (37.8°C) or greater) with cough or sore throat in the absence of a known cause other than influenza. Case definitions have been provided by the US CDC. The signs and symptoms of influenza caused by pandemic H1N1 influenza A virus are similar to those of seasonal influenza. Adults hospitalised with pandemic H1N1 influenza are more likely to have lower respiratory tract complications, shock/sepsis and organ failure than those with seasonal influenza. They are also more likely to be admitted to the intensive care unit, require mechanical ventilation or die. Young adults with pandemic H1N1 influenza are two to four times more likely to have severe outcomes than individuals of the same age with seasonal influenza.5 Many hospitalised patients have infiltrates suggestive of pneumonia or acute respiratory distress syndrome. Other complications reported in patients with pandemic H1N1 influenza A infection includes neurological complications, myocarditis, renal insufficiency, rhabdomyolysis, haemophagocytic syndrome and multisystem organ failure.6 Rates of lymphopenia have varied among studies. Among 426 patients with confirmed pandemic H1N1 infection in China, lymphopenia occurred in 68% of adults and 92% of children.7 Elevations of creatine kinase levels were reported in some patients with severe illness, including one patient with severe rhabdomyolysis. Lactate dehydrogenase was elevated in all 16 patients with severe illness in one study.8 Common findings using chest radiography and/or chest tomography included patchy consolidation or ground glass opacities, with or without consolidation; there was a lower lung zone predominance and the most commonly affected regions were the peripheral and central perihilar areas.9 Nucleic acid amplification tests, such as reverse transcriptase polymerase chain reaction (RT-PCR), are the most sensitive and specific tests for the diagnosis of influenza virus infection. Hence, the initial signs and symptoms of HAPO and viral pneumonia like swine-influenza are very similar. Moreover, even the chest radiograph has similar scattered non-homogenous opacities I the lung fields with no zonal predominance. This makes the diagnosis very challenging, especially in peripheral hospitals located in high altitude areas. A peripheral high altitude hospital like ours receive on an average two to three cases of HAPO every day and in our PubMed search, we did not find any case of H1N1 pneumonia reported from high-altitude. An unsuspecting clinician may tend to misdiagnose viral pneumonia as routine HAPO and there are only few clinical pointers like poor response to supplemental O₂, leuco/lymphopenia and of course travel and epidemiological history taking, which will clinch the diagnosis of a rare (in high altitude setting) and rapidly fatal disease like swine-influenza.