College Answer

Many potential causes should be considered.  High pressure pulmonary oedema (fluid overload: CPAP/PEEP, diurese, fluid restrict, remove blood; cardiac failure: diurese, vasodilate ± inotropes; acute ischaemia: nitrates, morphine ± betablockers, anticaogulants).  Low pressure pulmonary oedema/ARDS  (CPAP/PEEP, fluid restrict, diurese, treat underlyimg cause eg. sepsis).   Diffuse pneumonia (diffuse typical or atypical: CPAP/PEEP, likely to need invasive investigation [eg. lavage], diurese and fluid restrict, specific treatment [anti-agent therapy] according to underlying cause: bacteria [eg. strep or TB], viral [eg. CMV/influenza/SARS], protozoal [eg. pneumocystis], fungal [eg. cryptococcus]). Others: could uncommonly also be malignant (eg. Karposi’s sarcoma), pulmonary haemorrhage (eg. if low platelets: consider platelet transfusion) or auto- immune/vasculitic (consider steroids, immunosuppression).

Discussion

A good free full-text article is available to cover this terrain. In it, there is an excellent table, "Aetiology of pulmonary infections in HIV-infected patients". It also presents the incidence of the aetiology. Turns out, in 97% of cases the pulmonary infiltrates are infectious in nature. The bacterial pathogens are surprisingly mundane- its S.pneumoniae, H.influenzae and Legionella. Together they cover something like 60% of pulmonary infections. Pneumocystis accounts for another 20%, and viruses for 5%. Other fungal infections are surprisingly rare - 2% - and protozoal parasites represent only 0.5%.

Causes of diffuse pulmonary infiltrates in the AIDS patient, and a brief note on their specific treatment

Non-infectious:

• Pulmonary oedema - PEEP and preload reduction
• Diffuse alveolar haemorrhage - correction of coagulopathy
• Malignant (eg. lymphangitis carcinomatosis) - dexamethasone
• Autoimmune (vasculitis) - high dose steroids
• Inflammatory - ARDS - lung-protective ventilation

Infectious:

• Bacterial:
  • Streptococcus pneumoniae - ceftriaxone
  • Mycobacterium tuberculosis - standard cocktail
  • Mycoplasma pneumonia - azithromycin
  • Generally speaking, broad spectrum antibiotics which are narrowed when the pathogen is isolated
• Viral
  • CMV - ganciclovir or foscarnet
  • VZV - acyclovir
  • HSV - acyclovir
  • Influenza - possibly oseltamivir
  • Human metapneumovirus - supportive management
• Fungal:
  • Pneumocystis jirovecii - cotrimoxazole
    • This is the second most common cause of pneumonia, behind S.pneumoniae
  • Cryptococcus - fluconazole or amphotericin
• Protozoal:
  • Toxoplasma gondii - pyrimethamine plus cotrimoxazole or sulfadiazine