What is the cause of lung butter, a condition characterized by a butter-like substance or texture in the lungs?

"Lung Butter" - Understanding Pulmonary Alveolar Proteinosis

"Lung butter" is a colloquial term referring to pulmonary alveolar proteinosis (PAP), a rare lung disease characterized by the accumulation of phospholipoproteinaceous material in the alveoli that resembles a sandy-colored or light-brown buttery substance. 1

Pathophysiology and Causes

The primary cause of pulmonary alveolar proteinosis is:

• Deficiency of granulocyte-macrophage colony-stimulating factor (GM-CSF) which leads to:
  • Excessive production of surfactant
  • Diminished clearance of surfactant by alveolar macrophages 1

The accumulated material in the alveoli:

• Contains phospholipoproteins
• Stains deep pink with periodic acid-Schiff stain
• Appears as a granular lipoproteinaceous substance on histology
• Creates a characteristic sandy-colored or light-brown fluid that can be retrieved during bronchoalveolar lavage 1

Clinical Presentation

PAP typically affects individuals in their third or fourth decade of life with:

• Nearly one-third of patients being asymptomatic or minimally symptomatic
• Initial febrile episode in symptomatic patients
• Progressive dyspnea (most common presenting symptom)
• Minimally productive cough
• Rare hemoptysis 1

Diagnostic Findings

Imaging

• Chest X-ray: Bilateral, usually asymmetric, patchy airspace consolidation, more pronounced in lower two-thirds of lungs
• Advanced cases: Dense alveolar pattern ("bat's wing pattern" or "reverse pulmonary edema pattern")
• HRCT: "Crazy paving" pattern from airspace consolidation and thickened interlobular septae 1

Pulmonary Function Tests

• Restrictive dysfunction
• Reduced carbon monoxide diffusion capacity 1

Definitive Diagnosis

• Bronchoalveolar lavage yielding sandy-colored or light-brown fluid
• Bronchoscopic or surgical lung biopsy 1

Treatment Approach

Treatment decisions should be based on disease severity:

1. Observation for mild cases

   • Spontaneous improvement occurs in 30% of cases
   • Smoking cessation may hasten resolution of cough 1

2. Whole-lung lavage

   • Current treatment of choice
   • Uses normal saline solution
   • Some patients require repeated lavages at intervals of several weeks to months 1

3. Experimental therapies

   • Granulocyte-macrophage colony-stimulating factor administration has shown promise but remains experimental 1

4. Lung transplantation

   • Reserved for patients with progressive respiratory failure 1

Important Distinctions

Do not confuse pulmonary alveolar proteinosis with:

• Pulmonary alveolar microlithiasis: Characterized by intraalveolar deposition of calcium phosphate microliths (calcispherites) 1
• Pulmonary Langerhans cell histiocytosis: Characterized by proliferation of Langerhans cells that express S100, CD1a, and Langerin (CD207) 2
• Bronchiolitis obliterans: An airway-centric disease associated with exposure to flavorings like diacetyl 3

Clinical Pearls

• PAP should be considered in patients with bilateral alveolar infiltrates and minimally productive cough
• The characteristic BAL fluid appearance is often diagnostic
• Smoking cessation is an important adjunct to treatment
• The course is variable, with some patients experiencing spontaneous resolution while others progress to respiratory failure