Pneumocystis jirovecii Pneumonia (PCP) — Complete 10-Mark MD Exam Answer

1. Introduction and Historical Background

• Fishman's, Chapter 134 | Murray & Nadel's, Chapter 123

2. Organism Biology and Morphologic Forms

• P. jirovecii lacks ergosterol in its cell membrane (unlike most fungi) - instead contains cholesterol like mammalian cells. This explains intrinsic resistance to azoles and amphotericin B.
• The cell wall is low in chitin but rich in (1,3)-β-D glucan, making this a target for echinocandins and a basis for serologic testing (serum β-D-glucan assay).
• Cannot be cultured in vitro beyond a few replication cycles - no culture-based diagnosis is possible.
• Demonstrates strict host-species specificity - P. jirovecii infects only humans.
• Fishman's, Ch. 134

3. Life Cycle

• Aerosolized mature cysts are inhaled into terminal alveoli
• Excystation releases sporozoites into lung tissue
• Sporozoites → trophozoites → binary fission (asexual proliferation)
• Two haploid trophozoites → conjugation → precyst (diploid) → mature cyst (sexual phase)
• Mature cysts are essential for transmission; organisms do not bud (distinguishing from other yeasts)
• Fishman's, Ch. 134

4. Epidemiology and Risk Factors

• CD4+ count < 200 cells/μL (accounts for ~95% of cases in adolescents/adults) - Murray & Nadel's
• CD4+ percentage < 14%
• Prior episode of PCP
• Oropharyngeal candidiasis

• Prolonged high-dose corticosteroids (≥0.3 mg/kg for ≥2 weeks)
• Hematologic malignancies (particularly ALL, lymphoma)
• Solid organ transplant recipients
• Hematopoietic stem cell transplant (HSCT) recipients
• Cytotoxic chemotherapy (cyclophosphamide, methotrexate)
• Autoimmune diseases (SLE, vasculitis, inflammatory bowel disease)
• Biologic agents: TNF inhibitors, anti-CD20 (rituximab), anti-CD52 (alemtuzumab)
• Primary immunodeficiencies (SCID, hyper-IgM syndrome)

• Fishman's, Ch. 134

5. Pathogenesis

1. Attachment and colonization: P. jirovecii trophozoites attach closely to type I pneumocytes via surface glycoproteins (major surface glycoprotein, MSG/gp120), which undergo antigenic variation to evade host immunity. This attachment is mediated by fibronectin, vitronectin, and surfactant proteins.
2. Immune evasion: The organism is extracellular and resides within the alveolar space. CD4+ T-lymphocytes are the primary defense; CD4 depletion (as in HIV) is the single most important predisposing factor. Alveolar macrophages, neutrophils, and CD8+ T cells also contribute to defense but cannot compensate alone.
3. Alveolar injury: The combination of organism burden + host inflammatory response causes:
   • Exudative alveolitis with characteristic foamy, eosinophilic, honeycomb-like alveolar exudate (containing organisms, surfactant debris, and macrophages)
   • Type I pneumocyte damage → disruption of alveolar-capillary membrane
   • Impaired surfactant production and function
   • Interstitial edema and thickened alveolar walls
   • Severe cases progress to diffuse alveolar damage (DAD) with hyaline membrane formation
4. Paradoxically, patients with very low CD4+ counts (< 50 cells/μL) may have less inflammatory injury but higher organism burden; those with a recovering immune system (after ART initiation) can develop immune reconstitution inflammatory syndrome (IRIS) - a paradoxical worsening of clinical and radiographic features.
5. (1,3)-β-D-glucan released from the cell wall into the bloodstream during active infection serves as a diagnostic biomarker and mediates pro-inflammatory cytokine release.

• Fishman's, Ch. 134

6. Clinical Features

HIV-positive patients (subacute presentation - hallmark):

• Prodrome of several weeks - a key distinguishing feature vs. bacterial pneumonia (3-5 days)
• Classic triad: Fever + nonproductive cough + progressive dyspnea on exertion
• High fever, rigors, purulent sputum, and pleuritic chest pain are uncommon - their presence suggests bacterial pneumonia
• Physical examination: lungs may be normal; when abnormal, inspiratory crackles are the most common finding (associated with greater severity and increased mortality)
• Oxygen desaturation on exertion - sensitive but nonspecific indicator

HIV-negative patients (acute, more fulminant):

• Shorter prodrome (days rather than weeks)
• More severe hypoxia at presentation
• Higher ICU admission rates and mortality
• Murray & Nadel's, Ch. 123

7. Laboratory and Imaging

Laboratory:

• Serum LDH: Elevated in PCP (nonspecific - does not establish or exclude diagnosis)
• Serum (1,3)-β-D-glucan: Sensitive but not specific (elevated in other fungal infections); useful as screening/supportive tool
• ABG: Hypoxemia (PaO₂ < 70 mmHg in severe disease); widened alveolar-arterial (A-a) gradient > 35 mmHg defines severe disease requiring adjunctive corticosteroids
• S-LDH and β-D-glucan combined improve diagnostic accuracy

Chest Radiograph (CXR):

• Classic: Bilateral symmetrical reticular or granular opacities beginning in the perihilar region and extending peripherally
• May be normal in 0-39% of cases despite active infection - normal CXR does not exclude PCP
• Thin-walled cysts (pneumatoceles): Seen in 10-20% of cases - single or multiple; can be large; predispose to spontaneous pneumothorax (a classic complication of PCP)
• Less common: unilateral/asymmetrical opacities, lobar consolidation, nodules (±cavitation), miliary pattern, upper lobe predominance (particularly with aerosolized pentamidine prophylaxis breakthrough)

High-Resolution CT (HRCT):

• Bilateral ground-glass opacities - the hallmark CT finding; typically perihilar/central
• Useful when CXR is normal but clinical suspicion is high
• Can demonstrate pneumatoceles not visible on CXR

• Murray & Nadel's, Ch. 123 | Fishman's, Ch. 134

8. Diagnosis

Staining Methods:

Specimen Sources (in order of increasing invasiveness):

1. Induced Sputum (IS): First-line non-invasive approach. Sensitivity 50-90% in HIV-positive patients; lower in HIV-negative. A negative result mandates bronchoscopy.
2. Bronchoalveolar Lavage (BAL): Standard of care diagnostic procedure. Sensitivity ~97% for HIV-positive patients. Should be performed in the most affected lobe on CXR; if diffuse disease, right middle lobe; if upper-lobe predominant (pentamidine breakthrough), lavage both an upper lobe and RML. BAL + DFA staining is the diagnostic cornerstone.
3. Oral Wash + PCR: Sensitivity 54-100% in HIV-positive patients; 44-100% in HIV-negative. Less invasive than bronchoscopy; used when bronchoscopy is unavailable or high-risk.
4. Transbronchial Biopsy: Added diagnostic value when BAL is equivocal; rarely necessary alone. Sensitivity 43-96%.
5. Open/Surgical Lung Biopsy: Rarely performed today; reserved for cases with multiple potential pathogens.

Consensus Diagnostic Criteria (EORTC/MSGERC):

• Proven PCP: Detection of organism by staining in BAL, sputum, or lung tissue
• Probable PCP: ≥1 host criterion + clinical features (fever, cough, dyspnea) + suggestive imaging + positive non-stain test (β-D-glucan or PCR) - Fishman's, Ch. 134

9. Treatment

First-Line Treatment:

• TMP: 15-20 mg/kg/day
• SMX: 75-100 mg/kg/day
• Divided into 3-4 daily doses
• IV route: recommended for moderate-severe PCP (can transition to oral after clinical improvement)
• Mechanism: inhibits folate synthesis (TMP inhibits dihydrofolate reductase; SMX inhibits dihydropteroate synthase)

• Rash (common; can be severe: Stevens-Johnson syndrome, toxic epidermal necrolysis)
• Fever, GI complaints (nausea, vomiting)
• Elevated aminotransferases
• Hyperkalemia (TMP blocks ENaC)
• Bone marrow suppression (anemia, neutropenia) - typically in week 2

Alternative Regimens (for TMP-SMX intolerance/allergy):

Adjunctive Corticosteroids (exam-critical):

• PaO₂ < 70 mmHg on room air, OR
• A-a gradient > 35 mmHg

• Days 1-5: Prednisone 40 mg twice daily
• Days 6-10: Prednisone 40 mg once daily
• Days 11-21: Prednisone 20 mg once daily

• Murray & Nadel's, Ch. 123 | Fishman's, Ch. 134

10. Prophylaxis

Primary Prophylaxis Indications (HIV):

• CD4+ count < 200 cells/μL OR CD4+ percentage < 14%
• History of oropharyngeal candidiasis
• Irrespective of ART status

Discontinuation of Prophylaxis:

• When CD4+ count rises to > 200 cells/μL for at least 3 months as a result of ART
• Can consider discontinuation at CD4+ 100-200 cells/μL if HIV RNA is undetectable for ≥3-6 months

Secondary Prophylaxis:

• Any patient with prior PCP episode - continue indefinitely until CD4+ > 200 cells/μL for ≥3 months

Drug Options:

Prophylaxis in HIV-Negative Immunocompromised Patients:

• TMP-SMX (one SS or DS tablet daily/3x weekly) is recommended for solid organ transplant recipients, HSCT recipients, patients on high-dose steroids + T-cell depleting agents, and those on anti-CD20/CD52 biologics.
• Murray & Nadel's, Ch. 123 | Fishman's, Ch. 134

11. IRIS and Special Considerations

Summary Box (High-Yield for Exams)