What is the differential diagnosis for a hard, painful bump with a small amount of pus on the top of the head of an elderly patient?

Differential Diagnosis for Hard, Painful Scalp Bump with Purulent Drainage in Elderly Patient

The most likely diagnosis is a bacterial skin abscess (furuncle or carbuncle), which requires incision and drainage as primary treatment, with antibiotics reserved for patients with systemic signs of infection or significant comorbidities. 1

Primary Differential Diagnoses

Bacterial Skin Abscess/Carbuncle (Most Likely)

• Furuncles are infections of hair follicles caused by Staphylococcus aureus that extend through the dermis into subcutaneous tissue, forming small abscesses 1
• Carbuncles involve several adjacent follicles creating a coalescent inflammatory mass with pus draining from multiple openings, most commonly developing on the back of the neck in individuals with diabetes 1
• Clinical presentation matches: hard nodule, painful, purulent drainage, rapid onset 1
• Incision and drainage is the recommended treatment for carbuncles, abscesses, and large furuncles 1
• Gram stain and culture are recommended for carbuncles and abscesses, though treatment without these studies is reasonable in typical cases 1

Infected Epidermoid Cyst

• Can present as inflamed, painful bump with purulent drainage 1
• However, Gram stain and culture of pus from inflamed epidermoid cysts are NOT recommended 1
• Treatment is incision and drainage 1
• Less likely given the "overnight" appearance, as cysts typically have pre-existing history

Folliculitis (Less Likely Given Size)

• More superficial infection with pus limited to epidermis 1
• "Dime-size" suggests deeper involvement than simple folliculitis 1
• Often responds to antiseptics and topical antibacterials 2

Critical Considerations in Elderly Patients

When to Add Antibiotics to Incision and Drainage

Antibiotics directed against S. aureus should be added based on presence of SIRS criteria: 1

• Temperature >38°C or <36°C
• Tachypnea >24 breaths/minute
• Tachycardia >90 beats/minute
• White blood cell count >12,000 or <400 cells/µL

Additional indications for antibiotics in elderly: 1

• Markedly impaired host defenses
• Presence of SIRS
• Multiple comorbidities (diabetes, immunosuppression)

MRSA Coverage Considerations

• An antibiotic active against MRSA is recommended for patients with carbuncles or abscesses who have markedly impaired host defenses and SIRS 1
• Elderly patients commonly have risk factors requiring MRSA coverage 2

Less Likely but Important Differential Diagnoses

Cellulitis/Erysipelas

• Presents with erythema, warmth, and tenderness 2
• Typically lacks the discrete, hard, purulent nodule described 2
• Should be clinically distinguished from necrotizing fasciitis, which is life-threatening and requires surgical debridement 2

Secondary Infected Dermatosis

• Elderly patients have increased skin dryness and pruritus predisposing to secondary infections 2
• S. aureus and beta-hemolytic streptococci are most common causative organisms 2

Herpes Zoster (Early Stage)

• Can present with pain before rash appears 3
• However, purulent drainage and hard nodule are not typical features 3
• Would expect dermatomal distribution 3

Diagnostic Approach

Immediate Assessment

• Examine for systemic signs: fever, tachycardia, tachypnea, altered mental status 1
• Assess for diabetes or immunosuppression: carbuncles develop most commonly on the neck in diabetic individuals 1
• Check for surrounding cellulitis or lymphangitic spread 2
• Evaluate for multiple lesions suggesting furunculosis requiring systemic therapy 2

Laboratory Testing (When Indicated)

• Gram stain and culture of pus if performing incision and drainage 1
• Blood glucose/HbA1c if diabetes suspected 2
• Complete blood count if systemic infection suspected 1

Treatment Algorithm

Primary Treatment: Incision and Drainage

Strongly recommended for carbuncles, abscesses, and large furuncles 1

• Simply covering the surgical site with dry dressing is usually most effective 1
• Packing causes more pain without improving healing compared to sterile gauze coverage alone 1

Antibiotic Therapy Decision

WITHOUT systemic signs (stable, no SIRS): 1

• Incision and drainage alone is sufficient
• Systemic antimicrobials are usually unnecessary

WITH systemic signs or high-risk features: 1

• Add antibiotics active against S. aureus
• Consider MRSA coverage if risk factors present
• Typical duration: 5-10 days 1

Recurrence Prevention (If Applicable)

For recurrent abscesses, consider 5-day decolonization regimen: 1

• Intranasal mupirocin twice daily
• Daily chlorhexidine washes
• Daily decontamination of personal items (towels, sheets, clothes)

Critical Pitfalls to Avoid

• Do not attempt needle aspiration: ultrasonographically guided needle aspiration was successful in only 25% of cases overall and <10% with MRSA infections 1
• Do not prescribe antibiotics without drainage for abscesses requiring drainage 1
• Do not miss necrotizing fasciitis: life-threatening condition requiring urgent surgical debridement in majority of cases 2
• Do not overlook diabetes screening: carbuncles commonly occur in diabetic patients 1