Dermatology MCQ - Inflammatory Dermatoses - Subcorneal pustular dermatosis

A 60-year-old woman presents with a several-month history of a recurrent, pruritic eruption characterized by flaccid, superficial pustules arranged in annular and serpiginous patterns on her trunk and flexural areas. The pustules are easily ruptured and form superficial scale and crust. A potassium hydroxide (KOH) preparation from a pustule is negative for fungal elements. The most likely diagnosis is:

A. Generalized pustular psoriasis
B. Tinea corporis
C. Pemphigus foliaceus
D. Subcorneal pustular dermatosis (Sneddon-Wilkinson disease)
E. Acute generalized exanthematous pustulosis (AGEP)

Correct Answer: D. Subcorneal pustular dermatosis (Sneddon-Wilkinson disease)

Answer & Explanation

Explanation:

The description is classic for Subcorneal Pustular Dermatosis (SPD).

• Key Diagnostic Clues:

  • Pustule Characteristics: Flaccid, superficial pustules that are often described as "half-and-half" pustules (with pus in the lower half and clear fluid on top).

  • Configuration: A highly characteristic annular or serpiginous (circinate) arrangement.

  • Distribution: Predilection for the flexures (axillae, groin) and trunk.

  • Course: A chronic, relapsing course.

  • Negative KOH: Rules out a pustular tinea.

The other options are incorrect:

• A. Generalized pustular psoriasis: This is a much more acute and severe condition, with patients often systemically ill. The pustules are not typically flaccid and arranged in annular patterns; they arise on intensely erythematous, painful skin and become confluent.

• B. Tinea corporis: While it can be annular, it presents with a scaly, erythematous border, not with primary, flaccid pustules. A KOH preparation would be positive.

• C. Pemphigus foliaceus: This also presents with superficial flaccid blisters and erosions. However, the primary lesion is a blister that quickly ruptures to form scale-crust, not a visible pustule. The configuration is not typically annular/serpiginous. Direct immunofluorescence is positive for IgG intercellularly.

• E. Acute generalized exanthematous pustulosis (AGEP): This is an acute, severe drug reaction characterized by the rapid onset of dozens to hundreds of non-follicular pustules on a background of edematous erythema. It is not a chronic, relapsing condition and has a clear drug trigger.

Key Associations for Subcorneal Pustular Dermatosis

• Pathophysiology: An autoimmune neutrophilic dermatosis. The exact pathogenesis is unclear, but in some cases, it has been associated with autoantibodies against desmocollin 1, a desmosomal protein.

• Associated Conditions: SPD is frequently associated with monoclonal gammopathy, most commonly IgA paraproteinemia. It can also be associated with other conditions like pyoderma gangrenosum, inflammatory bowel disease, and rheumatoid arthritis. Screening for an underlying plasma cell dyscrasia (e.g., with serum protein electrophoresis) is mandatory.

• Clinical Presentation: The pustules are very superficial and rupture easily, leading to superficial crusting and scaling. New pustules often form at the edges of the resolving lesions, creating the characteristic annular pattern. It is often pruritic.

• Histopathology: The pathognomonic finding is a subcorneal pustule filled with neutrophils. The underlying epidermis is spongiotic but otherwise normal. Acantholysis (separation of keratinocytes) is typically absent or minimal, which helps distinguish it from pemphigus foliaceus.

• Differential Diagnosis: The main differentials are pustular psoriasis and pemphigus foliaceus. Direct immunofluorescence is negative in SPD, which helps rule out pemphigus.

• Prognosis: The disease is chronic and relapsing but is not life-threatening. The prognosis can be influenced by any associated underlying disease, such as multiple myeloma.

• Management:

  • First-line: Dapsone is the treatment of choice and is highly effective in most patients.

  • Second-line: Other options include retinoids (acitretin), phototherapy (PUVA), and colchicine.

  • For Refractory Cases: Immunosuppressants like methotrexate or biologic agents (e.g., TNF-alpha inhibitors, IL-1 inhibitors) may be used.