Pemfigus Vulgaris

01Pathophysiology

Pemphigus vulgaris is driven by IgG autoantibodies directed against desmoglein 3 (Dsg3), a transmembrane glycoprotein that forms the adhesive core of desmosomes, the structures responsible for cell-to-cell adhesion between keratinocytes. When these antibodies bind to Dsg3, they block its adhesive function and trigger intracellular signaling that leads to desmosome disassembly. The result is acantholysis, the loss of intercellular cohesion between keratinocytes, which creates an intraepidermal cleft.

The cleavage plane is suprabasal, meaning the split occurs just above the basal cell layer. The basal keratinocytes remain anchored to the basement membrane via hemidesmosomes but lose their connections to the cells above them. On histology, this produces the classic "tombstone" appearance, a row of rounded basal cells sitting on the dermal-epidermal junction with a cavity above them filled with acantholytic (rounded, detached) keratinocytes. This is the hallmark finding you need to recognize.

Because the cleft is high up in the epidermis, the blister roof is extremely thin, composed only of the upper epidermis without support from the basal layer. This is why blisters in pemphigus vulgaris are flaccid and rupture almost immediately, leaving behind raw, painful erosions rather than intact, tense blisters. This contrasts directly with bullous pemphigoid, where the split is subepidermal and the full-thickness epidermis forms a thick, tense blister roof.

The desmoglein compensation theory explains the distribution of lesions. Dsg3 is the dominant desmoglein in mucosal epithelium, while skin expresses both Dsg1 (predominantly in upper layers) and Dsg3 (predominantly in lower layers). In patients with antibodies against Dsg3 alone, only mucosal surfaces are affected because Dsg1 in the skin compensates for the loss of Dsg3 function. When antibodies target both Dsg3 and Dsg1, compensation fails in both locations, and the patient develops mucocutaneous disease. This concept explains why nearly all PV patients begin with oral lesions and only later develop skin blisters once anti-Dsg1 antibodies appear.

02Classification and Clinical Manifestation

Oral involvement is the presenting complaint in roughly 50 to 70 percent of patients and is present at some point in nearly all cases. The erosions are irregular, painful, and slow to heal. They lack the white reticular pattern seen in lichen planus. Any patient presenting with chronic, painful oral erosions unresponsive to standard treatment should raise suspicion for pemphigus vulgaris.

03Diagnostic Workup

The workup begins with a punch biopsy taken from the edge of a fresh blister (perilesional skin), including both the blister margin and adjacent uninvolved skin. Histopathology reveals the suprabasal cleft and tombstone row. A biopsy taken from the center of an old erosion is frequently non-diagnostic because secondary changes obscure the primary pathology; the biopsy site matters.

A second biopsy of perilesional, clinically uninvolved skin is sent for direct immunofluorescence. DIF is the gold standard because it demonstrates the intercellular IgG and C3 in the characteristic fishnet pattern. This finding is highly sensitive and distinguishes PV from bullous pemphigoid, where immunofluorescence shows linear deposition along the basement membrane zone rather than intercellular deposition. If a vignette describes intercellular IgG in a reticular pattern, the answer is pemphigus. If it describes linear IgG at the dermal-epidermal junction, the answer is pemphigoid.

ELISA testing for anti-Dsg1 and anti-Dsg3 is ordered to confirm the diagnosis and classify the subtype. These titers are also used to monitor disease activity during treatment. Rising titers often precede clinical flares, making them useful for guiding therapy adjustments.

Indirect immunofluorescence detects circulating antibodies in the patient's serum and can support the diagnosis, but DIF on tissue biopsy remains the definitive study.

04Management and Treatment

The first step when you suspect pemphigus vulgaris is to confirm the diagnosis with biopsy and DIF. Once confirmed, systemic corticosteroids are initiated for rapid disease control. Prednisone at 1 to 1.5 mg/kg/day is the standard starting dose. The goal is to halt new blister formation and allow existing erosions to heal.

Corticosteroids alone are insufficient for long-term management because the prolonged high doses required carry unacceptable toxicity (osteoporosis, diabetes, adrenal suppression, opportunistic infections). A steroid-sparing immunosuppressant must be introduced early. Current evidence, supported by the RITUX 3 trial and subsequent guidelines, positions rituximab as the preferred first-line steroid-sparing agent. Rituximab targets CD20-positive B lymphocytes, depleting the precursors of the antibody-producing plasma cells driving disease. It allows faster and more sustained steroid tapering compared to conventional immunosuppressants.

If rituximab is unavailable or contraindicated, mycophenolate mofetil or azathioprine are used. Before starting azathioprine, TPMT (thiopurine methyltransferase) enzyme levels must be checked. Patients with low or absent TPMT activity are at high risk for severe, potentially fatal myelosuppression. This is a commonly tested pharmacogenomic concept.

For refractory cases, IVIG can be added as a bridge therapy. It works by modulating Fc receptors and increasing catabolism of pathogenic IgG. Cyclophosphamide and plasmapheresis are last-resort options.

Monitoring includes periodic ELISA titers for anti-Dsg1 and anti-Dsg3. Declining titers correlate with clinical improvement and help guide steroid tapering. A rise in titers without clinical worsening may signal an impending flare, prompting preemptive treatment adjustment.

Pemphigus vulgaris is a chronic disease requiring years of therapy. Complete remission off therapy is achievable but may take 3 to 5 years or longer. Patients should be counseled about infection risk from immunosuppression, bone health from corticosteroids, and the need for regular follow-up.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common way students lose points on pemphigus vulgaris questions is by confusing it with bullous pemphigoid. Both are autoimmune blistering diseases, both involve immunoglobulin deposition, and both can present with widespread blisters. The critical distinction is the blister quality and the immunofluorescence pattern. PV gives you flaccid blisters with intercellular (fishnet) IgG; BP gives you tense blisters with linear IgG at the basement membrane. If you remember nothing else, remember: "PV = flaccid, intraepidermal, fishnet" versus "BP = tense, subepidermal, linear."

A second common trap is failing to recognize that oral erosions often precede skin involvement by weeks to months. Vignettes may present a patient with chronic, non-healing mouth sores who has been treated unsuccessfully for candidiasis or aphthous ulcers. The test-writer is checking whether you can identify pemphigus vulgaris from mucosal disease alone, before skin blisters ever appear.

Another tested concept is distinguishing PV from pemphigus foliaceus. The discriminator is straightforward: PF never involves the oral mucosa. If the stem describes a patient with flaccid blisters and crusting but explicitly states that the oral mucosa is normal, think PF (anti-Dsg1 only). If there are oral erosions, it is PV (anti-Dsg3).

Do not forget that a positive Nikolsky sign is not exclusive to PV. It can also be seen in SJS/TEN and staphylococcal scalded skin syndrome. The Nikolsky sign should raise your suspicion, but the diagnosis is confirmed by biopsy and DIF, not by a physical exam maneuver.

Finally, the pharmacogenomic pearl regarding TPMT testing before azathioprine is a frequently tested safety concept. If a vignette asks for the next best step before starting azathioprine, the answer is checking TPMT activity, not starting the drug and monitoring blood counts.

The core competency tested with PV questions is your ability to differentiate autoimmune blistering diseases based on blister morphology, cleavage plane, immunofluorescence pattern, and target antigen, and to sequence the diagnostic workup correctly (biopsy first, DIF for confirmation, ELISA for subtyping and monitoring).