What is the cause of pruritis (itching) that occurs daily with warm to hot showers and resolves 1 hour after showering in a patient with a history of ankylosing spondylitis (AS)?

Aquagenic Pruritus Associated with Polycythemia Vera

The most likely diagnosis is aquagenic pruritus secondary to polycythemia vera (PV), a myeloproliferative disorder that characteristically causes intense itching triggered by warm or hot water exposure, typically resolving within 30-60 minutes after bathing. 1

Primary Differential Diagnosis

Polycythemia Vera (Most Likely)

• Generalized pruritus exacerbated by hot baths is a characteristic feature of PV, occurring in approximately 48% of patients either at diagnosis or during disease progression. 1
• The pruritus typically begins during or immediately after warm/hot water exposure and resolves within 1 hour, exactly matching this clinical presentation. 1
• This aquagenic pruritus is often the most agonizing aspect of PV, resulting in sleep deprivation and interference with social and physical activities. 1
• The pathogenesis involves platelet-mediated release of pruritogenic substances including prostaglandins and serotonin. 1

Secondary Considerations

• Neuropathic pruritus can cause burning and stinging sensations but typically lacks the specific water-trigger pattern and time-limited resolution seen here. 2, 3
• Cholestatic pruritus from sclerosing cholangitis can worsen with hot showers, but patients with ankylosing spondylitis lack the typical liver disease association. 1
• Psychogenic or stress-related pruritus should only be considered after excluding physical causes. 1

Essential Diagnostic Workup

Initial Laboratory Testing

• Complete blood count with differential and blood film examination to identify elevated hemoglobin, hematocrit, and platelet count characteristic of PV. 4
• JAK2 mutation testing (present in >95% of PV cases) provides definitive diagnosis. 5
• Serum erythropoietin level (typically low or low-normal in PV, helping distinguish from secondary polycythemia). 1
• Ferritin level, as iron deficiency correlates with active pruritus in PV and may be pathogenetically contributory. 1

Additional Screening

• Liver function tests and renal function tests to exclude cholestatic or uremic causes. 4
• Lactate dehydrogenase and ESR if lymphoproliferative disorder is considered. 4

Treatment Algorithm for PV-Associated Aquagenic Pruritus

First-Line Therapy

• Low-dose aspirin (81 mg daily) produces prompt relief within hours in most patients with PV-associated pruritus through inhibition of platelet release of pruritogenic amines. 1
• This should be initiated immediately if PV is confirmed or strongly suspected. 1

Second-Line Options (If Aspirin Insufficient)

• Paroxetine (selective serotonin reuptake inhibitor) demonstrates response rates exceeding 80% in PV-associated pruritus. 1
• Interferon-α reduces pruritus in up to 81% of affected patients, with the added benefit of cytoreductive effects. 1, 5
• Normalization of platelet count with cytoreductive therapy may be necessary in patients who do not respond adequately to aspirin alone. 1

Adjunctive Measures

• Avoid hot baths or showers; use lukewarm water instead. 1
• Apply emollients to prevent skin dryness. 1
• Consider cooling gels (menthol-based) for symptomatic relief. 1

Critical Clinical Pitfalls

Common Diagnostic Errors

• Dismissing aquagenic pruritus as simply "dry skin" or "sensitive skin" without investigating underlying myeloproliferative disorders. 1
• Failing to recognize that PV can present with pruritus before other classic manifestations become apparent. 1
• Prescribing antihistamines as first-line therapy, which have unpredictable and variable responses in PV-associated pruritus. 1

Management Cautions

• Iron replacement therapy should not be used indiscriminately for PV-associated pruritus, as it has not been consistently effective and may worsen polycythemia. 1
• Conventional antihistamines and corticosteroids are largely ineffective for this type of pruritus. 1, 2
• Cholestyramine has limited evidence in non-cholestatic pruritus and should not be first-line. 1

Relationship to Ankylosing Spondylitis

The patient's history of ankylosing spondylitis is likely coincidental rather than causative. 1 While AS can have extra-articular manifestations including uveitis and carditis, aquagenic pruritus is not a recognized feature of spondyloarthropathies. 1 However, both conditions share inflammatory pathophysiology, and the presence of one autoimmune/inflammatory condition may slightly increase susceptibility to others. 1

Immediate Next Steps

Order CBC with differential, JAK2 mutation testing, and serum erythropoietin level immediately. 4, 5 If PV is confirmed, initiate low-dose aspirin 81 mg daily for rapid symptom control while arranging hematology referral for comprehensive management including potential cytoreductive therapy. 1, 5