Pruritus: A Comprehensive Guide for MCCQE1

Introduction

Pruritus (itching) is defined as an unpleasant sensation that provokes the desire to scratch. It is one of the most common dermatologic complaints encountered in Canadian primary care and internal medicine. For MCCQE1 preparation, it is crucial to distinguish between pruritus associated with primary skin lesions and pruritus on non-diseased skin (pruritus sine materia), which often signals an underlying systemic disease.

Understanding the CanMEDS roles, particularly Medical Expert (diagnostic reasoning) and Communicator (addressing patient quality of life), is essential when managing this condition.

MCCQE1 Objectives

According to the Medical Council of Canada, a candidate should be able to:

1. Identify the common causes of generalized and localized pruritus.
2. Differentiate between dermatologic causes and systemic causes (e.g., renal, hepatic, endocrine, hematologic).
3. Propose an appropriate investigation plan for a patient presenting with generalized pruritus without a primary rash.
4. Construct a management plan including non-pharmacologic and pharmacologic interventions.

Pathophysiology and Classification

Pruritus is transmitted via unmyelinated C-fibers. It can be classified into four clinical categories:

1. Pruritoceptive: Originating in the skin (e.g., scabies, urticaria).
2. Neuropathic: Due to damage of afferent pathways (e.g., post-herpetic neuralgia, notalgia paresthetica).
3. Neurogenic: Central origin without neural damage (e.g., cholestasis, opioids).
4. Psychogenic: Associated with psychiatric disorders (e.g., delusional parasitosis).

Clinical Approach to Pruritus

A systematic approach is vital for the MCCQE1. Use the following steps to evaluate a patient.

Differential Diagnosis

For MCCQE1, you must categorize differentials efficiently.

Mnemonic for Systemic Causes

Use the mnemonic H-I-T-C-H to recall systemic causes for the MCCQE1:

H - Hematologic (Polycythemia Vera, Lymphoma, Iron deficiency)
I - Infection (HIV, Parasites) / Idiopathic
T - Thyroid / Tumor (Malignancy)
C - Chronic Renal Failure / Cholestasis
H - Hepatic disease / Hydroxyethyl starch

Red Flags and Warning Signs

Management

Management involves treating the underlying cause and symptomatic relief.

1. General Measures (Non-Pharmacologic)

• Moisturize: Liberal use of emollients (e.g., petrolatum, urea-based creams) immediately after bathing.
• Bathing: Tepid water, limit duration, use mild soap-free cleansers (syndets).
• Environment: Humidifiers in the winter (crucial in Canadian climate).
• Clothing: Loose-fitting cotton; avoid wool and synthetics.

2. Pharmacologic Therapy

Canadian Guidelines & Context

• Canadian Dermatology Association (CDA): Emphasizes the “Soak and Seal” method for xerosis—bathing followed immediately by moisturizer application while skin is damp.
• Choosing Wisely Canada: Avoid testing for IgE mediated food allergies in the investigation of chronic urticaria or generalized pruritus unless there is a clear history of immediate reaction to food.
• Indigenous Health: Be vigilant for Scabies and Impetigo in overcrowded housing conditions; approach with cultural safety and consider treating household contacts simultaneously.

Study Checklist

Use this task list to ensure you have covered the essentials for your exam:

• Understand the physiology of the “itch-scratch” cycle.
• Memorize the systemic causes of pruritus (HITCH mnemonic).
• Review the presentation of Scabies and its management (Permethrin 5%).
• Know the first-line treatment for Uremic Pruritus (Phototherapy/Gabapentin/Emollients).
• Recognize the signs of Lymphoma (B-symptoms + Itch).

Sample Question

Case Scenario

A 62-year-old male presents to his family physician complaining of generalized itching for the past 4 months. He reports the itching is most severe after taking a hot shower. He denies any new soaps, detergents, or medications. Review of systems is positive for occasional headaches and a feeling of “fullness” in his head. He has no known allergies.

On physical examination, the skin appears plethoric (ruddy complexion) with scattered excoriations but no primary rash. The spleen is palpable 4 cm below the left costal margin.

Question

Which of the following is the most appropriate initial diagnostic test?

• A. Skin biopsy of an excoriated lesion
• B. Serum IgE levels
• C. Complete Blood Count (CBC)
• D. TSH and T4 levels
• E. Urea and Creatinine

Explanation

The correct answer is:

• C. Complete Blood Count (CBC)

Detailed Explanation: The patient presents with Aquagenic Pruritus (itching triggered by water), a classic sign of Polycythemia Vera (PV), a myeloproliferative neoplasm. The “ruddy” complexion (plethora) and splenomegaly further support this diagnosis. A CBC is the most appropriate initial test, which would likely show elevated hemoglobin, hematocrit, and possibly thrombocytosis or leukocytosis.

• Option A: Skin biopsy is not indicated as there are no primary skin lesions, only secondary excoriations.
• Option B: Serum IgE is useful for atopic conditions, which is unlikely given the age of onset and specific trigger (water).
• Option D: While hypothyroidism can cause pruritus (via xerosis), the specific constellation of aquagenic pruritus and splenomegaly points strongly to a hematologic cause.
• Option E: Renal failure causes uremic pruritus, but splenomegaly and plethora are not associated features.

Key Points to Remember for MCCQE1

1. Diagnosis of Exclusion: Generalized pruritus without rash requires a systemic workup.
2. Scabies: Always consider scabies in persistent pruritus, especially if nocturnal or if family members are affected.
3. Hodgkin’s Lymphoma: Chronic pruritus may be the presenting symptom in up to 30% of cases.
4. Elderly Patients: The most common cause is dry skin (xerosis); however, sudden onset pruritus in the elderly should prompt a malignancy screen.
5. Psychogenic: Diagnosis of exclusion (e.g., Delusional Parasitosis); do not label a patient psychogenic without ruling out organic causes.

References

1. Medical Council of Canada. Objectives for the Qualifying Examination. Available at: mcc.ca 
2. Canadian Dermatology Association. Clinical Practice Guidelines. Available at: dermatology.ca 
3. DynaMed. Pruritus. EBSCO Information Services. Accessed via Canadian Medical Association (CMA) portal.
4. Toronto Notes 2024. Dermatology Chapter. Toronto Notes for Medical Students, Inc.
5. Butler, D.F. Pruritus and Systemic Disease. Medscape Drugs & Diseases. Updated 2023.