Vomiting And/Or Nausea: MCCQE1 Preparation Guide

Introduction

Nausea and vomiting are among the most common presenting complaints in both primary care and emergency settings in Canada. For the MCCQE1, candidates must demonstrate the ability to differentiate between benign, self-limiting causes (e.g., viral gastroenteritis) and life-threatening emergencies (e.g., bowel obstruction, raised intracranial pressure).

As a future Canadian physician, you must apply the CanMEDS roles, particularly Medical Expert (diagnostic reasoning) and Communicator (empathetic history taking), to manage these patients effectively.

Pathophysiology Overview

Understanding the mechanism helps in choosing the correct antiemetic. The vomiting center in the medulla is triggered by four main pathways:

1. Chemoreceptor Trigger Zone (CTZ): Outside the blood-brain barrier; detects toxins, drugs (opioids, chemo), and metabolic derangements (uremia, DKA).
2. Vestibular System: Motion sickness (CN VIII).
3. Vagal Afferents: GI tract irritation (distension, infection).
4. Cerebral Cortex: Anxiety, pain, smell, increased intracranial pressure (ICP).

Clinical Approach: The MCCQE1 Framework

1. History Taking

2. Physical Examination

Focus on assessing volume status and ruling out surgical emergencies.

• Vitals: Tachycardia, hypotension (orthostatic), fever.
• Abdomen: Distension, surgical scars (adhesions), bowel sounds (high-pitched tinkering = obstruction; absent = ileus), peritoneal signs (rebound, rigidity).
• Neurological: Fundoscopy (papilledema), nystagmus, focal deficits.
• Rectal Exam: Impacted stool, occult blood.

Differential Diagnosis

The differential is broad. For MCCQE1, categorize by system to organize your thoughts.

Investigations

Laboratory

• CBC: Leukocytosis (infection/inflammation), Hemoconcentration (dehydration).
• Electrolytes:
  • Classic Vomiting Pattern: Hypokalemic, Hypochloremic, Metabolic Alkalosis.
  • Why? Loss of H+ and Cl- from stomach; renal compensation saves Na+ at expense of K+ and H+.
• BUN/Creatinine: Pre-renal azotemia (BUN:Cr ratio < 0.10 in SI units).
• Beta-hCG: Mandatory in all females of reproductive age.
• Glucose/Ketones: Rule out DKA.
• Lipase: Pancreatitis.

Imaging

• Abdominal X-ray (Series): First line for suspected obstruction (air-fluid levels) or perforation (free air).
• Abdominal Ultrasound: Biliary pathology, pyloric stenosis (infants), appendicitis (peds/pregnancy).
• CT Abdomen/Pelvis: Gold standard for obstruction, masses, diverticulitis.
• CT Head: If neurological signs present.

Management

Non-Pharmacological

• NPO: Bowel rest if obstruction or surgical abdomen suspected.
• Nasogastric (NG) Tube: Decompression for SBO or intractable vomiting.
• Rehydration:
  • Mild/Moderate: Oral Rehydration Therapy (ORT).
  • Severe: IV fluids (Normal Saline or Ringer’s Lactate). Add KCl once urine output is established.

Pharmacological (Canadian Formulary)

Canadian Guidelines

Nausea and Vomiting of Pregnancy (SOGC Guidelines)

The Society of Obstetricians and Gynaecologists of Canada (SOGC) provides specific guidelines often tested on MCCQE1:

1. Diagnosis: NVP affects 50-80% of pregnant women. Hyperemesis Gravidarum is intractable vomiting leading to weight loss >5%, dehydration, and ketonuria.
2. Management Hierarchy:
   • Lifestyle: Dietary changes (small frequent meals, ginger), acupressure (P6 point).
   • First Line Pharmacotherapy: Doxylamine/Pyridoxine (Diclectin).
   • Second Line: Dimenhydrinate, Diphenhydramine.
   • Third Line: Metoclopramide, Promethazine.
   • Fourth Line: Ondansetron (safety data generally accepted but used cautiously).

Pediatric Gastroenteritis (CPS Guidelines)

The Canadian Paediatric Society (CPS) emphasizes:

• ORT (Oral Rehydration Therapy) is preferred over IV for mild-moderate dehydration.
• Use commercial solutions (e.g., Pedialyte); avoid juices/sodas due to high osmolarity.
• Single-dose Ondansetron is effective in the ER to facilitate ORT success in children >6 months.

Key Points to Remember for MCCQE1

• Mallory-Weiss Tear: Longitudinal mucosal tear at GE junction due to forceful vomiting. Hematemesis. Diagnosis: Endoscopy. Tx: Supportive.
• Boerhaave Syndrome: Transmural esophageal rupture. Retrosternal chest pain, Hamman’s crunch (pneumomediastinum). Emergency: Surgical consult.
• Pyloric Stenosis: 2-6 week old, non-bilious projectile vomiting, “olive” mass. Labs: Hypochloremic metabolic alkalosis. Dx: Ultrasound.
• Metabolic Alkalosis Correction: Requires Volume (Na/Cl) and Potassium repletion to correct the alkalosis (“Contraction alkalosis”).

Study Checklist

Use this checklist to ensure you are ready for the exam:

• I can identify the metabolic derangement associated with prolonged vomiting.
• I know the SOGC first-line treatment for nausea in pregnancy.
• I can differentiate between bilious and non-bilious vomiting and their implications.
• I recognize the “Red Flags” requiring urgent surgical or neurological consultation.
• I am familiar with the Canadian guidelines for pediatric rehydration.

Sample Question

Clinical Scenario

A 4-week-old male infant is brought to the Emergency Department by his parents due to worsening vomiting over the past 3 days. The mother reports that the vomiting occurs immediately after feeding and has become “projectile” in nature. The vomit is non-bilious. The infant appears hungry immediately after vomiting and wants to feed again. He has had fewer wet diapers than usual. On physical examination, the infant appears mildly dehydrated. Abdominal palpation reveals a small, firm, mobile mass in the right upper quadrant.

Question

Which one of the following diagnostic modalities is the most appropriate next step to confirm the diagnosis?

• A. Upper GI series (Barium swallow)
• B. Abdominal Ultrasound
• C. Serum electrolytes
• D. Abdominal CT scan
• E. Endoscopy

Explanation

The correct answer is:

• B. Abdominal Ultrasound

Detailed Explanation

The clinical presentation is classic for Hypertrophic Pyloric Stenosis (HPS).

• Demographics: Typically presents between 2 and 6 weeks of age, more common in first-born males.
• Symptoms: Non-bilious, projectile vomiting. A key feature is the “hungry vomiter”—the infant feeds avidly despite vomiting.
• Physical Exam: The pathognomonic sign is the “olive-shaped” mass in the right upper quadrant (palpable hypertrophied pylorus).

Why B is correct: Abdominal Ultrasound is the gold standard imaging modality for diagnosing pyloric stenosis in Canada. It has high sensitivity and specificity (approaching 100%) and avoids radiation. Criteria include pyloric muscle thickness >3 mm and length >14 mm.

Why other options are incorrect:

• A. Upper GI series: Can demonstrate the “string sign,” but involves radiation and is usually reserved for cases where ultrasound is inconclusive.
• C. Serum electrolytes: While crucial for management (to identify hypochloremic metabolic alkalosis), they do not confirm the anatomical diagnosis. The question asks for the diagnostic modality to confirm the diagnosis.
• D. Abdominal CT: Unnecessary radiation; not the modality of choice for HPS.
• E. Endoscopy: Invasive and not indicated for HPS diagnosis.

References

1. Society of Obstetricians and Gynaecologists of Canada (SOGC). (2016). The Management of Nausea and Vomiting of Pregnancy. Clinical Practice Guideline No. 341.
2. Canadian Paediatric Society (CPS). (2018). Oral rehydration therapy and early refeeding in the management of childhood gastroenteritis.
3. Medical Council of Canada. (2023). MCCQE Part I Clinical Decision-Making and Multiple-Choice Question Objectives.
4. Toronto Notes. (2023). Gastroenterology & General Surgery Chapters. Toronto Notes for Medical Students, Inc.
5. UpToDate. (2024). Approach to the adult with nausea and vomiting. & Infantile hypertrophic pyloric stenosis.