Congenital Anomalies & Dysmorphic Features

Introduction to Dysmorphology for MCCQE1

Dysmorphology is the study of congenital birth defects that alter the shape or form of one or more parts of the body of a newborn. For MCCQE1 preparation, understanding the approach to a dysmorphic child is critical. Approximately 3-5% of Canadian newborns have a congenital anomaly. Early recognition allows for appropriate genetic counseling, prognosis determination, and management of associated complications.

This guide is structured to align with the CanMEDS framework, specifically the Medical Expert (diagnosis and management) and Health Advocate (screening and support) roles.

Classification of Anomalies

Understanding the terminology is the first step in mastering this topic for the MCCQE1. Anomalies are classified based on the mechanism of the defect.

Mechanisms of Morphogenesis

Patterns of Anomalies

• Syndrome: A pattern of multiple anomalies thought to be pathogenetically related (e.g., Down Syndrome).
• Sequence: A pattern of multiple anomalies derived from a single known prior anomaly or mechanical factor (e.g., Potter Sequence, Pierre Robin Sequence).
• Association: A non-random occurrence of multiple anomalies for which no specific etiology has been identified (e.g., VACTERL).

Clinical Approach to the Dysmorphic Child

For the MCCQE1, you must demonstrate a structured approach to data acquisition.

High-Yield Clinical Conditions for MCCQE1

The following conditions are frequently tested. Focus on the constellation of symptoms and Canadian management guidelines.

Important Sequences

Pierre Robin Sequence

• Triad: Micrognathia (small jaw) $\rightarrow$ Glossoptosis (tongue falls back) $\rightarrow$ Cleft Palate (U-shaped).
• Risk: Upper airway obstruction.
• Management: Prone positioning (gravity pulls tongue forward), nasopharyngeal airway, mandibular distraction osteogenesis in severe cases.

Potter Sequence

• Pathophysiology: Renal agenesis/dysplasia $\rightarrow$ Oligohydramnios $\rightarrow$ Fetal compression.
• Features: Pulmonary hypoplasia (cause of death), limb deformities (clubfoot), flattened facies.

Canadian Guidelines & Screening

Prenatal Screening (SOGC Guidelines)

• All pregnant women in Canada should be offered prenatal screening for aneuploidy (Trisomy 21, 18, 13).
• Options:
  1. FTS (First Trimester Screening): Nuchal translucency (NT) ultrasound + serum markers (PAPP-A, beta-hCG).
  2. NIPT (Non-Invasive Prenatal Testing): Cell-free fetal DNA in maternal blood. Higher sensitivity/specificity. Funded in many provinces for high-risk pregnancies or becoming primary screen.
  3. MSS (Maternal Serum Screen): Quad screen in second trimester if first trimester missed.

Newborn Screening

• Provincial programs screen for metabolic (PKU, MCAD), endocrine (Hypothyroidism), and hematologic (Sickle Cell) disorders.
• Critical Congenital Heart Disease (CCHD) screening via pulse oximetry is standard in Canadian hospitals pre-discharge.

Key Points to Remember for MCCQE1

• Hypotonia in a newborn + dysmorphic features $\rightarrow$ Think Down Syndrome or Prader-Willi.
• Cleft Palate usually implies a midline defect; check for cardiac and renal anomalies.
• Single Umbilical Artery (2 vessel cord) is associated with renal and cardiac anomalies.
• Fragile X Syndrome is the most common inherited cause of intellectual disability. Look for macroorchidism (post-pubertal), long face, and large ears.
• Advanced Paternal Age is associated with new dominant mutations (e.g., Achondroplasia, Neurofibromatosis).
• Advanced Maternal Age is associated with nondisjunction (Trisomies).

Sample Question

Explanation

The correct answer is:

• B. Placement of the infant in the prone position

Detailed Analysis

Diagnosis: The clinical presentation of micrognathia (small jaw), glossoptosis (posterior displacement of the tongue), and cleft palate constitutes the Pierre Robin Sequence.

• Why B is correct: The primary immediate threat to life in Pierre Robin Sequence is upper airway obstruction caused by the tongue falling back against the posterior pharyngeal wall due to the small mandible. Placing the infant in the prone position allows gravity to pull the tongue forward, relieving the obstruction in the majority of mild-to-moderate cases. This is the standard, non-invasive first-line intervention.

• Why A is incorrect: Immediate intubation is difficult in these patients due to the micrognathia and anterior larynx. It is reserved for severe obstruction that does not respond to positioning or nasopharyngeal airway placement. It is not the first step if the patient is breathing spontaneously but obstructed.

• Why C is incorrect: Surgical repair of the cleft palate is typically performed around 9-12 months of age. Doing it immediately does not solve the acute airway obstruction and is surgically unsafe in a newborn.

• Why D is incorrect: Oxygen addresses hypoxemia but does not treat the underlying airway obstruction. Without opening the airway, oxygen will not reach the lungs effectively.

• Why E is incorrect: While Pierre Robin Sequence can be associated with syndromes like Stickler syndrome or 22q11.2 deletion syndrome, genetic testing is not the immediate management priority for a compromised airway.

References

1. Canadian Paediatric Society (CPS). (2015). Fetal alcohol spectrum disorder: A guideline for diagnosis across the lifespan. Retrieved from www.cps.ca 
2. Society of Obstetricians and Gynaecologists of Canada (SOGC). Joint SOGC-CCMG Guideline: Update on Prenatal Screening for Fetal Aneuploidy, Fetal Anomalies, and Adverse Pregnancy Outcomes. J Obstet Gynaecol Can 2017.
3. Jones, K. L., et al. (2021). Smith’s Recognizable Patterns of Human Malformation (8th ed.). Elsevier.
4. Medical Council of Canada. MCCQE Part I Objectives: Congenital anomalies. Retrieved from mcc.ca .
5. Huether, S. E., & McCance, K. L. (2019). Understanding Pathophysiology (Canadian Edition). Elsevier Canada.