What is the differential diagnosis for a short PR (P-R) interval?

Differential Diagnosis for Short PR Interval

The differential diagnosis for a short PR interval (<120 ms) depends critically on whether a delta wave and QRS widening are present, with the most important distinction being between benign isolated short PR and potentially lethal Wolff-Parkinson-White (WPW) pattern. 1

Primary Categories

1. WPW Pattern (Short PR + Delta Wave + Wide QRS >120 ms)

• This is the most critical diagnosis to identify due to sudden cardiac death risk 1
• Results from an accessory pathway bypassing the AV node, causing ventricular pre-excitation 1
• The delta wave represents slurred initial QRS upstroke from early ventricular activation 1
• Occurs in approximately 1 in 250 athletes but carries risk of sudden death as first presentation 1
• Associated structural abnormalities include Ebstein's anomaly and hypertrophic cardiomyopathy 1

2. Isolated Short PR (No Delta Wave, Normal QRS)

This represents several possible mechanisms:

Enhanced AV Nodal Conduction

• Shortened A-H interval on electrophysiology studies indicates faster conduction through the AV node 2, 3
• Characterized by shortened functional refractory period of the AV node (mean 368 ± 36 ms vs normal 415 ± 50 ms) 3
• Effective refractory period of AV conducting system is also reduced (247 ± 26 ms vs normal 297 ± 51 ms) 3
• May represent partial AV nodal bypass or dual AV nodal pathways 2, 3

Lown-Ganong-Levine Syndrome

• Short PR with normal QRS and predisposition to paroxysmal supraventricular tachycardia 1, 4
• Can be familial with autosomal dominant inheritance and variable expressivity 4
• In asymptomatic patients, this requires no intervention 1, 5

Enhanced His-Purkinje Conduction

• Shortened H-V interval on electrophysiology studies 2
• Represents faster conduction through the His-Purkinje network 2

Sino-Atrial Conduction Defect

• Conduction defect in sino-atrial pathways with preserved sino-nodal conduction 6
• Rare mechanism described by Condorelli 6

3. Metabolic/Storage Diseases

Fabry Disease

• X-linked lysosomal storage disorder with glycosphingolipid accumulation in vascular endothelium 1
• Short PR interval (can be as short as 88 ms) occurs with sinus bradycardia and left ventricular hypertrophy 1
• Must be considered in patients with short PR and LVH, especially with multisystem symptoms 1
• Incidence approximately 1 in 40,000 to 60,000 males 1

Pompe Disease (Infantile)

• Short PR present in 75% of infantile cases 5
• Accompanied by extremely tall QRS complexes and cardiomegaly 5
• Critical pitfall: operators may decrease ECG gain due to high voltage, missing this diagnostic clue 5

4. Physiologic Variant

• In athletes, isolated short PR without delta wave is a normal variant requiring no evaluation 1, 5
• Results from enhanced vagal tone and training-related adaptations 1

Critical Clinical Algorithm

Step 1: Assess QRS Morphology

• If delta wave present + QRS >120 ms → WPW pattern → Mandatory comprehensive evaluation regardless of symptoms 1, 5
• If normal QRS + no delta wave → Isolated short PR → Further assessment based on clinical context 1, 5

Step 2: For WPW Pattern (Mandatory Workup)

• Detailed symptom history (palpitations, syncope, family history of sudden death) 1, 5
• Echocardiography to exclude Ebstein's anomaly and cardiomyopathy 1, 5
• Exercise stress testing for non-invasive risk stratification 1, 5
  • Low-risk features: abrupt complete loss of pre-excitation at high heart rates or intermittent pre-excitation at rest 1, 5
• Electrophysiology study if non-invasive testing inconclusive or patient is competitive athlete 1, 5
  • High-risk: shortest pre-excited RR interval ≤250 ms during atrial fibrillation → ablation indicated 1, 5

Step 3: For Isolated Short PR

• If asymptomatic athlete or patient without palpitations → No further evaluation needed 1, 5
• If symptomatic with palpitations → Consider electrophysiology study 2, 3
  • May reveal dual AV nodal pathways or predisposition to reentrant tachycardias 2
• If associated with LVH or multisystem symptoms → Consider Fabry disease 1
• If infant with cardiomegaly and very tall QRS → Consider Pompe disease and check creatine kinase 5

Common Pitfalls to Avoid

• Never dismiss WPW pattern as benign even in asymptomatic patients—sudden death can occur without warning 1, 5
• Do not confuse isolated short PR with WPW pattern—presence or absence of delta wave completely changes management 1, 5
• Ensure proper ECG calibration, especially when QRS voltage is extremely high (Pompe disease) 5
• Use age-appropriate PR interval norms, particularly in pediatric populations 5
• In patients with short PR and LVH, always consider Fabry disease before attributing findings to hypertension or athletic heart 1