Diagnostic Considerations
Conditions to consider in the differential diagnosis of sinus node dysfunction (SND) include the following:
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Carotid sinus hypersensitivity
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Neurocardiogenic syncope with a predominant cardioinhibitory component
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Physiologic normal bradycardia, especially among highly conditioned athletes
Clinical approach to the diagnosis
There are no standardized criteria for establishing a diagnosis of SND, and the initial clues to the diagnosis are most often gleaned from the patient’s history. However, the symptoms of SND are nonspecific and the electrocardiographic (ECG) findings may not be diagnostic. Hence, the key to making a diagnosis of SND is to establish a correlation between the patient's symptoms and the underlying rhythm at the time of the symptoms.
Patients may present with symptoms of fatigue, lightheadedness, presyncope, syncope, dyspnea on exertion, chest discomfort, and/or palpitations. A routine ECG and/or ambulatory ECG monitoring may confirm the diagnosis if typical ECG findings (eg, one or more periods of sinus bradycardia; sinus pause, arrest, and sinoatrial [SA] exit block; or alternating bradycardia and atrial tachyarrhythmias) can be correlated with the symptoms. In some patients, however, additional diagnostic testing may be required, and SND should not be diagnosed until any potentially reversible causes have been identified and treated.
Following a comprehensive history and physical examination, a resting 12-lead ECG, review of previous medical records and ECG tracings, and exercise stress testing are the keys to making a diagnosis of SND and establishing a symptom-rhythm correlation. A detailed history and ECG findings during symptoms are often sufficient to diagnose SND. Careful review of prior records, in particular previous ECG tracings, can provide subtle clues to changes in the ECG over time. For patients with clinically suspected SND in whom the diagnosis remains uncertain following the initial ECG, exercise stress testing is advised.
Carefully evaluate for potentially reversible causes and medication use to exclude remediable etiologies for apparent SND. In patients with medication use (eg, beta blockers, calcium channel blockers, digoxin, antiarrhythmics, and acetylcholine esterase inhibitors) suspected to result in symptomatic bradycardia, the patient should remain on an ECG monitor while the medications are withdrawn. If symptoms and ECG abnormalities persist following the withdrawal of the medications (ie, after 3-5 half-lives), then SND/sick sinus syndrome (SSS) may be diagnosed.
If the diagnosis of SND cannot be definitively diagnosed following a thorough history and physical examination, an initial 12-lead ECG, and/or an ambulatory ECG monitoring [Holter] for 1 to 14 days, perform an event monitor for up to 4 weeks to identify symptomatic episodes of arrhythmias and to monitor average heart rates over extended periods of surveillance.
Differential Diagnoses
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This 12-lead electrocardiogram (ECG) is from an asymptomatic girl aged 10 years, which was brought to our attention because of the irregularity of the P-P intervals. This ECG shows sinus arrhythmia at a rate of 65-75 beats per minute. The P waves all originate from the sinus node (SN) because they have a positive axis (upright) in leads I, II, and aVF. The PR interval is 104ms, and the QRS is narrow at 86ms, with a normal axis of 64°. The corrected QT (QTc) interval measures 402ms. Therefore, this is a normal ECG.
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Below is an electrocardiogram (ECG) of a girl aged 2 years who was referred to the clinic by a pediatrician for evaluation of a heart murmur. This ECG shows atrial rhythm originating most likely from the lower left atrium (P waves are inverted in lead I and are positive in II and aVF, with a frontal axis of 124°). The PR interval measures 113 ms, and the QRS is narrow at 90 ms. Right ventricular (RV) conduction delay is shown and is best seen in the precordial leads V1 and V2. The QRS frontal axis shows right axis deviation (reference range for a child aged 2 years is 0-110°). The patient does not have RV hypertrophy by voltage criteria. The inverted T waves in V1 are a normal finding at this age. An echocardiogram showed a moderately sized atrial septal defect. Nonsinus atrial rhythm is not a synonym of sinus node dysfunction.
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This is a 12-lead electrocardiogram (ECG) from a boy aged 12 years with a history of syncope. This patient was healthy until 1 month earlier, when he started to experience episodes of lightheadedness. The ECG shows sinus arrhythmia (bradycardia) at a rate of 50-79 beats per minute, with a PR interval of 136 ms. Two junctional escape beats are present after a prolonged pause. The QRS is narrow at 85 ms, with a normal frontal axis of 70°. The corrected QT interval (QTc) is 411 ms. A later electrophysiologic study showed prolonged sinus node recovery time (SNRT) and sinoatrial conduction time (SACT). Because of the patient's symptoms and his sinus node (SN) dysfunction, he received an atrial pacemaker. If this 12-lead ECG had been recorded from an asymptomatic patient, the findings would be considered within normal limits and no further workup would be indicated. In this case, the lightheadedness and, ultimately, the syncope defined sick sinus syndrome, with the patient requiring pacemaker therapy.
