Sick Sinus Syndrome: This condition is characterized by malfunction of the sinoatrial node, leading to various arrhythmias, including sinus bradycardia. The patient's age and symptoms of dizziness, possibly due to inadequate cardiac output, support this diagnosis.

Medication-induced Bradycardia: Many medications, such as beta-blockers, calcium channel blockers, and certain antidepressants, can cause bradycardia. Given the patient's age, polypharmacy is a common issue, making this a plausible cause.
Hypothyroidism: This condition can cause sinus bradycardia among other symptoms. Although not as directly linked to dizziness, it's a common condition in the elderly that can have systemic effects.
Deconditioning or Sleep Apnea: Both conditions can lead to bradycardia and may cause dizziness due to decreased physical fitness or intermittent hypoxia, respectively.

Acute Myocardial Infarction (MI) or Cardiac Ischemia: Although less common, sinus bradycardia can be a sign of inferior wall MI. Missing this diagnosis could be fatal, and the patient's age increases the risk of cardiac events.
Stoke or Transient Ischemic Attack (TIA): Dizziness can be a symptom of a stroke or TIA, and while the heart rate might not directly correlate, the combination of symptoms warrants consideration of cerebrovascular events.
Severe Hypoglycemia: This condition can cause bradycardia and dizziness. In diabetic patients, especially those on insulin or certain oral medications, hypoglycemia is a critical diagnosis not to miss.

Infiltrative Cardiomyopathies (e.g., Amyloidosis): These conditions can affect the heart's electrical conduction system, leading to bradycardia. While rare, they should be considered in the differential, especially if other systemic symptoms are present.
Neurocardiogenic Syncope: This condition involves a reflex of the nervous system that affects heart rate and blood pressure, potentially causing dizziness or fainting. It's less likely but could be considered if other diagnoses are ruled out.