Nocturia is voiding that is preceded and followed by sleep; ≥2 voids/night is clinically significant. It is associated with decreased survival and quality of life, poor sleep efficiency, depression, metabolic syndrome, and falls. A voiding diary classifies it into nocturnal polyuria, decreased bladder capacity, or global polyuria.
Diagnosis and Evaluation
- History — nocturia may signal systemic illness (hypertension, diabetes, heart/kidney disease). Drugs causing it include those raising urine output (SSRIs, tetracyclines, calcium-channel blockers, lithium, and mistimed diuretics — give mid-afternoon), CNS stimulants/insomnia-causing agents, and direct bladder toxins (ketamine, cyclophosphamide).
- Exam — peripheral edema (cardiac, nephrotic, venous) and obesity/short neck (possible obstructive sleep apnea).
- Voiding diary is the most useful tool.
Nocturnal Polyuria
Defined as nocturnal urine volume >20–33% of the 24-h total (age-dependent: ~14% at <25, ~34% at >65), >6.4 mL/kg, or >54 mL/hr.
- Causes: diuretics (timing), congestive heart failure, obstructive sleep apnea (hypoxia → ↑ ANP → ↑ sodium/water excretion; a common cause), diabetes mellitus, excessive nighttime fluids, and peripheral edema.
- Management: treat comorbidities (CPAP for OSA can improve nocturia); conservative measures (stop fluids 4 h before bed, compression stockings, mid-afternoon diuretics); and medical therapy — desmopressin (bedtime V2 antidiuresis; hyponatremia risk <1% at age <65 vs 8% at >65; melt dose 50 μg males, 25 μg females; monitor sodium baseline, day 7, day 28, then q6 months) or imipramine (use cautiously — arrhythmia, QTc prolongation, rare sudden death).
Decreased Bladder Capacity
Nocturnal urine volume exceeds nocturnal bladder capacity. The Nocturnal Bladder Capacity Index (NBCi) = actual nightly voids − predicted nightly voids, where predicted = (Nocturia Index − 1) and Nocturia Index = nocturnal urine volume ÷ maximum voided volume; NBCi >0 indicates a small nocturnal capacity. Causes (11) include stones, cystitis (bacterial/interstitial/TB/radiation), cancer, BOO, neurogenic bladder, low compliance, medications (xanthines, β-blockers), idiopathic nocturnal DO, anxiety, and learned voiding dysfunction. Management targets the cause; TURP is superior to tamsulosin for BPH-related nocturia.
Global Polyuria
24-h output >40 mL/kg causing both daytime frequency and nocturia. Causes: primary polydipsia, diabetes mellitus, and diabetes insipidus (central — ADH deficiency from hypothalamic/pituitary injury; nephrogenic — normal ADH but unresponsive kidneys).
Self-Test
1. Define nocturia and the clinically significant threshold. Voiding preceded and followed by sleep; ≥2 voids/night is significant.
2. How are the causes of nocturia categorized? Nocturnal polyuria, decreased bladder capacity, and global polyuria.
3. Give the definitions of nocturnal polyuria. Nocturnal urine >20–33% of the 24-h total (age-dependent), >6.4 mL/kg, or >54 mL/hr.
4. Medications for nocturnal polyuria, their MOA, and most serious AE? Desmopressin (V2-receptor antidiuresis; serious AE hyponatremia) and imipramine (TCA; serious AE arrhythmia).
5. Define global polyuria and list its causes. 24-h output >40 mL/kg; primary polydipsia, diabetes mellitus, diabetes insipidus.