NOCTURIA NOCTURNAL POLYURIA

Nocturia and nocturnal polyuria : Nocturia  is common and bothersome (sleep disturbance).
Prevalence of nocturia : men 40% aged 60 70yrs, 55% aged >70yrs; women 10% aged 20 40yrs, 50% aged >80yrs. Nocturia  is associated with a 2-fold increased risk of falls and injury in the ambulant elderly.
Men who void more than twice at night have a 2-fold increased risk of death (possibly due to the associations of nocturia with endocrine and cardiovascular disease).

The diagnostic approach to the patient with nocturia
Nocturia can be due to urological disease, but more often than not is non-urological in origin. Therefore  approach the lower urinary tract last’ (Neil Resnick, Professor of Gerontology, Pittsburgh13).

Causes of nocturia
Urological: benign prostatic obstruction, overactive bladder, incomplete bladder emptying.
Non-urological: renal failure, idiopathic nocturnal polyuria, diabetes mellitus, central diabetes insipidus, nephrogenic diabetes insipidus, primary polydipsia, hypercalcaemia, drugs, autonomic failure, obstructive sleep apnoea.

Assessment of the nocturic patient
Ask the patient to complete a frequency volume chart (FVC) a voiding diary that records time and volume of each void over a 24-h period for 7 days. This establishes:

- If the patient is polyuric or non-polyuric?
- If polyuric, is the polyuria present throughout 24h or is it confined to night-time (nocturnal polyuria)?

Polyuria is defined empirically as >3L of urine output per 24h (Standardisation Committee of ICS 2002).

Nocturnal polyuria is empirically defined as the production of more than one-third of 24-hr urine output between midnight and 8 a.m. (It is a normal physiological mechanism to reduce urine output at night. Urine output between midnight and 8 a.m. one-third of the 24-h clock should certainly be no more than one-third of 24-h total urine output and, in most people, will be considerably less than one-third.)

Polyuria (urine output of >3L per 24h) is due either to a solute diuresis or a water diuresis. Measure urine osmolality: <250mosm/kg = water diuresis, >300mosm/kg = solute diuresis. Excess levels of various solutes in the urine, such as glucose in the poorly controlled diabetic, lead to a solute diuresis. A water diruesis occurs in patients with primary polydipsia (an appropriate physiological response to high water intake) and diabetes insipidus (ADH deficiency or resistance). Patients on lithium have renal resistance to ADH (nephrogenic DI).