URINARY FREQUENCY

ROBERT G. BOLTE, MD

Urinary frequency is a symptom of several commonly encountered clinical pediatric problems such as urinary tract infection (UTI), urethritis, vulvovaginitis, diabetes mellitus (DM), drug side effect (with caffeine, theophylline, and diuretics), or psychogenic stress. Moreover, urinary frequency may suggest underlying disease processes with life-threatening potential such as diabetic ketoacidosis, diabetes insipidus (DI), or congenital adrenal hyperplasia that require emergent diagnosis and management. Therefore, an organized approach in the emergency department (ED) evaluation of this symptom is important for any clinician providing acute care to children.

Urinary frequency (pollakiuria) is defined as an increase in the number of voids per day. It is a symptom distinct from polyuria (excretion of excessive amounts of urine). Although the two symptoms can be related, most children who present to the ED with frequency have a normal daily urine output, although the individual voids are frequent and small. Frequency is also distinct from enuresis, which is defined as inappropriate urination at an age when bladder control should be achieved.

PATHOPHYSIOLOGY

More than 90% of newborns void during the first day of life. Infants void between 6 and 30 times each day. Over the next 2 years, the number of voidings per day decreases by about half, whereas the volume of urine produced increases fourfold. Children between 3 and 5 years of ages average 8 to 14 voids per day. By 5 years of age, the number of voids decreases to 6 to 12 times per day. Adolescents average 4 to 6 voids per day. In the school-aged population, urinary frequency is usually defined as voiding more often than every 2 hours.

Normal bladder mucosa is both pressure and pain sensitive. An uncomfortable sensation is produced when urine volume approaches the age-dependent capacity of the bladder. Voiding is initiated by relaxation of the striated muscles of the urinary sphincter. There is an associated contraction of the smooth muscle of the bladder, resulting in bladder emptying. This mechanism is mediated by sacral nerves II to IV. Uncontrolled, “uninhibited” bladder contractions are the normal mechanism for infant and toddler voiding. Uninhibited (parasympathetic-mediated) bladder contractions do not normally occur after toilet training. By 5 years of age, 90% of children have achieved direct voluntary mastery of the voiding reflex and exhibit the adult pattern of urinary control.

Urinary frequency may be caused by reduced bladder capacity, polyuria, or psychological stress. The urinary volume per voiding will be low if frequency is related to reduced bladder capacity or psychological stress. Moreover, there will not be associated polydipsia. If frequency is secondary to polyuria, the urine volume per voiding will be normal or high, and there usually is associated polydipsia (see Chapter 59 Polydipsia).

A reduced bladder capacity may also be secondary to inflammation of the bladder, changes in the bladder wall induced by distal obstruction, or extrinsic masses pressing on the bladder. When the bladder is inflamed, its pain/pressure sensitivity threshold is markedly decreased, so less stimuli are necessary to initiate the urge to void.

Distal infravesical obstruction leads to bladder muscle hypertrophy because of the increased effort needed to empty the bladder. This hypertrophied muscle has a higher resting tone, so smaller than normal urine volumes are necessary to initiate the desire to void. A decrease in the size and force of the urinary stream and/or straining to urinate may be noted. Eventually, the bladder muscle fatigues and cannot empty the bladder effectively. This decompensated bladder has an increased residual urine volume with a resultant decrease in the functional bladder capacity. This large, hypotonic bladder contracts poorly, resulting in small, frequent voids.

Extrinsic extravesical masses that impinge on the bladder may cause frequency by mechanically interfering with normal bladder expansion. Extrinsic masses may also stimulate frequent voiding by causing an irritable focus in the bladder wall.

Normal pediatric values for urine output are useful in determining the presence of polyuria. The traditional definition of polyuria is a urinary output of more than 900 mL/m²/day. An infant/toddler up to 2 years of age rarely exceeds 500 mL per day. Children 3 to 5 years of age void up to 700 mL per day. Children 5 to 8 years of age have an approximate maximum volume of 1,000 mL per day. Children 8 to 14 years of age void up to 1,400 mL per day. When polyuria is the cause of urinary frequency, the urine volume per void generally is more than 2 mL per kg.

Polyuria with dilute urine is classically associated with a decreased production of antidiuretic hormone or with impaired renal responsiveness to circulating antidiuretic hormone. Polyuria with dilute urine can also be seen when the stimulus for antidiuretic hormone release is absent (e.g., chronic water overloading). In all these situations, the specific gravity of urine seldom is greater than 1.005 and urine osmolality rarely exceeds 200 mOsm per kg. This contrasts with a normal urinary concentrating ability, which is confirmed by a specific gravity of greater than 1.020.

Polyuria with isotonic or slightly hypertonic urine occurs with an osmotic or solute diuresis. Unlike a water diuresis, there are increases in both urine flow rate and solute excretion. The urine osmolality is never lower than 300 mOsm per kg. However, the specific gravity of urine is variable, ranging from 1.010 when the solute is primarily electrolytes and urea (e.g., renal failure, administration of diuretics) to as high as 1.045 when the solute mass is large (e.g., DM, intravenous contrast agents).

Psychogenic/emotional stress may also induce urinary frequency. Cystometric studies have documented significant anxiety-related increases in intravesical pressure, usually accompanied by a desire to void.

DIFFERENTIAL DIAGNOSIS

A differential diagnosis of urinary frequency is outlined in Table 74.1. In-depth discussions of many of these subjects can be found in other chapters of this textbook (in particular, see Chapters 26 Fever, 33 Hypertension, 52 Pain: Dysuria, 59 Polydipsia, 88 Fever in Children, 92 UTI, Febrile, 97 Endocrine Emergencies, 100 Gynecology Emergencies, 108 Renal and Electrolyte Emergencies, 127 Genitourinary Emergencies, 134 Behavioral and Psychiatric Emergencies). The following discussion highlights selected topics in the differential diagnosis.

Frequency is often associated with UTIs; therefore, this diagnosis must always receive significant consideration in the differential, particularly in the febrile female patient younger than 2 years (see Chapters 26 Fever, 92 UTI, Febrile). Accurate diagnosis of pediatric UTI is important to ensure both appropriate initial treatment and follow-up evaluation.

The term urethral syndrome refers to an entity that can be seen in female adolescents, characterized by acute onset of frequency and dysuria with “insignificant” bacterial counts (less than 10⁵ per mL). Pyuria is generally, but not absolutely, present. Vaginitis is a common cause of the urethral syndrome. Chlamydia trachomatis is also a relatively common etiology. The urethral syndrome can also occasionally be associated with Neisseria gonorrhoeae.

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