The epidemiology of female urinary incontinence
By Hogne Sandvik

• Prevalence
• Why do prevalence estimates differ?
• Severity of incontinence
• Prevalence of "significant" incontinence
• Types of incontinence
• References

Prevalence

Several authors have investigated the prevalence of female UI. The most striking feature of these studies is the widely differing results, even when seemingly comparable settings and age groups have been studied. Figure 1 shows the prevalences (inter-study ranges and medians) found in 13 studies of female UI in the general population. Only studies that cover an age span of at least 30 years are included.(1-13)

Figure 1 Prevalence of female UI as reported in 13 different studies of the general population. Inter-study ranges are depicted by the bars, medians by the horisontal ticks.

The prevalence peak (the age group with the highest prevalence of UI) also varies considerably between the studies. Basically, the studies fall into two categories, those reporting the highest prevalence in old age,(1-3,13) and those reporting the highest prevalence around the menopause.(4-11) "High prevalence" studies, using a lower "threshold" for identifying the women as incontinent, tend to show a prevalence peak in midlife, but this tendency is not consistent.

The prevalence of UI has also been investigated in different settings and selected subsamples. Surveys of female patients visiting their general practitioner (GP) have revealed that 43% were incontinent in USA,(14) 44% in Sweden.(15) Hellström et al. found that 87% of females living in nursing homes were incontinent,(16) while somewhat lower estimates (approximately 50%) have been reported in American studies.(17,18)

Fewer studies have addressed the question of incidence and remission of UI, that is the dynamic, natural course of the disorder. In a retrospective Danish study yearly incidences between 0.5% and 1.4% were estimated for women aged 20-59 years.(10) However, in a prospective American study of community-dwelling women aged ³60 a yearly incidence of about 20% and a remission of 12% were found.(19) Approximately similar incidences were found in a prospective study in nursing homes.(18) Burgio et al. found that during three years the cumulative incidence of regular incontinence was 8% for middle-aged women.(20)

Why do prevalence estimates differ?

Some review papers have addressed the problem of the widely differing prevalence estimates.(21-23) In addition to the biases induced by investigating different age groups or otherwise selected populations, a major problem has been the use of different definitions of "incontinence". The ICS definition implies an objective demonstration of urine loss, and has never been used in community based surveys to its full extent.(1,24)

Some authors have tried to validate survey responses by inviting subsamples to a clinical evaluation. Diokno et al. invited both continent and incontinent responders for extensive investigations and found 83% agreement between self-reports about incontinence and the clinician assessment.(25) Two Swedish studies have reported that 4.6% and 5.7% of self-reported incontinence could not be verified in the clinic.(3,26) However, due to the time lag between the surveys and the clinical evaluation, these validation exercises may not be completely accurate. Diokno et al. operated with a time lag of 3-12 months,(25) while the Swedish papers do not give information about the time lag.(3,26) The short-term variability of self reports of UI,(27) and the yearly incidence and remission rates may explain the moderate lack of validity found in these studies.(19)

Some authors have introduced a minimum frequency of leakage in order to identify regular or significant incontinence. Two or more episodes in the previous month has been used as a threshold by many authors.(2,4,6,28-30) The use of such definitions imply that many prevalence studies of incontinence are in fact studies of period prevalence,(22) and differing reference periods may thus explain differing results.(21)

Some population based studies have included institutionalized patients,(3,26) some have excluded them,(11,31) while other authors have not given information about this.(5,6) This will obviously explain some of the differing prevalences found among older women. Some surveys, mostly British, have taken their samples from lists of patients in general practice.(4,7,9,28,32) These surveys usually get excellent response rates, but unless such lists include all persons in the population, they are unsuitable sampling frames.(21)

Biased response rates may invalidate the prevalence estimates.(33) Two studies reported diminishing response rates among older women,(3,10) while two other studies found no consistent age-dependent differences in response rate.(7,11) However, such known differences between responders and non-responders may be compensated during analysis. The major problem is unknown response bias, such as the possibility of different response rates between continent and incontinent women.(33) Incontinent women may not answer (or deny UI) because of embarrassment or concomitant handicaps. But incontinent women may also find the subject particularly relevant, and therefore respond to a greater extent than continent women. At present, we don't know how these factors may affect the response rates. To minimize response bias one should always aim at the highest possible response rates.

Data have usually been gathered by post, but some authors have conducted personal interviews,(5,29,31,32,34) or telephone interviews.(6) Interview methods make it possible to explore issues in greater detail, and they generally get higher response rates than postal questionnaires.(35) On the other hand interviews are less standardized. Yarnell et al. used five interviewers, and found that the prevalence reported by each interviewer ranged from 29% to 51%.(5) There is also the possibility that responses elicited by interview are more susceptible to social desirability bias than those elicited by post.(35)

Even when all the above-mentioned differences are taken into account, some variations in the prevalence of UI remain.(11) It is quite plausible that ethnic differences may exist, and this may restrict extrapolation of prevalence estimates to other populations.(13,20,30)

Severity of incontinence

Severity of UI has been estimated by detailed questions about amount and frequency of leakage,(5,8,11,31,32) or by asking about dampening of clothes, extra laundry, restrictions in activity, or the use of protective pads.(4,6,9,10,14) The first approach may be considered a simple attempt to operationalize the quantity of urine lost, but in none of the studies has the severity measure been validated.(21) The second approach also reflects perceptional differences, personal hygiene and coping ability.

Incontinent patients find it difficult to state the amount of urine lost in terms of teaspoons, tablespoons, or cups.(36) Most people have never urinated into such measuring devices. In a focus group study incontinent women preferred questions that quantify urine loss by the number and wetness of absorbent pads used each day.(36) However, many incontinent females do not use such pads, even if they suffer from significant UI.(11,20,37)

Prevalence of "significant" incontinence

Even though the definition of "significant" incontinence varies between authors (depending on frequency and amount of leakage, soaking of clothes, use of pads etc.), its prevalence is considerably more consistent across different studies. In table 1 are listed some community based studies where the authors have specified the prevalence of "significant", "serious", "severe", or "troublesome" incontinence. Three studies (marked *) have used the ICS definition (without objectively demonstrating urine loss).

It seems that 4% - 8% of adult females suffer from severe incontinence, as defined by the different authors. Two studies found that the prevalence of significant incontinence tended to increase with advancing age,(5,38) while another found the opposite tendency.(11)

If the ICS definition is used, prevalence estimates are considerably higher, 10% - 20%. One study found a steady increase in prevalence with advancing age,(3) while another did not.(10)

Table 1 Prevalence of significant incontinence, as defined by the different authors. ^#Yarnell et al. did not give an exact estimate. *Incontinence as defined by ICS.

The use of protective pads also reflects the severity of incontinence.(20) Jolleys found that 6% of all women wore protection against leakage,(9) while Sandvik & Hunskaar found that 3% of all adult women used specially designed incontinence pads.(39) The use of pads is much more prevalent in older age groups than among younger women,(39,40) probably reflecting that UI is more severe among older women than among the younger ones.

Types of incontinence

In surveys based on questionnaires or interviews only incontinence symptoms can be registered. It is not possible to diagnose incontinence as a sign or as a condition by history alone. Diagnoses such as motor urge incontinence or genuine stress incontinence require the use of urodynamic equipment.(24)

Many investigators have tried to correlate the symptom of stress incontinence to the urodynamic diagnosis genuine stress incontinence, and the symptom of urge incontinence to the urodynamic diagnosis motor urge incontinence. Most authors conclude that patient history alone is a poor predictor of genuine stress incontinence or detrusor instability.(41-46)

Several authors have reported the prevalence of stress, urge, and mixed symptoms. In table 2 are shown the distribution of incontinence types found in ten different surveys. Diagnoses other than stress, urge, and mixed are excluded.

Table 2 Distribution of different types of incontinence in the general population. Diagnoses other than stress, urge, and mixed are excluded.

Approximately 50% are classified as stress incontinence, although this percentage is lower among the older women. The prevalence of urge incontinence seems to increase with advancing age, and motor urge incontinence (detrusor instability) is often claimed to be the most common type of incontinence in the elderly.(48-51) However, these claims are based upon studies in selected groups and probably do not reflect the epidemiology of the general population.(52,53) When Diokno et al. attempted to validate their population survey by urodynamic evaluation of a subsample, they found detrusor instability in only 12.2% of the incontinent women.(25)

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