Primary nocturnal enuresis and secondary nocturnal enuresis have been considered separate entities with different causes. Robson and colleagues sought characteristics that would distinguish primary from secondary enuresis.

Children who presented at a tertiary care center for enuresis evaluation were enrolled in the study. Enuresis was defined as bed-wetting occurring at least once per month; patients with secondary enuresis had a history of nighttime dryness for at least six consecutive months. The minimum age of participants was 3.5 years. Patients were evaluated for factors such as daytime voiding habits, urinary tract infection, constipation, the age at which the child was able to void on his or her own, squatting behavior (in girls), and attention-deficit/hyperactivity disorder. Children also underwent uroflow and postvoid ultrasound studies.

A total of 170 patients were enrolled. In these patients, primary analysis showed no differences in characteristics between the children with secondary enuresis (27.6 percent) and those with primary enuresis (72.4 percent) except constipation, which was more prevalent in the primary enuresis group. Once constipation had been adjusted for, the only remaining significant difference between the two groups was the age at which the children had begun voiding on their own: patients with primary enuresis started an average of 0.22 years later than patients with secondary enuresis (2.35 versus 2.13 years, respectively). There was a high prevalence of daytime voiding disturbances in both groups.

The authors conclude that primary and secondary nocturnal enuresis have a common etiology. The causes of primary enuresis have been thought to be sleep arousal problems, small bladder capacity, and nocturnal overproduction of urine. Secondary enuresis has been thought to be caused by urge and voiding dysfunction, stress, constipation, diabetes, and sleep apnea. Relief of constipation and addressing daytime voiding disturbances may be effective in treating both conditions, obviating the need for medications or enuresis alarms.