Is the bladder affected by ALS?

The bladder is composed primarily of smooth muscle, which is not controlled by motor neurons. The sphincter muscles also consist predominantly of muscle tissue that is not controlled by motor neurons. Therefore, ALS usually does not cause urinary incontinence (nor fecal incontinence).

Despite this general exclusion of the bladder and sphincter muscles, bladder dysfunction can still occur in certain situations. The bladder is closely related to the pelvic floor. The external portions of the sphincter muscles also contain muscle fibers that are controlled by motor neurons and may be affected by ALS. Due to this interaction between the bladder muscles (which are not affected by ALS) and the pelvic floor muscles (which may be affected), a small proportion of patients experience bladder dysfunction. Experience shows that this problem usually occurs in connection with pronounced spasticity of the legs, which is often associated with spasticity of the (adjacent) pelvic floor. The increased muscle tension (spasticity) of the pelvic floor leads to difficulties in emptying the bladder, which manifests as urge incontinence. Those affected experience a sudden urge to urinate that is difficult to control; due to the increased pelvic tension, urination is limited to small amounts, which are passed frequently and with greater urgency.

The discrepancy between bladder filling and the inhibition of bladder emptying (the tense pelvic floor inhibits bladder emptying) is referred to as bladder dyssynergy. Several medications are available to relax the pelvic floor and treat bladder dyssynergy, which can help alleviate bladder dysfunction.