03 Investigations

Routine cystometry usually demonstrates a large capacity bladder without the usual sensations during the filling phase, and filling is often stopped at 500 ml on grounds of safety, although the subject’s capacity is much greater. The patient is then unable to pass urine – technically, and what is often written in the report is that there is “no rise in detrusor pressure”. Thus little can be concluded from this result except that the patient has a large bladder capacity and cannot pass urine.

Sphincter EMG

The diagnostic investigation is urethral sphincter EMG using a concentric needle electrode, to detect a particular type of abnormal activity that sounds like helicopters or underwater recordings of whales. Despite injection of local anaesthetic first, which may itself be painful, this test is uncomfortable and carried out a few centres worldwide since it requires a special interest in urology and neurophysiology – an unusual combination! But if the EMG is highly abnormal it does offer an explanation for why someone has urinary retention and confirms the diagnosis of Fowler’s syndrome, which is a condition which responds well to sacral neuromodulation (see below). Such abnormal activity is not always easy to find and it may be necessary to adjust the position of the needle several times to locate it.

A patient was recently filmed for Channel 4 having this test and you will see she does not look to be in discomfort or unhappy. In fact when she laughs the signal becomes even more abnormal.


Details of EMG abnormality for the expert

Although the EMG activity sounds superficially like myotonia, detailed analysis show that the characteristic descending sound is due to a decelerating component of a complex repetitive discharge (Fowler, Kirby et al. 1985) (Figure 2A). When a number of generators of this type of activity are heard, the sound has been likened to that of underwater recordings of whale song (Butler 1979) Complex repetitive discharges without deceleration produce a sound like helicopters over the audio-amplifier of the EMG machine (Figure 2B). Jitter analysis of the components of the complex repetitive discharges shows that this is so low that it must be due to ephaptic transmission between muscle fibres (Fowler, Kirby et al. 1985) generating repetitive, circuitous, self-excitation.  It is this abnormal activity which is thought to prevent relaxation of the sphincter and cause urinary retention or voiding dysfunction (Fowler, Kirby et al. 1985). Other studies have demonstrated an association between complex repetitive discharges and increased post-void residual urine (Jensen and Stien 1996) or voiding dysfunction (FitzGerald, Blazek et al. 2000) whilst others point out some of this type of activity can be found in women without any urinary symptoms.

Urethral pressure profile

If the EMG abnormality is hard to find or not very pronounced, the investigator may have difficulty in answering the question “is there enough abnormality to explain this girl’s urinary retention?” The nature of EMG testing is that it can provide only limited information about the severity of an abnormality and expected resulting loss of function.

The test which tells most about function is the “urethral pressure profile”. An 8Fr urethral catheter is withdrawn at 2mm/s while infusing saline at 2ml/min, and the maximum urethral closure pressure (MUCP) measured. The test involves having a catheter inserted which is the gradually pulled out whilst the pressure is being measured. The formula of 92-age (in years) is used to derive the expected pressure based on the work of Edwards and Malvern in 1974 (Edwards and Malvern 1974). Typically women with Fowler’s syndrome are found to having resting values in excess of 100 cm of water.

Urethral ultrasound

Research in 2002 by Dr Wiseman and others in Uro-Neurology at Queen Square (Wiseman, Swinn et al. 2002) suggested that ultrasound measurement of sphincter volume was helpful to detect the hypertrophy of the striated sphincter resulting from sustained overactivity. However what we have found since is that with our current machine the test does not produce consistent results although it is very likely that with the newer machines that are becoming available, sphincter ultrasound may again prove to be a valuable test. However we continue to carry out this investigation because it gives an excellent opportunity for an expert to examine the sphincter and sometimes identify possible other causes of urinary retention.