The increase in penile circumference can be measured with a mercury-filled strain gauge whose electrical resistance is proportional to the length of the gauge. The circumference is measured by gauges at the base and at the coronal sulcus. The rigidity of the penis is recorded by assessing the force that is needed to cause it to buckle.
Normal values have been obtained for the frequency of erections during sleep, the total duration of the erections and the increase in penile circumference. Knowledge of the sleep structure is required to interpret these measurements, which should therefore only be carried out as part of polysomnography. Severe fragmentation of REM sleep, as in obstructive sleep apnoeas, may prevent erections from taking place.
Penile erection monitoring has been used to distinguish organic from psychogenic causes of impotence. In the latter, erections during sleep are retained, in contrast to organic disorders. Different patterns of abnormalities are seen in vascular and neuropathic disorders.
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