陰莖體積描記儀
有膨脹問題的男子，也就是說勃起困難的男子，應該進行一個特殊的夜間檢測，以排除生理方面的病因。一個健康男子在睡眠期的規律性複現的所謂快速動眼睡眠期(rapid-eye-movement phase, REM)會有陰莖勃起。顯然，對夜間具有這種無意識的陰莖勃起的男子，可以排除生理方面的病因。因為在夜間的測試中，這個男子住進了睡眠測驗室，他的陰莖套連著所謂的陰莖體積描記儀(penile plethydmograph)，通過這個描記儀，記錄了睡眠期間的陰莖膨脹增加和消退的程度。第二天早晨，可以在描記圖上讀取記錄結果。如果顯示了預期的勃起記錄，說明無生理方面的病因；如果未顯示勃起或出現非常微弱的勃起，可能有某些血管方面的病症，則需要進一步的檢查和治療。

The Penile Plethysmograph
Men who have tumescence problems, i.e. erection difficulties, should undergo a special overnight test in order to rule out physical causes. Healthy men have erections in their sleep during the regularly recurring so-called REM (Rapid-Eye-Movement) phase. Obviously, in men who have these unconscious nightly erections a physical cause can be ruled out. For the overnight test, the man checks into a sleep lab. Before he goes to sleep, a  so-called penile plethydmograph is attached to his penis where it records the increasing and decreasing degrees of tumescence during sleep. The next morning, the test results can be seen in a recording chart. If it shows the expected erections, there is no physical problem. If it shows no or very weak erections, there may be some vascular problem needing further investigation and treatment.