Thursday, 13 January 2011

The penis and sexual pleasure

Now this is an interesting study: "Self-ratings of genital anatomy, sexual sensitivity and function in men using the 'Self-Assessment of Genital Anatomy and Sexual Function, Male' questionnaire" (PDF).

It's a study of self-assessed ratings of sexual pleasure, orgasm intensity, and similar items by area. The study included 81 healthy men. One weakness of the study was that only eleven men were uncircumcised; this shouldn't bias the results, but it does mean that the foreskin's scores are known with less precision than those for other areas.

The key results were as follows:

Overall discrimination between genital areas was highly significant (mixed-model anova, P = 0.001) for ratings of 'sexual pleasure', 'orgasm intensity' and 'orgasm effort', but was not significant for 'discomfort/pain'. Ranked by degree of 'sexual pleasure', the area 'underside of the glans' was highest, followed by 'underside of the penile shaft', 'upper side of the glans', 'left and right sides of the glans', 'one or both sides of the penis', 'upper side of the penile shaft', 'foreskin' (11 subjects), 'skin between the scrotum and anus', 'back side of the scrotum', 'front side of the scrotum', and 'around anus', but not all pair differences were significant. The rank order was similar for 'orgasm intensity', but less similar and with fewer significant pair differences for 'orgasm effort'.

What this means, effectively, is that the foreskin is the least sensitive of all areas of the penis, when "sensitive" is defined as "capable of producing sexual pleasure or orgasm".

This is no great surprise to me. It's entirely consistent with the bulk of evidence indicating that the foreskin can be removed without adverse effects on sexual satisfaction, which would seem inconsistent with the foreskin playing a major role in sexual pleasure. But it may come as a surprise to those who were mislead by studies such as Sorrells et al., which (erroneously) presented the foreskin as the most sensitive part of the penis. Why?

Well, as Prof. Morris and I noted (among other points) in our critique of Sorrells' paper:

The authors conclude that ‘circumcision ablates the most sensitive parts of the penis’, although they only tested the ability of subjects to detect the lightest touch. Meissner’s corpuscles, being light-touch receptors, would be expected to cause such a measurement to exaggerate the sensitivity of the prepuce. However, sensitivity, particularly when discussing erogenous sensation, depends on several different modes of stimulation and their interaction. In addition, sexual sensation depends upon the types of mechanical stimulation generated during intercourse, which might in turn be influenced by circumcision status. Thus circumcision has the potential to either increase or decrease sexual sensation.

Put bluntly, the two studies measured completely different things. Sorrells et al tested response to having a nylon filament pressed against the skin; Schober assessed response to sexual stimulation. And the two are not the same at all.

Wednesday, 12 January 2011

Circumcision, HPV, and HIV

After a period of relative quiet, I found two interesting new studies in my PubMed alert this morning.

First, Wawer et al. published Effect of circumcision of HIV-negative men on transmission of human papillomavirus to HIV-negative women: a randomised trial in Rakai, Uganda. This is a fascinating study, not least because it is yet another study cleverly piggy-backed on one of the randomised controlled trials that effectively proved that circumcision reduces the risk of HIV. Results have been published previously showing that circumcision reduced the risk of (and increased clearance of) HPV infection in men; what's unusual about this study is that it directly measured the effect on infection in women (to be precise, the female partners of the men in the trial). And as previous observational studies indicated, the risk was reduced, from 38.7% to 27.8%.

The significance of this, of course, is that HPV is the virus responsible for cervical cancer in women (and about half of penile cancers in men). According to the World Health Organisation, cervical cancer is the second biggest cause of cancer deaths worldwide, resulting in 288,000 deaths annually. Could circumcision programmes reduce those deaths by a third, as a happy side-effect of HIV prevention programmes?

It does raise an interesting ethical question, though: when weighing costs and benefits of circumcision, how much weight should be given to something that does not affect the male directly, but rather a possible partner in the future? It's a difficult question, and I don't claim to have the answers.

Moving on, Anderson et al. have published what looks to be an interesting review entitled HIV Infection and Immune Defense of the Penis. Their subject matter, of course, is the mechanism by which circumcision protects against HIV. I haven't had the opportunity to read the full text, but from the abstract it appears that they reject the traditional view of the foreskin's mucosal layer as an HIV target, instead preferring a model in which the subpreputial cavity traps HIV and HIV-infected cells, bringing them into contact with the urethra. It's an interesting hypothesis, but I personally suspect that attempts to find a single explanation are doomed to failure; I believe that the protective effect seen is a result of multiple mechanisms acting together.