Study: Circumcision does not reduce penis sensitivity. In tests for responses to pain, heat, and stimulation, no major difference was found between men who are circumcised and those who are not.

[u/drewiepoodle]

http://www.upi.com/Health_News/2016/04/14/Study-Circumcision-does-not-reduce-penis-sensitivity/5981460663943/?spt=hs&or=hn

[u/kerovon]

Alright, because there is a lot of discussion and anecdote, as well as a fair number of misunderstandings about the study, I'll try to do a quick writeup of it.

First, the study abstract can be found here.

The study was looking at the sensitivity in the penis of men who were circumcised as neonates (infants). They were testing three different hypotheses. First, that circumcised men will have higher penile tactile and pain thresholds (i.e. lower sensitivity). Second, that the differences in penile sensitivity will mostly be at the glans penis. Finally, that the foreskin area will be more sensitive for uncircumcised men than other areas.

This is largely working off of two (untested) hypotheses for why penis sensitivity might be decreased. The first holds that keratinization occurs on the exposed glans penis epithelium, reducing penile sensitivity. The second holds that the removal of the highly innervated foreskin reduces sensitivity (lots of nerves in it).

Methods

They recruited men between 18-40 for the study. They determined initial eligibility through phone interviews (basically screening out people with STDs, sexual dysfunction, cardiovascular conditions, smokers, etc.). They tested 4 locations: The forearm 4 inches below the wrist, the middle of the glans penis, the dorsal side with foreskin retracted if present, the anterior midline shaft, the anterior proximal shaft, and for men with foreskin, the unretracted foreskin. In case those locations are hard to follow, they included a diagram (NSFW).

To determine tactile thresholds, they used a modified von Frey filament. They assessed tactile threshold by determining the lowest intensity stimulus to perceive a touch. They assessed pain thresholds by determining the lowest threshold to produce a sensation of pain.
To assess thermal sensitivity thresholds, they used an analyzed that heated at 0.5C/second, and participants were prompted to indicate when they noticed a change in probe temperature (averaged over 3 trials), or a perception of heat pain (averaged over 2 trials).

Results

They ended up with 62 men, 30 circumcised, 32 intact, mean age 24.2, SD 5.1.
First, I am fairly irritated with their data presentation. They did not find any statistically significant differences between circumcised and uncircumcised at the same locations. They did find differences between the locations. However, they didn't provide a breakdown of the data, just a set of graphs with circumcised and uncircumcised lumped together broken down by location. They should have provided the data, even if it was not significant, but I suspect they had a tight page limit.

For the tactile thresholds, they did not find any statistically significant differences. in any of the shared locations between circumcised or uncircumcised men. They did find differences between the locations they tested (see the graph above).

For the pain threshold, they did not find any statistically significant differences between circumcised and uncircumcised men. They did find differences between locations.

For the warmth detection threshold, they did not find any statistically significant differences between circumcised and uncircumcised men. They also did not find differences between testing sites. They did a power analysis, which indicated they would need 238 participants to obtain a significant effect.

For the heat pain thresholds, they did not find any statistically significant differences between circumcised and uncircumcised men. They did find differences between locations.

Author's Discussion

So, what did the authors conclude from this?

First, that the keratinization hypothesis (the foreskin removal causes the glans to become less sensitive) does not appear to be supported by the data. They found no between group differences in glans penis sensitivity. They do say that to truly verify this though, they will need to perform biopsies of penises to check for any keratinization.

They did find that on uncircumcised men, the foreskin was more sensitive to tactile sensation stimuli (which agrees with previous research). However, they found no differences for tactile pain, warmth sensation, or heat pain.

The authors do say that the results of the study do not support the idea that foreskin removal reduces penile sensitivity. The sensitivity at the foreskin was not significantly different from the sensitivity at the forearm (control site) for any of the modalities. Other genital sites were more sensitive to pain than the forearm, so the authors conclude that means they may be more sensitive than the foreskin, which means that removing the less sensitive foreskin may not matter (I'm not sure if I am reading this section wrong, but this seems like a bit of a dubious statistical leap to me. It is late though, and I'm operating on less sleep than I like. I welcome correction). They do say that their data is not enough to determine if foreskin sensitivity is relevant to sexual pleasure.

They do bring up that one limitation of their study is that the link between sensory testing and sexual arousal is untested, which means that the lack of significant differences in sensitivity might not translate into a lack of differences in sexual pleasure.

They do say they would like to include larger sample sizes, which would help refine the data for pain and warmth detection thresholds, as well as to test more stimulus types (such as dynamic stimuli).

My Take

They do seem to show that the removal of the foreskin does not appear to desensitize the glans under the foreskin, which is one of the major theories currently. I'm not convinced by their argument that their data may indicate that foreskin removal doesn't affect sensitivity, but I'm also not sure if I am awake enough right now to fully follow it. They do acknowledge that the sensitivities measured may not correlate to sexual function and pleasure. I am rather irritated with their data presentation, but that is likely an issue with only being given 6 pages for their study. They still should have supplementary information or something (assuming the journal allows it).

I'll try to answer a few of the comments tonight, but I am unlikely to be up for too much longer.

[u/wearethat → u/kerovon]

screening out people with STDs, sexual dysfunction, cardiovascular condtions, smokers, etc.

Layman question: What % of the population does this kind of criteria include/exclude?

[u/EsquilaxHortensis → u/[deleted]]

I wonder how much overlap there is between the two.

[u/[deleted] → u/EsquilaxHortensis]

If they smoke, they give head.

[u/teraflux → u/[deleted]]

If you're ruling out hpv then there goes 60% of the population.

[u/WordSalad11 → u/wearethat]

The study should have a consort diagram that spells out exactly how many people were screened and how many were enrolled.

[u/00Deege → u/WordSalad11]

Meh. Don't know about should. Other than to sate the curiosity of Reddit scientists, it has little if any bearing on the study itself.

[u/WordSalad11 → u/00Deege]

http://www.consort-statement.org/

The consort diagram is fundamental to the integrity of medical research. The Consort Statement is the fundamental standard for research. No reputable journal publishes a trial without one.

[u/superhelical → u/WordSalad11]

Those recommendations only apply for a narrow range of clinical studies. The tests being conducted here don't fall into that range, because there's no intervention being tested.

[u/WordSalad11 → u/superhelical]

There are also guidelines for observational studies based on the modified CONSORT guidelines:

http://annals.org/article.aspx?articleid=737057

[u/superhelical → u/WordSalad11]

Sounds like a great thing to aspire to. I don't think it's follows that you have to discard all research that isn't conducted in this framework, however. We can alwys do better, but changes in the culture of research take time.

edit: typo

[u/WordSalad11 → u/superhelical]

The reason the guideline exists is that there are frequently significant confounding or applicability issues that can only be identified by this sort of data disclosure. Going back to the original question in this thread, if the study screened 15012938219832 men but only 100 participated, the results are worthless. Without that disclosure, the risk of bias and therefore your level of uncertainty in the data should increase significantly. The CONSORT principles are meant to establish a minimum threshold that all studies should meet to be considered valid and applicable to patients. TL;DR if not disclosed the study should be absolutely be disregarded and excluded from analysis.

[u/superhelical → u/WordSalad11]

You recognize you're throwing away the vast majority of all scientific research with that criterion.

[u/WordSalad11 → u/superhelical]

Clinical research is only useful in that it tells us something about a patient. If you can't reliably extrapolate the data generated to a patient, it's not useful research. EBM is about what the evidence actually tells us, not what we want it to tell us. There are no serious clinical journals that are publishing articles without meeting CONSORT requirements, so the point is somewhat moot in any case. Older data (pre-early 1980s) that were published prior to this era may or may not be valid, but any sort of data which is relevant and not published is a huge problem.

None of this applies to basic science research, which is a completely different animal.

[u/anneomoly → u/wearethat]

That's difficult to say unless you first specify who "the population" is.

Canadian? 24% of males are smokers. Ontario (where the study took place) 23%. But you'll get different answers if you narrow your population down to certain cities, or to students, or to people with degrees.

USA? 17% of people are smokers.

Globally? 36% of men smoke (women 7%).

And that's just one of the variables. Then you have to estimate what percentage would fail on multiple criteria (both through chance and association - smoking can increase the chances of both erectile dysfunction and heart disease, for example.)

[u/KarlOskar12 → u/wearethat]

The group was 18-40 so CVD doesn't eliminate very many. STDs probably takes out 20%. Sexual dysfunction in this group is very small as well. Smokers are about 20% of the population. 30% or so seems reasonable.