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Penile sensitivity in men: a composite of recent findings

  • David L Rowland
    Correspondence
    Reprint requests: David L. Rowland, Ph.D., Department of Population Dynamics, Room 4503, 615 North Wolfe Street, Johns Hopkins School of Public Health, Baltimore, MD 21205
    Affiliations
    Department of Psychology, Valparaiso University, Valparaiso, Indiana, USA

    Department of Population Dynamics, Johns Hopkins University School of Public Health, Baltimore, Maryland, USA
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      Abstract

      Objectives. To present, in standardized form, age- and dysfunction-related data from 13 studies on vibrotactile penile epiglandal thresholds in men to allow cross-study comparisons, a capability previously prevented by the use of varying methods and units of measurement.
      Methods. On the basis of the summarization and standardization of multiple studies located through online searches of bibliographic data bases, penile sensory thresholds were first plotted as a function of age, and then as a function of dysfunctional or disease status. In a third plot, both age- and dysfunction-related differences were illustrated within a single plot.
      Results. An increasing curvilinear threshold as a function of age was confirmed in the pattern derived from multiple studies. Furthermore, even when controlling for age, penile thresholds of men with erectile dysfunction lay one or more standard deviations above those of sexually functional counterparts, and this deviation became even higher during penile tumescence. When relationships among age, dysfunctional/disease status, and penile threshold were examined simultaneously, potential compounding effects of age and dysfunction emerged.
      Conclusions. From a research perspective, standardized information on penile vibrotactile thresholds will facilitate ongoing study aimed at elucidating the role of penile sensitivity to both erectile and ejaculatory response. From a clinical perspective, standardized information on populations of sexually functional and dysfunctional men may assist in a preliminary differential diagnosis.
      A number of recent studies indicate that penile sensitivity may be related to sexual function in men. For example, penile vibrotactile thresholds are higher in men with erectile dysfunction than in age-matched control subjects.
      • Rowland D.L.
      • Leentvaar E.J.
      • Blom J.
      • et al.
      Changes in penile sensitivity following papaverine-induced erection in sexually functional and dysfunctional men.
      ,
      • Rowland D.L.
      • Haensel S.M.
      • Blom J.
      • et al.
      Penile sensitivity in men with premature ejaculation and erectile dysfunction.
      ,
      • Padma-Nathan H.
      Neurologic evaluation of erectile dysfunction.
      Aging and diabetes, conditions often associated with diminished sexual capacity, are characterized by significantly elevated penile thresholds as well.
      • Rowland D.L.
      • Greenleaf W.
      • Mas M.
      • et al.
      Penile and finger sensory thresholds for young, aging, and diabetic males.
      ,
      • Breda G.
      • Xausa D.
      • Giunta A.
      • et al.
      Nomogram for penile biothesiometry.
      ,

      Morrissette DL, Goldstein M, Raskin D, et al: Sensory thresholds and intensity and pleasure ratings in sexually functional and dysfunctional diabetics. Diabetologica (in press).

      ,
      • Newman H.F.
      Vibratory sensitivity of the penis.
      Such findings suggest that low penile sensitivity (ie, high thresholds) may be indicative of, if not contributing to, decreased sexual functioning.
      • Rowland D.L.
      • Greenleaf W.
      • Dorfman L.J.
      • et al.
      Aging and sexual function in men.
      Not all studies support the idea that an elevated threshold is a mark of decreased sexual function. Penile thresholds have been found to increase with increasing penile tumescence, the sexually more ready state.
      • Rowland D.L.
      • Leentvaar E.J.
      • Blom J.
      • et al.
      Changes in penile sensitivity following papaverine-induced erection in sexually functional and dysfunctional men.
      ,
      • Edwards A.E.
      • Husted J.R.
      Penile sensitivity, age, and sexual behavior.
      ,
      • Rowland D.L.
      • Geilman C.
      • Brouwer A.A.
      • et al.
      New device for penile vibrotactile stimulation description and preliminary results.
      In addition, thresholds have been found to be lower in men lacking normal levels of androgen, a hormone of vital importance to sexual response.
      • Burris A.S.
      • Gracely R.H.
      • Carter C.S.
      • et al.
      Testosterone therapy is associated with reduced tactile sensitivity in human males.
      ,

      Rowland DL, Greenleaf W, and Davidson JM: Penile sensitivity in hypogonadal men. Presented at the Conference on Reproductive Behavior, Monterrey, California, June 2–5, 1985.

      Despite the growing number of studies investigating the relationship between penile sensitivity and sexual function, a summary of data on penile thresholds in men of varying ages, sexual histories, and diseases has yet to be compiled; that is, although normative data on vibrotactile sensory threshold and various disorders
      • Edwards A.E.
      • Kopple J.D.
      • Kornfield C.M.
      Vibrotactile threshold in patients undergoing maintenance hemodialysis.
      ,
      • Mirsky I.A.
      • Futterman P.
      • Broh-Kahn R.
      The quantitative measurement of vibratory perception in subjects with and without diabetes mellitus.
      have been reported for many areas of the human anatomy, comparable data on penile sensitivity are limited. Furthermore, although age-related data on penile vibrotactile sensitivity have been published,
      • Breda G.
      • Xausa D.
      • Giunta A.
      • et al.
      Nomogram for penile biothesiometry.
      ,
      • Padma-Nathan H.
      Editorial erectile dysfunction.
      those data were based on a measurement scale specific to one type of instrument. From a research standpoint, standardized information on penile vibrotactile thresholds might facilitate ongoing study aimed at elucidating the role of penile sensitivity in both erectile and ejaculatory response. From a clinical perspective, standardized information on populations of sexually functional and dysfunctional men has potential value because determination of penile sensitivity is often counted among the simpler, noninvasive techniques that can aid in a preliminary differential diagnosis.
      • Goldstein I.
      • Krane R.J.
      Diagnosis and therapy of erectile dysfunction.
      However, the comparison of findings on penile thresholds from one study to the next has been impeded by three obstacles. First, vibrotactile threshold data are often presented in different units of measurement. Some studies, for example, report data in units specific to the instrument used to apply the vibrotactile stimulation rather than standardized units relevant to the physical parameters of the stimulus. Second, parameters of the vibrotactile stimulation have varied in several ways, including the frequency (hertz), duration, and force of the vibratory stimulus, the size of the surface area stimulated, and the direction of the vibration (either horizontal or vertical to the surface)—parameters all known to affect thresholds.
      • Verillo T.
      • Fraioli A.J.
      • Smith R.L.
      Sensation magnitude of vibrotactile stimuli.
      ,
      • Horch K.
      Coding of vibrotactile stimulus frequency by Pacinian corpuscle afferents.
      Finally, the psychophysical method used to determine thresholds has ranged from simple biased procedures of graduating intensity to highly controlled bias-free procedures requiring forced choice discriminations. Thus, even when threshold values are presented on an absolute scale (eg, microns of movement at the tip of the stimulator), thresholds for sexually functional groups have shown substantial disparity. As a result, comparisons between groups or conditions within a study have been feasible, but direct comparisons across studies have not been possible.
      Herein, an initial attempt at standardizing penile threshold data from different populations and/or under different conditions obtained from studies employing diverse methods has been made. Such standardization provides a means of circumventing problems that arise with the use of varying measurement scales and experimental methods.

      Material and methods

      An online bibliographic search of four data bases (Medline, BioAbstracts, PsychInfo, Current Contents) was conducted to identify studies dealing with penile sensitivity, vibrotactile stimulation, and penile biothesiometry since 1967. Although more than 30 studies were identified, only 12 provided sufficient numeric data to warrant inclusion in this review. In addition, unpublished data from one conference presentation were included. Although it is possible that all reports on penile thresholds were not located through this search, a fairly representative sample of the range of studies examining penile sensitivity in relation to sexual functioning has been identified.
      Penile vibratory threshold was first plotted against age, a variable of known importance to vibrotactile sensitivity (Fig. 1). Sensory data were limited to those drawn from stimulation of the glandal or epiglandal (10 mm or less) region of the penis. Threshold mean values of sexually functional groups for men in a middle age range (45 to 50 years) were set to a value of 0 to establish an overall point of reference; each standard deviation unit was represented by a value of 1.0. Although selection of this age group as a reference is arbitrary, it does represent the time interval preceding the age-related onset of significant sexual decline seen in many men.
      • Davidson J.M.
      • Chen J.J.
      • Crapo L.
      • et al.
      Hormonal changes and sexual function in aging men.
      Values within this age range were then used as the standard against which the mean values ± standard errors of all other age groups were compared. In a second set of comparisons (Fig. 2), the mean values and standard deviations of sexually functional men in age categories ranging from 25 to 60 years served as reference points (ie, mean values were set to zero). Threshold values ± standard errors of dysfunctional subgroups and/or groups under various experimental conditions were then plotted. These groups included, but were not limited to, men with various sexual dysfunctions, hormonal profiles, or diseases. In those instances in which a dysfunctional group was examined under repeated measures (eg, flaccid versus erect penis induced through intracavernosal papaverine), distribution characteristics of the functional comparison group under comparable conditions were used as the point of reference. In situations in which the control group was not adequately age-matched with the dysfunctional group (more than 5 years difference), the variance between groups was reduced by 24% on the basis of four studies that, on examining the relationship between age and sensitivity, found correlations ranging from 0.23 to 0.74 (mean r2 = 0.24).
      • Rowland D.L.
      • Leentvaar E.J.
      • Blom J.
      • et al.
      Changes in penile sensitivity following papaverine-induced erection in sexually functional and dysfunctional men.
      ,
      • Rowland D.L.
      • Greenleaf W.
      • Mas M.
      • et al.
      Penile and finger sensory thresholds for young, aging, and diabetic males.
      ,
      • Edwards A.E.
      • Husted J.R.
      Penile sensitivity, age, and sexual behavior.
      ,
      • Xin C.
      • Chung S.
      • Choi D.
      • et al.
      Penile sensitivity in patients with primary premature ejaculation.
      In doing so, there was a slight risk of underestimating differences between groups, but this conservative strategy was preferred over possible exaggeration of differences. Finally, in a third plot (Fig. 3), information from the first two plots was combined so that both age- and dysfunction-related differences in penile vibratory thresholds could be viewed together. For this plot, regression analysis was used to generate the best fitting line for data obtained from sexually functional men of different age groups (Fig. 1). This regression line was then used as the age-controlled norm against which dysfunctional subgroups were compared.
      Figure thumbnail GR1
      FIGURE 1Relationship between penile threshold and age (mean + or − SE) in sexually functional men. Plotted points are based on three studies.
      Figure thumbnail GR2
      FIGURE 2Dysfunctional and experimental conditions (mean + or − SE) compared with sexually functional control subjects when age is held constant. Control groups under nonexperimental conditions are positioned on the zero line; control groups under experimental conditions are so indicated. CON = control; ED = erectile dysfunction; PE = premature ejaculation; DB = diabetes; INF = infertile; UHYP = untreated hypogonadal; THYP = testosterone-treated hypogonadal; BP = under bupropion (antidepressant) treatment; (E) = during erection.
      Figure thumbnail GR3
      FIGURE 3Age- and dysfunction-related differences (mean + or − SE) plotted simultaneously, with control group values under nonexperimental conditions situated on the regression line. This line, represented by the equation y=−1.6174+0.00079472 x2, depicts the function of age on penile sensitivity, obtained from the data in . CON = control; ED = erectile dysfunction; PE = premature ejaculation; DB = diabetes; INF = infertile; UHYP = untreated hypogonadal; THYP = testosterone-treated hypogonadal; BP = under bupropion (antidepressant) treatment; (E) = during erection.
      A number of strategies were employed in culling the data from published reports. When available from the text or a table, actual numerical values for mean and standard deviations (or standard errors that could be used to calculate standard deviations) were used. When data were published in graphic form, two researchers independently estimated the values (means, standard deviations, and so forth) for the relevant groups (r = 0.94). If the estimates varied between researchers, their mean estimations were used to characterize the group. When standard deviations or errors were not reported, estimates were made from the sample range according to the procedure described by Hurlburt.
      • Hurlburt R.
      When no measure of dispersion was reported, the data were not included in the overview. For one study,
      • Rowland D.L.
      • Greenleaf W.
      • Mas M.
      • et al.
      Penile and finger sensory thresholds for young, aging, and diabetic males.
      data were reanalyzed into smaller age intervals specifically for the purpose of this report.

      Results

      Penile thresholds as a function of age are presented in Figure 1. These data can be represented by the functional form of the quadratic regression equation to describe an increasing curvilinear function. In this equation, the coefficient of the linear term is set equal to zero such that (Eq. [1]) (1)Math Eq Insertion of the ordinary least squares coefficient estimates (t-statistics are given in parentheses) from the regression analysis yielded the following equation: (2)Math Eq(−6.357)(10.944).
      The fit of this regression line was evident by the value of R2 = 0.89. In Figure 2, the reference values were adjusted for age, and thresholds for various dysfunctions and/or experimental conditions are illustrated. In Figure 3, the effects of age and subgroup thresholds are represented within a single plot, using values for sexually functional control subjects of varying ages on the basis of the regression equation (Eq. [2]) as the line of reference. Table I adds pertinent information regarding differing stimulus characteristics that might affect straightforward comparison among studies.
      • Rowland D.L.
      • Leentvaar E.J.
      • Blom J.
      • et al.
      Changes in penile sensitivity following papaverine-induced erection in sexually functional and dysfunctional men.
      ,
      • Rowland D.L.
      • Haensel S.M.
      • Blom J.
      • et al.
      Penile sensitivity in men with premature ejaculation and erectile dysfunction.
      ,
      • Rowland D.L.
      • Greenleaf W.
      • Mas M.
      • et al.
      Penile and finger sensory thresholds for young, aging, and diabetic males.
      ,
      • Breda G.
      • Xausa D.
      • Giunta A.
      • et al.
      Nomogram for penile biothesiometry.
      ,

      Morrissette DL, Goldstein M, Raskin D, et al: Sensory thresholds and intensity and pleasure ratings in sexually functional and dysfunctional diabetics. Diabetologica (in press).

      ,
      • Newman H.F.
      Vibratory sensitivity of the penis.
      ,
      • Rowland D.L.
      • Greenleaf W.
      • Dorfman L.J.
      • et al.
      Aging and sexual function in men.
      ,
      • Edwards A.E.
      • Husted J.R.
      Penile sensitivity, age, and sexual behavior.
      ,
      • Rowland D.L.
      • Geilman C.
      • Brouwer A.A.
      • et al.
      New device for penile vibrotactile stimulation description and preliminary results.
      ,
      • Burris A.S.
      • Gracely R.H.
      • Carter C.S.
      • et al.
      Testosterone therapy is associated with reduced tactile sensitivity in human males.
      ,

      Rowland DL, Greenleaf W, and Davidson JM: Penile sensitivity in hypogonadal men. Presented at the Conference on Reproductive Behavior, Monterrey, California, June 2–5, 1985.

      ,
      • Xin C.
      • Chung S.
      • Choi D.
      • et al.
      Penile sensitivity in patients with primary premature ejaculation.
      ,
      • Rowland D.L.
      • Myers L.
      • Culver A.
      • et al.
      Bupropion and sexual function a prospective study in diabetic men with erectile dysfunction.
      Table IVibrotactile stimulus parameters for studies included in the overview
      Key: NA = not available.
      StudyPlacementHertzArea (mm2)DurationMethod
      Three types of psychophysical methods have been used; the method of ascending/descending limits is most prone to experimenter and subject bias, the forced choice method least prone.
      Measurement Scale
      Breda et al.
      • Breda G.
      • Xausa D.
      • Giunta A.
      • et al.
      Nomogram for penile biothesiometry.
      Glans10080NANAVolts
      NA
      Burris et al.
      • Burris A.S.
      • Gracely R.H.
      • Carter C.S.
      • et al.
      Testosterone therapy is associated with reduced tactile sensitivity in human males.
      Glans12080NALimitsMicrons
      Dorsal
      Edwards and Husted
      • Edwards A.E.
      • Husted J.R.
      Penile sensitivity, age, and sexual behavior.
      Glans Ventral6030NALimitsDecibels
      Morrissette et al.

      Morrissette DL, Goldstein M, Raskin D, et al: Sensory thresholds and intensity and pleasure ratings in sexually functional and dysfunctional diabetics. Diabetologica (in press).

      <10 mm glans120300500 msForced choiceMicrons
      Ventral
      Newman
      • Newman H.F.
      Vibratory sensitivity of the penis.
      Glans12080ContinuousLimitsVolts
      Dorsal
      Padma-Nathan et al.
      These data are identical to those reported in Goldstein et al.16
      Glans Dorsal12080NALimitsVolts
      Rowland et al.

      Rowland DL, Greenleaf W, and Davidson JM: Penile sensitivity in hypogonadal men. Presented at the Conference on Reproductive Behavior, Monterrey, California, June 2–5, 1985.

      <10 mm glans120300500 msForced choiceMicrons
      Ventral
      Rowland et al.
      • Rowland D.L.
      • Greenleaf W.
      • Mas M.
      • et al.
      Penile and finger sensory thresholds for young, aging, and diabetic males.
      <10 mm glans120300500 msForced choiceMicrons
      Ventral
      Rowland et al.
      • Rowland D.L.
      • Leentvaar E.J.
      • Blom J.
      • et al.
      Changes in penile sensitivity following papaverine-induced erection in sexually functional and dysfunctional men.
      <5 mm glans Ventral1203001 sRandom/limitsMicrons
      Rowland et al.
      • Rowland D.L.
      • Geilman C.
      • Brouwer A.A.
      • et al.
      New device for penile vibrotactile stimulation description and preliminary results.
      <5 mm glans1002751 sLimitsMicrons
      Ventral
      Rowland et al.
      • Rowland D.L.
      • Haensel S.M.
      • Blom J.
      • et al.
      Penile sensitivity in men with premature ejaculation and erectile dysfunction.
      ,
      • Rowland D.L.
      • Greenleaf W.
      • Dorfman L.J.
      • et al.
      Aging and sexual function in men.
      <5 mm glans1203001 sRandomMicrons
      Ventral
      Rowland et al.
      • Rowland D.L.
      • Myers L.
      • Culver A.
      • et al.
      Bupropion and sexual function a prospective study in diabetic men with erectile dysfunction.
      <10 mm glans Ventral120300500 msForced choiceMicrons
      Xin et al.
      • Xin C.
      • Chung S.
      • Choi D.
      • et al.
      Penile sensitivity in patients with primary premature ejaculation.
      Glans5080ContinuousLimitsMicrons
      NA
      legend Key: NA = not available.
      Three types of psychophysical methods have been used; the method of ascending/descending limits is most prone to experimenter and subject bias, the forced choice method least prone.
      These data are identical to those reported in Goldstein et al.
      • Goldstein I.
      • Krane R.J.
      Diagnosis and therapy of erectile dysfunction.

      Comment

      Although “normative” age-related values on penile vibrotactile thresholds have been reported in a number of studies,
      • Rowland D.L.
      • Greenleaf W.
      • Mas M.
      • et al.
      Penile and finger sensory thresholds for young, aging, and diabetic males.
      ,
      • Breda G.
      • Xausa D.
      • Giunta A.
      • et al.
      Nomogram for penile biothesiometry.
      ,
      • Padma-Nathan H.
      Editorial erectile dysfunction.
      these values have been derived using different instruments, stimulus parameters, and in some cases, measurement scales. Such differences have produced substantial variations in penile sensory thresholds and, accordingly, have made comparison across studies difficult. The present overview not only enables comparison of penile thresholds across studies using diverse methods, but it also provides a set of standard values for evaluation of future research and clinical findings. Once the mean values and standard deviations have been determined for control and experimental (eg, patient) groups in any experiment, conversion to standard scores will allow comparison with any of the findings summarized in this report.
      This overview of multiple studies confirmed the age-related curvilinear pattern of increasing threshold suggested by previous research
      • Breda G.
      • Xausa D.
      • Giunta A.
      • et al.
      Nomogram for penile biothesiometry.
      ,
      • Padma-Nathan H.
      Editorial erectile dysfunction.
      ,
      • Goldstein I.
      • Krane R.J.
      Diagnosis and therapy of erectile dysfunction.
      and described the general relationship between these variables with a regression line. The magnitude of the increase in the seventh and eighth decade of life may be functionally significant, because mean values for these age groups consistently reached 2 and sometimes 3 standard deviations above middle-age thresholds. Interestingly, there was substantial variation among individuals within these higher age groups, suggesting the presence of at least two different subpopulations. In addition, erectile dysfunction was consistently associated with penile sensory thresholds about 1 to 2+ standard deviations above the average. Furthermore, although erection was characterized by diminished sensitivity (higher thresholds) in all subjects, sexually dysfunctional men showed much greater insensitivity to stimulation during erection, about 3 standard deviations beyond that of sexually functional men during erection. When these differences were placed in the context of age-related changes (Fig. 3), the potential compounding effects of age, erectile dysfunction or disease, and erection on penile sensitivity became apparent. Although no threshold data exist on an aging population of men with erectile problems during penile tumescence (eg, induced through intracavernosal vasoactive substances), extrapolation on the basis of patterns from younger men would suggest that men in their 70s might have penile thresholds 5 to 6 standard deviations above the reference group of sexually functional men at 50 years.
      Several precautions are advised in using and interpreting these comparisons. As mentioned earlier, some data were estimated from figures and therefore measurement error has been slightly increased. In addition, we may have underestimated group differences when ages differed by more than 5 years by reducing the between-group variance by 24%, the average amount of variance attributed to age. Furthermore, stimulus parameters and psychophysical procedures varied substantially across studies: vibrator placements differed slightly, with some studies reporting direct stimulation of the glans penis and others reporting stimulation slightly posterior to the glans; and uncircumcised men whose foreskins were not retracted were used in at least two studies. Although these factors undoubtedly distort absolute threshold values for vibrotactile stimulation, transformation of raw values to fit a standardized curve minimizes the impact of these differences. Nevertheless, such problems emphasize the need for future studies not only to standardize procedures as much as possible but also to specify with greater precision the relationship between the reported measurement unit and the actual sensory stimulation. Beyond these methodologic problems, sample sizes for some groups were small and therefore these findings await further confirmation.
      Finally, the clinical usefulness of penile sensory data, particularly from the glans area of the penis, has recently been questioned.
      • Bemelmans B.
      • Hendrikx L.
      • Koldewijn E.
      • et al.
      Comparison of biothesiometry and neurophysiological investigations for the clinical evaluation of patients with erectile dysfunction.
      Although this review does not address this issue directly, the patterns evident here are generally consistent with the idea that sensory thresholds in the glans area may be indicative of a larger neurophysiologic condition that affects penile functioning. Furthermore, despite presumed lower sensitivity of the glans compared with other penile regions, we and others have found that thresholds in the glans area correlate fairly well with those from the penile shaft.
      • Rowland D.L.
      • Geilman C.
      • Brouwer A.A.
      • et al.
      New device for penile vibrotactile stimulation description and preliminary results.
      ,
      • Padma-Nathan H.
      Editorial erectile dysfunction.
      Thus, although there is need for standardization of data taken from other areas of the penis, patterns similar to those described above are likely to emerge.

      Conclusions

      By enabling comparison across studies, standardization of penile vibrotactile thresholds indicates greatly reduced sensitivity in men with erectile problems. This condition is further compounded by the decreasing sensitivity associated with aging and by the diminished sensitivity that occurs during increasing tumescence. These results reiterate the importance of ensuring adequate penile stimulation during sexual activity in men having these conditions.

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