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BJD British Journal of Dermatology

C L I N I C A L A N D L A B O R A T O R Y I N V E S TI G A T I O N S

Assessment of skin barrier function in podoconiosis: measurement of stratum corneum hydration and transepidermal water loss J.S. Ferguson,1 W. Yeshanehe,2 P. Matts,3 G. Davey,4 P. Mortimer5 and C. Fuller6 Department of 1Dermatology and 5Lymphovascular Medicine, St George’s Hospital, London, U.K. 2 Department of Dermatology, Bahir Dar University, Bahir Dar, Ethiopia 3 Procter and Gamble – Skin Care, London, U.K. 4 Brighton and Sussex Medical School, University of Sussex, Brighton, U.K. 6 Department of Dermatology, Chelsea and Westminster Hospital, London, U.K.

Summary Correspondence John S. Ferguson. E-mail: [email protected]

Accepted for publication 3 November 2012

Funding sources Some funding for travel and accommodation was provided by the lymphoedema research fund at St George’s Hospital. The Procter and Gamble Company Ltd kindly lent us equipment.

Conﬂicts of interest None declared. DOI 10.1111/bjd.12134

Background Podoconiosis is a common cause of lymphoedema in barefoot workers in Ethiopia and other countries. It has severe consequences for patients’ physical function, quality of life and economic status. Aims To investigate stratum corneum (SC) hydration and transepidermal water loss (TEWL) in patients with podoconiosis compared with controls. Methods In total, 55 patients and 20 controls were recruited. For each study subject, SC and TEWL measurements were taken, along with foot and lower leg circumferences. Measurements were compared between the patient and control groups. Results Foot circumferences tended to be higher in patients with podoconiosis, with the mean foot:leg circumference ratio being 1Æ19 (95% confidence interval 1Æ11–1Æ28) times that for controls (P = 0Æ001). There was no detectable difference between patients and controls in TEWL values (P > 0Æ05); however, SC hydration was significantly lower in patients vs. controls for the foot (P = 0Æ004) and lower leg (P = 0Æ046) sites. Conclusions Patients with podoconiosis have significantly lower SC hydration in the skin of their lower legs and feet than controls, which may lead to cracking and splitting, and increased risk of lymphoedema and infection.

Podoconiosis (nonfilarial elephantiasis) is a common cause of lymphoedema1 in those who work barefoot in the red volcanic soil of upland Ethiopia,2,3 and has been described in various other countries of the developing world where people work barefoot.4 It is known to be familial, and Tekola Ayele et al.5 have recently described a specific association with human leucocyte antigen. It has been described as offering a unique ‘tropical model for gene–environment interactions’.6 As in many types of lymphoedema, untreated patients suffer episodes of acute cellulitis (erysipelas, adenolymphangitis), with an associated risk of morbidity and mortality. On clinical examination these patients often have dry, hyperkeratotic skin with deep cracks in the heels and between the toes (Fig. 1a). Many patients go on to develop gross oedema and fibrotic nodular scarring (Fig. 1b).7 Podoconiosis has severe consequences for patients’ physical function, quality of life and economic status.8

Understanding the pathogenesis of this disease will contribute to future developments in targeted therapy. Observation of the xerosis and cracking on the feet of patients early in disease led us to hypothesize that mechanical failure of the plantar stratum corneum (SC) barrier may play a pivotal role in either the initial pathogenesis or the onset of episodes of cellulitis, or both. It is probable that a damaged, cracked, SC will allow both soil particles and microbes into the dermis.9 Episodes of cellulitis may follow and are known to contribute to the worsening of lymphoedema.1 Both transepidermal water loss (TEWL, as a metric describing SC barrier function) and electrical capacitance (as a derivative measure of SC hydration) have been validated as measures of SC quality and have been used in a wide variety of peerreviewed studies for over 30 years.10,11 Lower values of TEWL and higher values of SC hydration are, in general, characteristic features of a healthy, intact skin barrier.12,13 2012 The Authors

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BJD 2012 British Association of Dermatologists 2013 168, pp550–554

Skin barrier function in podoconiosis, J.S. Ferguson et al. 551

(a)

(b)

recruited from general hospital outpatients and the community leaders’ meeting in Debre Elias. The study was conducted according to the principles of the Declaration of Helsinki and was approved by the Bahir Dar University Ethics Committee. All new (previously untreated) patients attending the clinic over a period of 14 days during the study period were approached and invited to participate. Study information sheets were read to the patients and controls in the local language (Amharic). The patients were then asked to sign a consent form written in Amharic. Foot and lower leg circumferences were recorded for all study subjects. A simple measuring tape was used and the maximum circumference was recorded for both sites. Both SC hydration and TEWL are highly sensitive to changes in ambient temperature and relative humidity. Therefore, patients and controls were asked to sit still, in the shade, with leg sites exposed for at least 20 min to equilibrate with ambient climatic conditions. They were instructed to avoid crossing or touching their legs (or any other similar movement), which might affect the measurement sites. Local ambient temperature and percentage relative humidity were recorded. Stratum corneum hydration

Fig 1. (a) Heel cracks on the foot of a patient with podoconiosis. (b) Patients with podoconiosis exhibiting the different phenotypes seen in this disease.

It is known that a variety of instrument- and environmentrelated variables (e.g. ambient air temperature, relative humidity and direct air flow) and factors originating from the individual (e.g. age, sex, race, anatomic site, sweating, hair, and skin surface temperature) influence TEWL and SC hydration and, thus, need to be controlled for in studies that measure them. Values of SC hydration and TEWL tend to be higher on the palms, soles and forehead than on the back, abdomen and legs.14 It has been demonstrated that low values of SC hydration and high values of TEWL are features of impaired skin barrier function in atopic dermatitis, ichthyosis vulgaris and psoriasis.13,15–18 Rising TEWL and falling SC hydration have also been shown in these conditions to correlate well with increasing disease severity.15–18 To the best of our knowledge, TEWL has not previously been measured in patients with lymphoedema, and although SC hydration has been studied in such patients, it was used only as a surrogate measure for tissue oedema.19,20 We set out to investigate SC hydration and TEWL in patients with podoconiosis compared with controls.

Patients and methods In total, 55 new patients were recruited from the podoconiosis clinic of the International Orthodox Christian Charities project in Debre Markos, Ethiopia. Also, 20 controls were 2012 The Authors BJD 2012 British Association of Dermatologists 2013 168, pp550–554

SC hydration can be determined rapidly and noninvasively by measuring surface electrical properties.10 Various instruments are now commercially available for this purpose. In the current study, we used the handheld Moisture Meter SC (Delfin Technologies Ltd, Kuipio, Finland), which has been specifically developed and validated as a portable wireless device for fieldwork. The instrument measures changes in SC capacitance commensurate with changes in dielectric constant due to hydration, and expresses results as rescaled interval units (due to the complexity of expressing capacitance). The protocol is described in Appendix 1. The instrument includes a pressure sensor to help reduce intermeasurement variability. The instrument was placed on the skin for 4 s, after which the numerical hydration value was shown on an integrated LED display. Measurements were taken in triplicate for each site, the mean value being used for analysis. Transepidermal water loss TEWL is a measure of water vapour flux,11 with units expressed as g m)2 h)1 of water. TEWL can be determined rapidly and noninvasively using evaporimetry (an instrumental measure of water flux through the SC). In this study, we used the portable, handheld wireless Vapometer instrument from Delfin Technologies Ltd (see Appendix 1). For fieldwork, the Vapometer has the advantage of being a closed-chamber device, thus reducing to a minimum any variability due to environmental factors such as air movement, etc. The instrument was placed on the skin surface for approximately 10 s, with the exact time varying slightly from site to site, dependent on measurement stability determined by the instrument. Single measures of TEWL were recorded for each site.

552 Skin barrier function in podoconiosis, J.S. Ferguson et al. Table 2 Stratum corneum transepidermal water loss: the average ratio of patients:controls

Statistical methods We recorded all data in pseudoanonymized field sheets and transferred the results onto a spreadsheet. Where replicate measures were taken, mean values were used in the analysis for each body site. Graphical examination of the data and use of the Shapiro– Francia21 test showed that neither the control nor the patient groups’ data followed a normal distribution for any outcome measure. However, a natural logarithm transformation allowed assumptions of normality and equality of variances to be met sufficiently to allow parametric analysis methods, using an unpaired Student’s t-test.

Site

Average ratio of patients:controls

95% confidence interval

P-value

Thigh Shin Foot

0Æ92 1Æ01 0Æ84

0Æ73–1Æ16 0Æ83–1Æ23 0Æ65–1Æ09

0Æ49 0Æ93 0Æ18

Table 3 Stratum corneum hydration: the average ratio of patients:controls

Results This study included 55 patients with podoconiosis (30 male, 25 female). The median age of the patients was 45 years (range 16–83 years). Results were gathered at an average temperature of 23Æ3 C, and an average relative humidity (%RH) of 40Æ46%. Twenty controls (14 male, six female, median age 30 years, range 10–56 years) were measured at an average temperature of 23Æ6 C and 36Æ7% RH. A few patients and controls did not have data collected from all areas due to excess body hair interfering with the results. One control withdrew as he had to attend a clinic appointment. Foot circumferences tended to be higher in patients with podoconiosis (Table 1), with the mean foot:leg circumference ratio being 1Æ19 (95% confidence interval 1Æ11–1Æ28) times that of controls (P = 0Æ001). There was no detectable difference between patients and controls in TEWL values (P > 0Æ05) (Table 2); however, SC hydration was significantly lower in patients vs. controls for the foot (P = 0Æ004) and lower leg (P = 0Æ046) sites (Table 3).

Site

Average ratio of patients:controls


P-value

Thigh Shin Foot

0Æ83 0Æ75 0Æ61

0Æ66–1Æ04 0Æ57–0Æ99 0Æ44–0Æ84

0Æ11 0Æ046 0Æ004

risk of infection. Indeed, previous data suggest that increased toe web maceration (breakdown of SC structure due to hyperhydration) leads to a greater incidence of acute cellulitis.9 There were some limitations to the methodology, which may have affected some results. Selection of an appropriate control group for a case–control study like this is difficult. We used a pragmatic sample control group. Ideally, we would have preferred to use case-matched controls, but this was not practical. Although efforts were made to reduce sweating in patients and controls by encouraging them to equilibrate for 20 min before readings were taken, we still observed some sweating in a small number of patients and controls, a problem that was more noticeable around midday and early afternoon (when ambient temperature reached a maximum). Sweat is a significant confounding factor in measures of TEWL and SC hydration. Similarly, hair on the legs of patients and controls may also have compromised the integrity of contact of the probes with the skin surface, introducing further variability. Therefore, to reduce these affects to a minimum, areas with profuse hair were avoided and the majority of data were gathered in the morning. Ambient climatic conditions were very similar for the control and patient groups. It is notable that the use of the new, portable bioengineering instrumentation we describe herein was overall a success, even in the challenging conditions we experienced in the field. The instruments are light, portable, noninvasive and easy to use, requiring minimal training.

Discussion We believe that this is the first time SC barrier function has been studied in lower leg lymphoedema. Patients with podoconiosis had significantly drier skin on their feet and lower legs vs. controls. Because SC mechanics change dramatically with water activity22 (i.e. drier SC has a significantly lower elastic modulus, leading to brittleness and fragility), this observation is entirely consistent with the severe cracking evident in the thick plantar skin of the majority of the patients. Cracks in the skin allow exogenous material, including microbes, to breach the epidermis, with significant Table 1 Ratios of mean foot:leg circumferences Average foot:leg ratio in patients (n = 53 ⁄ 55)

Average foot:leg ratio in controls (n = 16 ⁄ 20)

Foot:leg ratio in patients ⁄ foot: leg ratio in controls


P-value

0Æ89 (26Æ4 cm ⁄ 29Æ7 cm)a

0Æ75 (22Æ9 cm ⁄ 31Æ1 cm)a

1Æ19

1Æ11–1Æ28

0Æ001

a

Nonparametric data included as a guideline.

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Skin barrier function in podoconiosis, J.S. Ferguson et al. 553

It is not possible to determine from our study whether the significantly lower SC hydration measured in the feet and legs of patients is a cause or an effect of lymphoedema, and to what extent it is significant to podoconiosis. Only patients with lymphoedema caused by podoconiosis were examined, but it is possible that patients with lymphoedema caused by other pathologies exhibit similar changes in barrier function. We plan to compare SC barrier function in patients with podoconiosis with that in other types of lower limb lymphoedema. This would help us to determine whether the clear SC hydration changes we have measured are a unique feature of podoconiosis. Comparison with previous SC hydration and TEWL data from studies of other diseases would also be interesting.15–20 However, because the equipment, populations and environment are different in each study, it is unlikely that they would be meaningful. In summary, this study has shown that patients with podoconiosis have significantly lower SC hydration in the skin of their lower legs and feet than controls. With ensuing mechanical failure of the SC leading to cracking and splitting (particularly in the extremely thick, dry plantar skin of these patients), we hypothesize strongly that exogenous material (including microbes and soil particles) will breach the skin barrier, promoting increased risk of lymphoedema and infection. Further work is needed to determine whether skin barrier impairment is related to the initial induction of podoconiosis and ⁄or the propagation of exacerbating episodes of cellulitis, and also to assess whether it is present in other causes of lymphoedema. Of note, impaired skin barrier function may prove to be determined genetically, thus supporting the geogenetic hypothesis for the aetiology of podoconiosis.

What’s already known about this topic? • Podoconiosis is a common cause of lymphoedema in Ethiopia, and is found among individuals who work barefoot in areas that share the same volcanic soil type. • Podoconiosis has a known association with human leucocyte antigen. • The disease is known to have a massive impact on the quality of life of sufferers and on their economic activity.

What does this study add? • This study suggests that patients with podoconiosis have impaired skin barrier function. • Patients were shown to have significantly reduced stratum corneum hydration compared with controls, particularly on the feet and shins. • Impaired barrier function may have a role in the pathogenesis of the disease.


Acknowledgments Special acknowledgment for statistical advice to Robert Dawe, Dermatology Department, Ninewells Hospital Dundee, Scotland, U.K. We would like to acknowledge the Procter and Gamble company for the loan of the hardware used in this study, and the Staff at the International Orthodox Christian Charities project in Debre Markos for their time and support.

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Appendix 1 Moisture meter protocol 1 The device was switched on, and when the ‘ready’ message was displayed the probe head was applied to the skin surface, the operator being careful to employ just enough downward pressure to maintain the green status bar (indicating optimum probe–skin contact). 2 After 5 s integration time stratum corneum (SC) hydration status was shown in arbitrary units on an integrated LED display.

3 Triplicate measurements were performed in a tight triangle pattern on the measurement site, the operator taking note of errors due to excess pressure, hair, etc. 4 Mean values of the triplicate measurements were used as SC hydration status. Vapometer protocol 1 The device was switched on and the operator ensured that the ‘normal’ operating mode was selected (rather than ‘nail mode’ for detecting transepidermal water loss from nails). 2 The device performed a brief calibration routine before counting down for 3 s, after which the operator placed the probe onto the skin surface (only moderate pressure being needed to achieve an efficient seal with the skin surface). 3 The device counted up to 10 s while water flux was measured and calculated. 4 The device was removed from the skin surface and water flux was shown in units of g m)2 h)1 on an integrated LED display. 5 The device button was pressed again, whereupon ambient temperature and relative humidity were displayed before powering down. 6 Only single measurements per site were taken, with the procedure starting from step 1 each time.
