In Locus Minoris Resistentiae—Or Not?

Abstract

For dermatologists, assessing the safety and efficacy of sun exposure in human skin has been a challenge since the introduction of artificial sources of ultraviolet radiation (UVR) as a therapeutic modality early in the 20th century. This includes evaluation of acute responses such as inflammation and chronic effects including photoaging, photocarcinogenesis, and immunosuppression to name several.In this issue of the journal, Waterston et al (2004) studied the cutaneous response to ultraviolet B (UVB) radiation in normal human subjects of Fitzpatrick phototypes I and II by first measuring erythema responses at multiple anatomical sites. They administered a series of incremental doses of energy between 38 and 300 mJ per cm 2 at 12 pairs of body sites. The study design permitted comparative assessment of both within subject and between subject variations in regional sensitivity to UVB. Erythema was measured using a reflectance instrument and the minimal erythema dose (MED) defined as UVB dose needed to produce barely detectable visible redness determined at each site in each subject. Additional studies were conducted in four of these subjects to determine the induction of melanin pigmentation by measuring the melanin index (MI) at the irradiated sites.In a second protocol, human subjects of Fitzpatrick phototypes I-IV were irradiated at three identical sites daily for 5 days with ascending standard erythemal doses of UVB. Ninety-six hours later the irradiated sites and three matching non-irradiated sites on the contralateral side of the body were challenged with incremental doses of UVB and 24 hours later blood flow measurements obtained with a contact laser Doppler flowmeter to obviate the overlapping spectra for hemoglobin and melanin. The purpose of these studies was to define the ability of the subjects to ''photoadapt'' to repeated erythema doses administered at different anatomic sites.In the third experiment, erythema responses to diathranol, a chemical irritant used therapeutically in psoriasis, were defined by applying increasing doses of this agent under occluded patches and then measuring erythema responses with laser Doppler flowmetry.Finally, baseline blood flow determinations and MI were obtained at nine body sites to determine whether local anatomic variations in blood flow could influence UVB-induced erythema and melanogenesis.The authors have shown striking differences in responsiveness to UVB among human subjects of the same Fitzpatrick phototype, thereby raising questions about the utility of this system in assessing human erythema susceptibility. For example, among subjects some anatomic sites such as the forearm showed remarkable variation in erythema responses and the chest and upper back appeared to be most susceptible and the legs, not surprisingly, the least sensitive to UVB. Within individual subjects there were distinct ordering patterns of site responsiveness that have implications for site selection for phototesting patients prior to initiating phototherapy.MI and MED showed an inverse ...

Invisible dermatoses

“…The skin has a long memory, as demonstrated by the concept of 'locus minoris resistensiae'. [15] 4. Structural changes from metabolic changes: [1] In metabolic diseases, deposits in the skin may not always be detectable.…”

Approach to the so-called “Invisible Dermatosis”: When Subtle Histopathological Findings Guide Diagnosis

“…3,4 Regarding the first group, authors emphasized generalized pruritus where many underlying causes, such as invisible mycosis fungoides, can be found 5 and diseases where "clinically uninvolved skin" is not completely normal like in psoriasis, pemphigus or lupus, or the areas of recently healed skin that may favor the subsequent appearance of different dermatosis acting like a "locus minoris resistentiae." 6 This idea will fit with the concept of disease scar, such as psoriasis, 7 and also with the presence of different dermatosis in areas previously involved by herpes zoster reactivation. 8 We will not review this first group to focus on the second one.…”