Investigation of the Association of the Second-to-Fourth Digit Ratio with Skin Sebum Levels in Females with Acne Vulgaris

Is prenatal testosterone-estrogen balance associated with psoriasis?

The etiopathogenesis of psoriasis is not understood; however, psoriasis is affected by hormones, particularly the sex steroids. The second-to-fourth digit (2D:4D) ratio, which is an indicator of prenatal sex hormone balance, has been studied in various diseases that are affected by hormones. A total of 369 individuals comprising 172 patients with psoriasis and 197 healthy volunteers were included in the study. Patients with psoriasis were divided into type-1 and type-2, according to age of onset and family history. The 2D:4D ratio of both hands was recorded for all participants. Females in the psoriasis group had a lower 2D:4D ratio in both hands compared with those in the control group, with no significant difference. Males in the psoriasis group had a higher 2D:4D ratio for both hands compared with those in the control group (P = .009 and P < .001 for the right and left hands, respectively). Further, male patients with type-1 psoriasis had a lower 2D:4D ratio compared to those with type-2 psoriasis. Our results suggest that an alteration of the estrogen-testosterone balance due to prenatal testosterone activity is an independent predisposing factor for psoriasis in males.

Neuroendocrinology and neurobiology of sebaceous glands

The nervous system communicates with peripheral tissues through nerve fibres and the systemic release of hypothalamic and pituitary neurohormones. Communication between the nervous system and the largest human organ, skin, has traditionally received little attention. In particular, the neuro-regulation of sebaceous glands (SGs), a major skin appendage, is rarely considered. Yet, it is clear that the SG is under stringent pituitary control, and forms a fascinating, clinically relevant peripheral target organ in which to study the neuroendocrine and neural regulation of epithelia. Sebum, the major secretory product of the SG, is composed of a complex mixture of lipids resulting from the holocrine secretion of specialised epithelial cells (sebocytes). It is indicative of a role of the neuroendocrine system in SG function that excess circulating levels of growth hormone, thyroxine or prolactin result in increased sebum production (seborrhoea). Conversely, growth hormone deficiency, hypothyroidism, and adrenal insufficiency result in reduced sebum production and dry skin. Furthermore, the androgen sensitivity of SGs appears to be under neuroendocrine control, as hypophysectomy (removal of the pituitary) renders SGs largely insensitive to stimulation by testosterone, which is crucial for maintaining SG homeostasis. However, several neurohormones, such as adrenocorticotropic hormone and α-melanocyte-stimulating hormone, can stimulate sebum production independently of either the testes or the adrenal glands, further underscoring the importance of neuroendocrine control in SG biology. Moreover, sebocytes synthesise several neurohormones and express their receptors, suggestive of the presence of neuro-autocrine mechanisms of sebocyte modulation. Aside from the neuroendocrine system, it is conceivable that secretion of neuropeptides and neurotransmitters from cutaneous nerve endings may also act on sebocytes or their progenitors, given that the skin is richly innervated. However, to date, the neural controls of SG development and function remain poorly investigated and incompletely understood. Botulinum toxin-mediated or facial paresis-associated reduction of human sebum secretion suggests that cutaneous nerve-derived substances modulate lipid and inflammatory cytokine synthesis by sebocytes, possibly implicating the nervous system in acne pathogenesis. Additionally, evidence suggests that cutaneous denervation in mice alters the expression of key regulators of SG homeostasis. In this review, we examine the current evidence regarding neuroendocrine and neurobiological regulation of human SG function in physiology and pathology. We further call attention to this line of research as an instructive model for probing and therapeutically manipulating the mechanistic links between the nervous system and mammalian skin.

Second-to-fourth digit ratio and seborrheic dermatitis in males: a cross-sectional study

BACKGROUND

Seborrheic dermatitis is a common disease characterized by the erythematous plaques with oily-yellow desquamation. Increased sebaceous gland activity by androgenic hormones has played a role in the etiology of the disease. The second-to-fourth digit (2D:4D) ratio is thought to be a marker of prenatal androgen exposure.

OBJECTIVES

To investigate the association between 2D:4D ratios and seborrheic dermatitis in a male population.

METHODS

Healthy male controls and patients with seborrheic dermatitis were included in this study. One hundred seborrheic dermatitis patients and 120 healthy controls, aged 17-59, were enrolled. A digital Vernier caliper was used to measure the finger lengths. Seborrheic dermatitis severity was assessed using the Seborrheic Dermatitis Area and Severity Index (SDASI).

RESULTS

The 2D:4D ratios of the patients (x = 0.977) were significantly lower than those of the controls (x = 1.050) for right hands (t = 6.948; p = 0.000; > 0.05). No similar relationship was found between the 2D:4D ratio for left hands (t = 0.901; p = 0.368; > 0.05). Seborrheic dermatitis severity was negatively correlated with 2D:4D ratios of right hands (r = -0.391; p = 0.000-0.05).

STUDY LIMITATIONS

One of the main limitations of this study was the small sample, which got a head of us from acquiring certain findings about the 2D:4D ratio and seborrheic dermatitis. The other limitation is that the patient selection did not reflect the general population, as a single clinic was studied.

CONCLUSION

To the authors' knowledge, this is the first study examining the relationship between 2D:4D ratios and seborrheic dermatitis. The result of this study may indicate a line of investigation and can support the theory of prenatal androgen exposure.