Botulinum toxin A: a contradictory role in hair loss

Botulinum toxin A can increase blood flow to the follicle, thus stimulating hair growth. However, in patients repeatedly treated for forehead wrinkles, it can induce hair loss.

Prof. Natasha Mesinkovska (UC Irvine School of Medicine, CA, USA) illustrated that therapy with botulinum toxin for hair loss was assessed in 6 studies since 2006: 4 showed improvement, 2 inconclusive results [1]. Most patients had androgenetic alopecia (AGA). In a pilot study including 50 patients with AGA, botulinum toxin therapy led to a significant increase in hair density [2]. The primary outcome was change in hair count in a fixed 2 cm area. After 3 sessions, hair count increased by 18% between the start of the trial and week 48 in the 40 male subjects that completed the trial. Hair loss was also significantly reduced, by a mean of 39%. The treatment response rate was 75%.

Yet, why is botulinum toxin effective in androgenetic hair loss? It is plausible that botulinum toxin interferes with the molecular mechanisms seen in AGA: the predisposed scalp exhibits high levels of dihydrotestosterone (DHT) and an increased expression of the androgen receptor. Conversion of testosterone to DHT within the dermal papilla plays a central role in AGA. The elevated DHT concentrations can at least in part be explained by a microvascular insufficiency in AGA: pO₂ is lower in bald patients compared with patients with hair. In a low-oxygen environment, conversion of testosterone to dihydrotestosterone is enhanced, whereas more testosterone is converted to oestradiol in a high-oxygen environment. As Prof. Mesinkovska pointed out in her talk, botulinum toxin increases both blood supply and transcutaneous pO₂. It relaxes the muscles of the scalp, eases the muscle tension in the scalp, thereby reducing pressure on the vessel. Therefore, blood flow to the follicle is increased and DHT is removed.

In 2020, a second study with botulinum toxin in Chinese AGA patients was published [3]. In this study 63 patients with AGA were treated every 3 months for a total of 4 times in 30 sites (100 U botulinum toxin in each site). Hair growth and density were significantly augmented, and the area of hair loss was attenuated after each treatment through head photographs.

Yet, data on a detrimental effect of botulinum toxin on hair growth is available as well. In particular, botulinum toxin seems to induce frontal alopecia accompanied by gradual regression of the frontal hairline in patients that are treated periodically with botulinum toxin injections for forehead wrinkles. In patients with botulin-induced frontal alopecia (BIFA), skin is normal and there is no atrophy, scarring or progressive hair miniaturisation at trichoscopy, or inflammatory responses [4]. One of the explanations for botulinum-induced hair loss is a decreased neurological stimulation of the hair follicle due to blockade of the function of autonomic fibres. “This is something to watch out for in our patients,” Prof. Mesinkovska said. Taken together, many questions are left to answer regarding alopecia and the role of botulinum toxin.

1. Mesinkovska N. HOT Topics: Alopecia. Session S027, AAD VMX 2021, 23-25 April.
2. Freund BJ, Schwarz M. Plast Reconstr Surg 2010;126:246e-8e.
3. Zhou Y, et al. BioMed Res Int 2020, article ID1501893.
4. Di Pietro, Piraccini BM. Skin Appendage Disord 2016;2(1-2): 67-9.

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Posted on 7 June, 202128 March, 2024