Mike Murphy's Blog

These days, we see many claims of ‘paradoxical hypertrichosis’. Most of the time, these claims are inaccurate – they are not seeing this at all. So, what do these terms actually mean?

Let’s define these two terms…

Paradoxical hypertrichosis

PH (I can’t be bothered writing this out all the time!) occurs when hairs are observed near to an area which has been previously treated by either laser or IPL. The important thing to note is that it grows in areas OUTSIDE of the treated areas. This is not known to occur on treated areas (how could you tell?)

The cause of PH is still unknown. It may be caused by hormonal changes in the victim, stimulation by some outside force or divine intervention!

Synchronised hair growth

When hair follicles are treated with laser or IPL, some of them will be completely destroyed – they cannot grow back. However, many follicles will receive a relatively low fluence – a “sub-lethal” dose. These follicles will be partially damaged and will go back into the catagen phase. No matter where they are in the anagen phase, they will return to the catagen phase.

This occurs each and every time the area is treated. Unfortunately, it is inevitable. As a consequence, the follicles’ natural asynchronous growth pattern is disrupted. What happens is many of the follicles begin to grow at roughly the same time – in phase!

When this happens, we see ‘synchronised growth’ – may follicles growing at the same time.

Expert analysis

A review¹ (in 2010) of such occurrences by a team of American doctors found that the incidence of PH is quite low – typically between 0.6% to 10% and usually is found on the face and neck, and appears to be more common in darker skin tones.

However, they were wrong! They had identified synchronised growth – not HP!! Even the experts get it wrong. In their conclusion they state “subtherapeutic thermal injury causing induction of the hair cycle” as being a potential cause of the growth. This implies that the treated follicles had received insufficient fluence to kill the follicles, resulting in an alteration of their growth cycle. We know this is true today (perhaps they didn’t in 2010) but the fact that they claim this indicates that they were looking at hair growth in the treatment sites.

PH occurs outside of the treatment sites, as properly stated in a 2024 paper² where they state that PH was the “appearance of excess hair around previously treated areas”. In other words, not within the treatment sites.

So, if there is an area of the body where, say, 25% of the follicles are in the active anagen growth phase, and we kill half of them, then the half which survives will be forced into a catagen phase. At the next treatment session, the same thing will happen again, resulting in many follicles eventually being forced into catagen after a number of sessions.

After about five or six sessions, many of the follicles are in a relatively close synchronised growth cycle.

What happens now is very upsetting. At some point, many of these follicles will grow new hairs which will all appear at about the same time. But now there could be 60% or 70% or even more of the hairs visible, compared with only 25% previously.

THIS is synchronised growth – the appearance of many more hairs than before the treatment started. The fact is that there are simply more hairs visible to our eyes – NOT more hairs! Incidentally, this also occurs after repeated waxing session too.

A random photo of Neptune (because you’ve all seen hairy bits before!!)

So what???

Well, this is actually quite useful. Why, I hear you ask…

It is useful because it means that many follicles are growing at around the same time, which is not normal – but can be advantageous.

If more follicles in the ‘right’ stage of the anagen phase, then it will be easier to kill many more follicles, in one session, given the correct fluence etc.

But, the appearance of HP is unpredictable (otherwise it wouldn’t be a ‘paradox’!!) and it always occurs outside of treatment areas. However, it is just more hair to treat. There’s nothing particularly special about it – it’s just hair!

Hope this clears up a bit of misunderstanding,

Mike.

References

1. Desai, Shraddha MD; Mahmoud, Bassel H. MD, PhD; Bhatia, Ashish C. MD, Faad; Hamzavi, Iltefat H. MD. Paradoxical Hypertrichosis After Laser Therapy: A Review. Dermatologic Surgery 36(3):p 291-298, March 2010. | DOI: 10.1111/j.1524-4725.2009.01433.x

2. Inoue Y, Nishioka H, Inukai M, Shimizu Y, Kimura M, Akita H, Okumoto T. What are the Factors That Induce Paradoxical Hypertrichosis After Laser Hair Removal? Aesthet Surg J. 2024 Apr 4;44(5):NP347-NP353. doi: 10.1093/asj/sjae018. PMID: 38299374.