An ideal patient for conventional laser hair removal is one who has thick dark terminal hairs, light skin and normal hormonal status. Patient selection should not be compromised during laser hair removal as these can decrease response to treatment. These are explained in the following sections.
Dark skin types necessitate that sufficient caution is taken for the safe application of a laser hair removal by any wavelength. Clinical studies show that in skin types 4-6, there is increased amount of epidermal melanin which acts as a competing chromophore to melanin in hair bulb and shaft. This leads to a higher frequency of adverse effects. To minimize these adverse reactions, most clinicians use less fluence which can reduce efficacy and response in laser hair removal.
The long-pulsed Nd: YAG laser remains the recommended choice in very dark individuals and tanned patients due to its longer wavelength. Safety of patients with type 5-6 skin is a challenge for laser hair removal due to high density of competing chromophore in the epidermis. A wavelength which is less absorbed by melanin maybe less effective clinically as target chromophore for hair removal laser is melanin in hair bulb and bulge.
A study reported that diode laser was better than the alexandrite laser because emission from the former could penetrate deeper into the dermis.
Compared to intense pulsed light, long-pulsed Nd: YAG laser has been found to be more effective - as reported by both subjects and clinicians. Safety and efficacy of laser hair removal is compromised in patients with darker skin types with short-pulse durations and high fluences.
Tanning of skin
Complications like first or second degree epidermal burns from short wavelengths exist. Some physicians are compelled to use lower fluences to prevent burns at the cost of efficacy. Reports of complications from short wavelengths in the form of first- or second-degree epidermal burns result in use of suboptimal laser fluence and reduces efficacy of the procedure. This is a limitation of laser hair removal in dark skin types as hair reduction can be achieved but at the cost of epidermal burns.
Hair type and color
Terminal hairs, not vellus hairs, are considered suited for laser hair reduction as they absorb laser energy more. Good response to laser hair reduction occurs when the targeted hair has a high concentration of chromophores. Thin fine hairs have less pigment, and hence, are poor choices for laser hair reduction even with best fluences and multiple treatments compared to thick terminal hairs. Hairs less than 30 microns in diameter are not ideal for laser hair removal. When vellus or thin hairs are treated, responses may be poor due to relatively less chromophore in them. This is true when treating areas such as upper lip where chromophore in vellus hairs is less for laser wavelength absorption.
Pigmentation of hair
Melanin is the chromophore for laser absorption. Persons with black, brown, red, dark or blonde hairs achieve long-lasting results but those with light blonde or white hairs experience only temporary reduction for up to 12 weeks. However, considerable variations in treatment results are often seen among patients with dark hair. Most patients with brown or black hair obtain a 2- to 6-month growth delay after a single treatment. Though permanent hair loss is not expected in all individuals, lessening of hair density and thickness are. White and gray hairs have no melanin and are not known to respond to lasers. Nd: YAG laser works less effectively than alexandrite and intense pulsed light due to its poor affinity for melanin, which illustrates the role of chromophores in determining responsiveness of hairs to lasers. Some studies have shown that externally applied chromophores such as carbon suspension cause temporary reduction in white or gray hairs.But this is not proven to be an effective hair removal method the dermal papilla and stem cells are not destroyed by this method. These results suggest that photodynamic therapy may damage the nonpigmented hair matrix but not stem cells or dermal papillae. Repeated photodynamic therapy may impair the hair-regeneration capacity via a bystander effect on bulge stem cells or dermal papillae.
Stage of hair cycle
Hair in early anagen phase is most susceptible to laser treatment. Areas with high anagen hair percentages respond well to laser hair removal. After a session of laser hair reduction, re-growth of hair may be delayed up to 6 or 8 weeks; subsequent sessions after very brief intervals are associated with poor outcome. One has to plan the subsequent sessions after taking the hair growth cycles of various body areas into consideration. Duration of anagen phase is different in different areas of the body. An interval of 1-2 months between sessions is optimum and this depends on the body location.
Laser hair removal at short intervals results in inadequate time for initiation of anagen phase that may not allow laser energy absorption in the hair bulb or bulge. Prolonged intervals result in deeper migration of anagen bulb to the subcutis, thus reducing efficacy when shorter wavelength lasers are used. Clinically, this is a limitation as it is difficult to gauge by examination if the hair is in early anagen phase.
How to enable adequate energy diffusion to the bulge is a dilemma. Perifollicular edema and erythema are the only clinical features which can help to gauge the optimum endpoint.