Laser therapies

Meet the patient — four walks into the laser clinic

A newborn with a pink patch on the forehead in the V1 trigeminal dermatome; a fair-skinned woman tired of shaving her axillae; a South-Asian man with beard pseudofolliculitis who wants the hair gone; and a patient with a multicolour tattoo he now regrets. Four different concerns, but each consultation reduces to the same three questions: which chromophore am I trying to hit, what wavelength reaches it, and is this lesion safe to laser?^[1]

Hold those three questions and a dozen devices become a short list. Forget them and you burn skin of colour, scar a patient on isotretinoin, or — worst of all — laser a melanoma and delay the diagnosis that would have saved a life.^[1][10]

Selective photothermolysis — the one principle that organises everything

What transformed lasers from blunt thermal instruments into precise surgical tools was the principle Anderson and Parrish published in Science in 1983. They showed that if three conditions are met, a target in the skin is destroyed while the tissue around it is preserved.^[1]

That single insight made it possible to ablate a 100-micrometre blood vessel, shatter a tattoo particle, or coagulate a hair follicle without burning the overlying epidermis, and it underpins every laser decision in modern dermatology.^[1]

The beam, and the physics that makes it surgical

The word LASER is an acronym: Light Amplification by Stimulated Emission of Radiation. Unlike ordinary white light, a laser beam is monochromatic (a single wavelength), coherent (waves in phase), and collimated (parallel, low divergence) — three properties that allow a large amount of energy to reach a microscopic target with minimal scatter.^[1]

Once the beam enters skin, three interactions decide the clinical effect. Absorption by the target chromophore is what you want, and the absorption coefficient at the chosen wavelength sets how much energy deposits per unit depth. Scattering by collagen deflects photons and is wavelength-dependent, which is why longer wavelengths penetrate deeper — Nd:YAG at 1064 nm reaches deep reticular dermis and leg veins, while KTP at 532 nm treats only superficial vessels. Reflection at the surface (4 to 7 percent) is reduced by coupling gel and a matched handpiece.^[1]

The pulse duration is the single most controllable safety variable, because heat diffuses out of a target at a finite rate. The thermal relaxation time (TRT) is the time for the target to cool to half its peak temperature; keep the pulse shorter than the TRT and heat stays confined, run it longer and heat spreads and collateral damage follows. Small targets have short TRTs:^[1]

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This is why a Q-switched nanosecond ruby pulse shatters a melanosome without burning the epidermis, while a long-pulsed millisecond alexandrite heats the whole hair follicle to coagulation — the same wavelength, a different pulse duration, a different target.^[1]

The four chromophores — and the wavelength that hits each

Every clinical laser decision reduces to one question: which chromophore am I hitting, and what wavelength and pulse duration does it demand? Skin holds four clinically relevant chromophores.^[1]

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Depth of penetration rises with wavelength within the optical window, which is why device choice is dictated as much by lesion depth as by chromophore.^[1]

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Epidermal cooling — the second great enabler of safety

Because the epidermis is rich in melanin and water, it absorbs a fraction of every vascular, hair, and pigment pulse; without protection this produces a burn and, weeks later, pigmentary change. Cooling lowers the basal epidermal temperature so it tolerates the brief thermal pulse while the deeper target is still heated above its injury threshold.^[1][10]

Three methods are in routine use. Contact cooling uses a chilled sapphire or copper window pressed to the skin before, during, and after the pulse; dynamic cryogen spray fires a timed burst of tetrafluoroethane milliseconds before the laser pulse, evaporating on the surface; and forced refrigerated air delivers a continuous cold stream. Cooling is essential for hair-removal and vascular work in all but the fairest skin.^[1]

The devices by chromophore and use

A working classification groups devices by the chromophore they target, which in turn sets the indications they treat. The categories below cover virtually every laser an examiner will name.^[1]

Vascular lasers (oxyhaemoglobin). The pulsed-dye laser (PDL) at 585 to 595 nm is the gold standard for vascular lesions — its yellow light matches an oxyhaemoglobin absorption peak, treating port-wine stain, infantile haemangioma, facial telangiectasia, rosacea erythema, and scar redness. KTP at 532 nm is excellent for fine facial telangiectasia and spider naevi but absorbs more melanin, so use with caution in darker skin. Long-pulsed Nd:YAG at 1064 nm penetrates deeper for reticular leg veins and larger vessels and is the vascular laser of choice in Fitzpatrick V and VI. Intense pulsed light (IPL), broadband rather than a true laser, treats diffuse erythema and photorejuvenation.^[9][14]

Pigment and hair-removal lasers (melanin). Q-switched ruby (694 nm), alexandrite (755 nm), and Nd:YAG (1064 nm, with the 532 nm doubled beam for red-orange ink and epidermal lentigines) are the tattoo and pigment workhorses. For hair removal, the long-pulsed alexandrite suits fair skin (I to III), the diode suits olive skin (III to IV), and the long-pulsed Nd:YAG at 1064 nm is safest in dark skin (V to VI) because its longer wavelength minimises competing epidermal melanin absorption.^[2][5]

Resurfacing lasers (water). CO2 at 10600 nm ablates and leaves a coagulation zone that contracts dermal collagen — the best single-session result for wrinkles, acne scars, and photoaging, at the cost of two weeks of downtime. Er:YAG at 2940 nm gives precise shallow ablation with minimal residual thermal damage. Fractional lasers create a grid of microscopic treatment zones surrounded by viable tissue, so re-epithelialisation is rapid and scarring risk is far lower than fully ablative resurfacing.^[3][4]

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Picosecond lasers deliver pulses a thousand times shorter than Q-switched nanosecond pulses; at these durations the dominant effect is photoacoustic, fracturing ink into fragments fine enough for macrophage clearance with less photothermal damage. They are superior for multicolour and resistant tattoos and, with diffractive lens arrays, for acne scarring.^[11]

The 308-nm excimer laser delivers targeted narrowband ultraviolet B to a discrete lesion, repigmenting stable vitiligo and clearing localised plaque psoriasis with a fraction of the cumulative UV dose of whole-body phototherapy.^[6][7]

Clinical applications by indication

Vascular lesions. Port-wine stain is the classic PDL indication — these congenital capillary malformations grow darker and thicker with age, so early treatment in infancy exploits thinner skin and smaller vessels. Six to twenty or more sessions yield roughly 50 to 70 percent lightening with complete clearance in only 10 to 20 percent. PDL is also first-line for superficial infantile haemangioma, especially ulcerated lesions, though deep or segmental haemangiomas still need oral propranolol. Diffuse facial telangiectasia and rosacea erythema respond to sub-purpuric PDL or KTP.^[9][14]

Pigmented lesions. Epidermal pigmentation — solar lentigines, cafe-au-lait macules, epidermal melasma — responds to Q-switched or picosecond lasers. Melasma is treated cautiously: dermal melasma may paradoxically darken, so low fluence, test patches, and concurrent hydroquinone are essential. Post-inflammatory hyperpigmentation in skin of colour demands conservative settings, aggressive cooling, and pre-treatment with a tyrosinase inhibitor; multiple low-fluence sessions outperform one aggressive treatment.^[10]

Tattoo removal. Q-switched ruby, alexandrite, and Nd:YAG remain the workhorses; picosecond lasers improve clearance of multicolour, brightly coloured, and resistant tattoos. Black and dark-blue ink clears fastest; green, yellow, and white respond poorly; flesh-toned cosmetic ink (titanium dioxide) may undergo paradoxical darkening to grey-black and must always be test-treated. Amateur tattoos clear in fewer sessions than professional ones, with sessions spaced six to eight weeks apart and typically five to fifteen needed.^[5][11]

^[1]

Hair removal. Long-pulsed lasers target melanin in the anagen hair bulb and bulge, with the best results in dark, coarse hair on light skin where chromophore contrast is maximal; blonde, grey, red, and white hair respond poorly because the follicle lacks pigment. Six to ten sessions at four-to-eight-week intervals achieve durable 70 to 90 percent reduction rather than permanent eradication, and maintenance is expected.^[2]

^[2]

Resurfacing and rejuvenation. Ablative CO2 remains the benchmark for deep rhytides, atrophic acne scars, and photoaging, delivering a single-session result with five-to-ten-year durability. Fractional CO2 and Er:YAG deliver comparable improvement over three to five sessions with a fraction of the downtime and have become the default for most scar and rejuvenation work. Burn and surgical scars benefit from fractional CO2 or PDL, which improve texture, pliability, and erythema even in mature scars.^[3][4][13]

Targeted phototherapy. The 308-nm excimer laser repigments stable, localised vitiligo (face and neck respond best, extremities poorly) and clears localised plaque, scalp, and palmoplantar psoriasis with fewer treatments and a lower cumulative UV dose than whole-body narrowband UVB.^[6][7]

Pre-treatment assessment — the visit that prevents the complication

Most laser indications are diagnosed clinically and need no investigation, but several targeted work-ups are essential, and the pre-treatment assessment is where the harm is prevented.^[1]

The single most important rule — and a frequent exam stem — is that no undiagnosed or atypical pigmented lesion should be lasered. A melanoma misdiagnosed as a lentigo and treated with a Q-switched laser will be masked, biopsied late, and prove fatal. Any lesion meeting the ABCDE criteria, or with atypical dermoscopy, must be biopsied first. Dermoscopy is performed on every pigmented lesion before any laser.^[1]

Before any vascular treatment, distinguish a port-wine stain (congenital, present at birth, grows darker and thicker, never involutes) from an infantile haemangioma (appears in the first weeks of life, proliferates, then involutes), because management diverges — propranolol dominates the latter. A port-wine stain in the V1 ophthalmic distribution mandates evaluation for Sturge-Weber syndrome (leptomeningeal angiomatosis, seizures, glaucoma): an ophthalmology review for glaucoma and an MRI if neurological signs are present.^[14]

The focused pre-treatment assessment documents Fitzpatrick skin type (the master determinant of fluence and pigmentary risk), the colour, depth, and density of the target, sun-exposure and tan status, a full medication history concentrating on isotretinoin in the preceding six months and photosensitisers, a history of herpes simplex or keloids, and pregnancy status. A test patch is performed in skin of colour (Fitzpatrick IV to VI) or any high-risk lesion, read at 24 to 48 hours before full treatment.^[10]

Safety — protect the eye, protect the skin, exclude the contraindicated

Laser safety rests on protecting the eye, protecting the skin, and excluding patients in whom the procedure is contraindicated. The beam is intense enough to cause permanent retinal, corneal, or iris injury in a fraction of a second, and reflected (specular) beams carry the same hazard as the primary beam.^[1]

Wavelength-specific protective goggles — matched to the laser's wavelength and optical density — must be worn by the patient, the operator, and every person in the treatment room; generic sunglasses are not acceptable. For periorbital work, internal metal eye shields are placed after topical anaesthesia. Windows are covered, warning signs are displayed, and the door is locked during firing. The laser plume (vaporised tissue smoke) can carry viable viral particles, so a smoke evacuator is mandatory for ablative work.^[1]

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The contraindications — when to put the handpiece down

Acute emergencies — the burn and the eye

Laser treatment is elective and ambulatory, but two genuine emergencies can arise, and the operator must be prepared for both.^[1]

Acute thermal injury (over-treatment) from excessive fluence, inadequate cooling, or treatment of tanned or dark skin produces a partial- or full-thickness burn within minutes. Immediate management is that of any thermal burn: stop further treatment, cool the area with running cool water or compresses for 15 to 20 minutes, apply a topical corticosteroid, cover with a non-adherent dressing, and give oral analgesia. Assess burn depth; deep injuries, especially on the face, warrant referral to a plastic or burns service, and the patient is counselled about near-certain post-inflammatory pigment change.^[10]

Ocular exposure — a reflected or direct beam reaching the eye — can cause instant, permanent retinal, corneal, or iris injury, most feared with invisible near-infrared beams (Nd:YAG, diode, alexandrite) where there is no blink reflex. Any suspected intraocular exposure is an ophthalmological emergency: stop the procedure, keep the patient still, cover both eyes, and arrange immediate assessment. Prevention is paramount and entirely reliable when wavelength-specific goggles and intraocular metal shields are used correctly.^[1]

^[10]

Complications — pigmentary, thermal, infectious, scarring

Complications fall into pigmentary, thermal, infectious, scarring, and cosmetic categories, and their incidence rises with darker skin type, higher fluence, inadequate cooling, a recent tan, and isotretinoin exposure.^[9]

^[9]

Pigmentary changes are the commonest adverse event in Fitzpatrick IV to VI. Post-inflammatory hyperpigmentation appears three to four weeks after treatment and may persist for months; it is minimised by conservative fluence, aggressive cooling, hydroquinone pre-treatment, and sun protection. Hypopigmentation is less common but more recalcitrant, from melanocyte destruction, and is a particular risk of overly aggressive hair removal and Q-switched treatment in dark skin.^[10]

Thermal burns, blistering, and crusting result from excessive fluence, inadequate cooling, or a recent tan, and are managed as thermal injuries. Infection — reactivated herpes simplex, bacterial folliculitis, or candidiasis — is prevented by antiviral prophylaxis in at-risk patients and aseptic technique. Hypertrophic scarring and keloids are rare but devastating, concentrated on the chest, jawline, and neck and after resurfacing during or soon after isotretinoin; the dictum is to defer resurfacing for at least six months after cessation.^[8]

Ocular injury is prevented entirely by correct eye protection and is a sentinel never-event. Paradoxical darkening of cosmetic tattoo ink (titanium dioxide reduction to grey-black) is avoided by a mandatory test patch on flesh-toned or white ink. A systematic review confirms PDL for port-wine stain is safe overall, with transient pigmentary change and blistering the most common effects and scarring rare.^[9]

Pre- and post-treatment care

Careful preparation and aftercare determine the outcome as much as the laser itself. Photoprotection starts before treatment: apply a broad-spectrum sunscreen of SPF 50 or higher and use sun-protective measures — wide-brimmed hat, shade, avoiding tanning — from two to four weeks before the procedure.^[15] In skin of colour and for pigment work, pre-treatment with topical depigmenting agents such as hydroquinone is a recognised prophylaxis strategy against post-inflammatory hyperpigmentation, alongside retinoids and alpha hydroxy acids.^[16] Patients with a history of herpes simplex undergoing facial resurfacing receive antiviral prophylaxis: valaciclovir 500 mg twice daily, started the day before or the morning of the procedure and continued for 10 to 14 days. In two prospective trials (one randomised), totalling over 200 patients, this regimen prevented HSV reactivation in every patient.^[17][18]

Immediately after ablative resurfacing the wound is an open, weeping surface managed with occlusive dressings and frequent emollient; non-ablative and fractional patients need only gentle cleansing, moisturiser, and sunscreen. Sun avoidance and SPF 50 sunscreen are non-negotiable for at least four to six weeks (longer in dark skin) to prevent pigmentary change.^[4]

Special populations

These are the groups where the strategy shifts.^[1]

• Children — port-wine stains are best treated in infancy, when skin is thinner, vessels smaller, and the stain lighter; topical anaesthesia, distraction, and (for extensive work) sedation or general anaesthesia address the practical barriers.^[14]
• Skin of colour (Fitzpatrick IV to VI) — competing epidermal melanin raises pigmentary and burn risk; choose the longest effective wavelength (long-pulsed Nd:YAG 1064 nm), use lower fluence, longer pulse durations, aggressive cooling, hydroquinone pre-treatment, a mandatory test patch, and longer intervals.^[10]
• Pregnancy — elective laser is deferred; hormone-driven melasma and spider naevi may worsen, and there are no safety data on fetal exposure.^[2]
• Immunocompromised patients — slower healing and higher infection risk warrant conservative settings, antiviral prophylaxis, and a low threshold for antibiotic cover.^[1]
• Patients on isotretinoin — defer ablative and fractional resurfacing for at least six months after cessation because of the markedly increased scarring risk.^[8]

Adjuncts — microneedling, drug delivery, and combination therapy

Laser results are often enhanced by combination. Microneedling mechanically creates micro-wounds that trigger collagen remodelling for acne scars, fine wrinkles, and stretch marks, and can be combined with platelet-rich plasma or vitamin C for synergy with fractional laser. Laser-assisted drug delivery uses fractional ablation to create channels for topical agents, an emerging application for vitiligo, alopecia areata, and scar modulation. For burn and surgical scars, PDL plus fractional CO2 outperforms either alone.^[12][13]

The history, briefly

The mantra, and the memory device

^[1]

The mantra: match the wavelength to the chromophore, keep the pulse shorter than the relaxation time, protect the eye, defer the isotretinoin patient, and never laser a lesion you have not diagnosed.^[1][8]

Etymology for viva gold: LASER began as an acronym — Light Amplification by Stimulated Emission of Radiation — coined by Gordon Gould in 1957, and like radar before it, graduated from acronym to ordinary noun. Photothermolysis stitches Greek photos (light) plus therme (heat) plus lysis (loosening or destruction): light-driven heat that destroys a chosen target.^[1]

Ward-round test — three stems, thirty seconds each

Stem 1 — the newborn with a pink V1 forehead patch (answer)ShowHide

A newborn has a pink patch on the forehead in the V1 ophthalmic distribution. What is the lesion, what syndrome must you exclude, and which laser is first-line? Model: This is a port-wine stain (capillary malformation), present at birth and growing darker and thicker with age. A V1 distribution mandates evaluation for Sturge-Weber syndrome — leptomeningeal angiomatosis with seizures and glaucoma — via an ophthalmology review for glaucoma and an MRI if neurological signs are present. First-line treatment is the pulsed-dye laser at 585 to 595 nm, best started in infancy when the skin is thin and the vessels small; expect 50 to 70 percent lightening over many sessions, with complete clearance in only 10 to 20 percent.^[9][14]

Stem 2 — resurfacing requested six weeks after stopping isotretinoin (answer)ShowHide

A patient stopped isotretinoin six weeks ago and requests ablative CO2 resurfacing for acne scars. What is the right advice, and why? Model: Defer the resurfacing for at least six, ideally six to twelve, months after isotretinoin cessation. Isotretinoin impairs wound healing — particularly sebaceous-gland function and re-epithelialisation — and resurfacing during or soon after therapy carries a markedly increased risk of hypertrophic scarring, a devastating and largely irreversible complication. The six-month rule is conservative practice supported by most guideline bodies; a small challenging study does not overturn it. Document the advice and rebook.^[8]

Stem 3 — a South-Asian man wants facial hair removal (answer)ShowHide

A Fitzpatrick V man with beard pseudofolliculitis wants laser hair removal. Which device, and what safety measures? Model: Choose the long-pulsed Nd:YAG at 1064 nm — its longer wavelength minimises competing epidermal melanin absorption, making it the safest hair-removal device in dark skin. Run lower fluence, longer pulse durations, aggressive epidermal cooling, and a mandatory test patch read at 24 to 48 hours, with longer intervals between sessions to watch for post-inflammatory hyperpigmentation. The competing melanin in Fitzpatrick V skin is the hazard; the longer wavelength is the mitigation. Expect durable 70 to 90 percent reduction over six to ten sessions, with maintenance.^[2][10]

References18ShowHide

1. [1]Anderson RR, Parrish JA. Selective photothermolysis: precise microsurgery by selective absorption of pulsed radiation Science, 1983.PMID 6836297
2. [2]Gan SD, Graber EM. Laser hair removal: a review Dermatol Surg, 2013.PMID 23332016
3. [3]Bogdan Allemann I, Kaufman J. Fractional photothermolysis--an update Lasers Med Sci, 2010.PMID 19787413
4. [4]Janik JP, Markus JL, Al-Dujaili Z, et al. Laser resurfacing Semin Plast Surg, 2007.PMID 20567665
5. [5]Gurnani P, Williams N, Al-Hetheli G, et al. Comparing the efficacy and safety of laser treatments in tattoo removal: A systematic review J Am Acad Dermatol, 2022.PMID 32763326
6. [6]Sun Y, Wu Y, Xiao B, et al. Treatment of 308-nm excimer laser on vitiligo: A systemic review of randomized controlled trials J Dermatolog Treat, 2015.PMID 25428573
7. [7]Mudigonda T, Dabade TS, Feldman SR. A review of protocols for 308 nm excimer laser phototherapy in psoriasis J Drugs Dermatol, 2012.PMID 22206083
8. [8]Khatri KA, Iqbal N, Bhawan J. Laser skin resurfacing during isotretinoin therapy Dermatol Surg, 2015.PMID 25993614
9. [9]Shi MD, Yang K, Li SB, et al. Complication rates and safety of pulsed dye laser treatment for port-wine stain: a systematic review and meta-analysis Lasers Med Sci, 2023.PMID 38141129
10. [10]Sowash M, Alster T. Review of Laser Treatments for Post-Inflammatory Hyperpigmentation in Skin of Color Am J Clin Dermatol, 2023.PMID 36781686
11. [11]Wu DC, Goldman MP, Wat H, et al. A Systematic Review of Picosecond Laser in Dermatology: Evidence and Recommendations Lasers Surg Med, 2021.PMID 32282094
12. [12]Alster TS, Graham PM. Microneedling: A Review and Practical Guide Dermatol Surg, 2018.PMID 28796657
13. [13]Altemir A, Boixeda P. Laser Treatment of Burn Scars Actas Dermosifiliogr, 2022.PMID 35963335
14. [14]Sebaratnam DF, Rodríguez Bandera AL, Wong LF, et al. Infantile hemangioma. Part 2: Management J Am Acad Dermatol, 2021.PMID 34419523
15. [15]Lain E, Alexis AF, Andriessen A, et al. A Practical Algorithm for Integrating Skincare to Improve Patient Outcomes and Satisfaction With Energy-Based Dermatologic Procedures J Drugs Dermatol, 2024.PMID 38709701
16. [16]Wong ITY, Richer V. Prophylaxis of Post-Inflammatory Hyperpigmentation From Energy-Based Device Treatments: A Review J Cutan Med Surg, 2021.PMID 32929988
17. [17]Beeson WH, Rachel JD. Valacyclovir prophylaxis for herpes simplex virus infection or infection recurrence following laser skin resurfacing Dermatol Surg, 2002.PMID 11966791
18. [18]Gilbert S, McBurney E. Use of valacyclovir for herpes simplex virus-1 (HSV-1) prophylaxis after facial resurfacing: A randomized clinical trial of dosing regimens Dermatol Surg, 2000.PMID 10632686