The word laser is an acronym for light amplification by the stimulated emission of radiation.2 Laser light is unique in that it is composed of a single wavelength and all emitted photons are in the same phase. These properties are termed monochromatic and coherent, respectively. The wavelength of light is determined by the optical medium, which can be a solid (e.g., alexandrite), liquid (e.g., pulsed dye laser) or gas (e.g., carbon dioxide laser). When laser light hits the skin, it is absorbed, reflected, transmitted, or scattered. The desired treatment effect occurs when the light is absorbed and the energy is converted to heat (photothermal), acoustic waves (photomechanical) or used in chemical reactions (photochemical). The majority of dermatologic lasers employ photothermal reactions.

In 1983, the theory of selective photothermolysis was introduced, revolutionizing the application of lasers.3 This theory posits that because each chromophore has a unique photoabsorption spectrum, it can be selectively targeted and destroyed using specific wavelengths. Endogenous chromophores include melanin, hemoglobin and water.2 Tattoo ink is an exogenous chromophore frequently targeted with laser therapy.

A basic understanding of laser physics is required to troubleshoot complications and difficult-to-treat patients. By varying wavelength, fluence, pulse duration and spot size, the desired effect can be achieved (see Table 1 for definitions). The wavelength of light used determines which tissue chromophores will be targeted. The energy per area emitted by the laser is the fluence. High fluence can result in untoward damage to tissue, while insufficient fluence will lead to incomplete treatment. The concept of pulse duration is vital in minimizing damage to the surrounding tissue. The ideal pulse duration is generally the same4 or shorter2 than the thermal relaxation time (TRT) of the target. Immediately following photon absorption, the chromophore loses this newly absorbed energy in the form of heat. TRT is the time required for a chromophore to lose 50% of its heat to the surrounding tissue and is directly proportional to the square of its diameter. Thus, there is a positive relationship between pulse duration, TRT and diameter of the target. Destruction of targets with a small diameter, such as melanosomes, requires small pulse durations in order to limit damage to surrounding tissue. Those with a larger diameter, such as blood vessels, can be treated with longer pulse durations. For this reason, Quality– or Q-switched (QS) lasers (see Table 1 for definitions) are often used for pigmented lesions, while pulsed dye lasers (PDL), with pulse durations in the millisecond range, are used for vascular lesions.

Spot size and wavelength can each impact the depth of penetration of the laser. Depth of penetration is determined by both the amount of light absorption in the tissue and the scatter.2 In general, larger spot sizes (10–15 mm) and longer wavelengths (1,000–1,200 nm) have less scatter than smaller spot sizes (3–5 mm) and shorter wavelengths (300–400 nm) and penetrate deeper into the tissue. However, when wavelengths exceed approximately 1,300 nm, such as the CO₂ resurfacing laser at 10,600 nm, depth is limited to the superficial epidermis due to absorption by water, the primary targeted chromophore in this range.

The ideal laser settings will provide enough thermal energy and selectivity to damage the targeted chromophore while minimizing injury to the surrounding skin.5 However, the energy required to damage the target often exceeds that which is needed to harm the overlying epidermis. In order to overcome this obstacle, cooling devices have been implemented. Many laser handpieces are now equipped with cryogen spray cooling or sapphire contact plates that cool the skin immediately prior to the laser pulse. The epidermis is then selectively cooled, raising the threshold for thermal damage, while temperatures for the intended (deeper) dermal targets are relatively unaffected. Pre-cooling with ice-packs, gel and aluminum rollers can also be used but these are generally not as effective as the incorporated cooling devices. In addition to increasing the treatment efficacy of lasers, cooling also plays a prominent role in providing anesthesia during therapy.

Lasers are useful medical devices but can be dangerous. The operating staff and patient are susceptible to a number of hazards, including burns, accidental fires, infection and ocular injury. General safety measures, such as a visible door sign when the laser is in use and controlled access into the laser areas, are important. All lasers should be kept in standby mode when not in use and have a rapid deactivation mode to prevent unintentional firing. To avoid accidental fires, the use of oxygen and alcohol-based cleansing methods should be minimized. Each laser should have its own outlet, limiting the use of extension cords. Since human papilloma virus (HPV)6 and human immunodeficiency virus (HIV)7 particles have been found in aerosolized material after laser therapy, a smoke evacuator should be employed when lasers with significant potential to splatter aerosolized particles (such as CO₂ resurfacing) are used.

Lasers can subject the unprotected eye to significant damage, including visual loss. The degree of ocular damage is determined by the wavelength of light as well as the amount and duration of energy exposure.8 Within the eye, there is a broad absorption spectrum resulting from three primary chromophores: melanosomes, found in the retinal epithelium, iris, sclera and choroid; hemoglobin, found in retinal vasculature; and water, found primarily in the cornea and lens. The symptoms associated with damage to these ocular structures are summarized in Table 2. Especially important to remember is that absorption of light outside the visual spectrum is visually undetectable. Additionally, due to their lack of sensory innervation, absorption damage to the retina, iris, choroid and sclera can be painless. Thus, ocular exposure to wavelengths in the near infrared range (700–1,400 nm), which is primarily absorbed by these pigmented eye structures, can lead to pronounced injury without symptoms.8,10

To prevent untoward injury, wavelength-specific eye protection should be worn any time the laser is in operation. Metal contact lenses should be placed on the patient when treating periorbital skin, and metal goggles should be worn for the treatment of facial lesions. Plastic goggles should be avoided in these cases due to the risk of melting. Pediatric and smaller patients should have size-specific goggles; if these are not available, the use of a thick layer of white gauze (which non-specifically absorbs and reflects laser light) can be used.

An appropriately trained physician should evaluate all patients prior to laser treatment to determine whether they will be appropriate candidates for therapy. Some patients are at increased risk of sustaining complications (see Patient Factors section below), and these factors should be considered prior to choosing laser parameters. In some cases these risk factors may prove to be contraindications to laser treatment, as the risk of complications may outweigh the benefits. Verbal and written informed consent, including an explanation of the potential risks and benefits, alternative treatment options, number of treatment sessions, and cost per session should be obtained prior to the planned procedure which can help avoid the patient feeling pressured into making a quick decision.11

The indication for laser therapy must be appropriate. While treatment of rhytides may seem straightforward, treatment of other lesions, such as those on photo-damaged skin, may present more challenges to the non-dermatologist. Often, only minor differences distinguish a solar lentigo from a melanoma, or an actinic keratosis from a nonmelanoma skin cancer. These subtleties may not be obvious to the untrained clinician. Any lesion in question should be biopsied prior to treatment.

Troubleshooting complications requires a firm understanding of laser mechanics. A skilled operator will not only choose appropriate laser settings, but will be attentive in monitoring the tissue reaction to the laser treatment. An immediate white/gray discoloration can indicate significant thermal damage requiring modification of laser settings. Ideally, test spots should be done at least 2 weeks prior to the planned procedure.

Patient factors must be taken into account in the assessment of an individual’s risk profiles. Patients with dark or tanned skin are at increased risk of hypopigmentation if treated with lasers that target melanosomes. Any history of abnormal scarring, vasculopathy or vitiligo should be elicited, as these conditions may indicate a propensity toward hypertrophic scarring, bruising and dyspigmentation, respectively. Treatment should be avoided in areas with concurrent infection or inflammation as it can exacerbate these conditions. Connective tissue diseases, which can be a marker for photosensitivity, may be a contraindication to laser therapy.12 One study reported new-onset discoid lupus erythematosus thought to be from thermal injury sustained after argon laser treatment for facial telangiectases.13 Debate exists in the literature, however, as others have successfully treated discoid lupus with similar lasers.14 Additionally, there are several medications (summarized in Table 3) that may confer an increased risk of complications. These should be considered and avoided if possible.15,16,19,20