Lasers represent a relatively new group of devices used within the field of cutaneous surgery. The field of laser surgery continues to advance with the addition of new devices and the expansion of indications for laser therapy. A laser's wavelength, energy, and pulse duration are used to target a desired chromophore. The most common chromophores are water, hemoglobin, and pigment (e.g., melanin or tattoo pigment). These chromophores are selectively targeted to achieve various clinical outcomes. 

Laser therapy can be divided broadly into four different indications: vascular lasers, pigment lasers, resurfacing lasers (ablative or non-ablative), and hair removal lasers.[1] Complications will be broadly discussed; however, each laser presents unique challenges, risk factors, and complications that are beyond the scope of this article. For specific complications, please refer to the complication section of each laser. In general, the complications from laser surgery can be mitigated by combining proper technique with appropriate patient and device selection.[2]

Burns, scarring, dyspigmentation, ocular injury, and infection can occur as a result of almost any laser therapy.[1][2][3][4][5] These complications are expected side effects based on the theory of selective photothermolysis and, in some cases, can even be used to the surgeon’s advantage, such as when reducing pigmentation in melasma. When a chromophore is stimulated by its target wavelength, the chromophore begins to heat up. Newer devices use a pulse duration that is less than the thermal relaxation time of the chromophore. Thermal relaxation time is the amount of time necessary for a particle to cool down to a baseline temperature. The goal of selective photothermolysis is to heat the particle enough to destroy the chromophore without injuring the surrounding tissue.[2][4] It follows that surrounding non-targeted tissue can be burned from overheating. Burns can result from long pulse durations, excessive fluence (an indication of the amount of energy delivered over the area), and improper delivery of cooling. The risk of burns is higher for lasers that use a continuous beam. Newer devices have attempted to mitigate this risk by pulsing the beam in the millisecond range, using multiple rapid bursts in a quasi-continuous manner or by using extremely short pulses with high powers.[2] The addition of various cooling devices can also help to limit heating of non-targeted tissue. Many modern lasers have built-in cooling systems to aid in the efficacy and safety of the device. Cooling of the skin can be done before treatment, intra-operatively, or immediately after delivery of a laser pulse. Cooling techniques can be divided into two categories: contact and non-contact. Types of contact cooling include using a sapphire tip to actively cool the skin during surgery or using ice packs before and after the procedure. Non-contact techniques include the use of cryogen sprays and forced refrigerated air.[6] The choice of cooling techniques is typically limited by the type of laser and physician preference. Laser surgery can induce scarring as a result of burns during treatment, abnormal wound healing, or secondary infections. Ablative lasers are used to create microscopic zones of ablated tissue that subsequently heals in and stimulates collagen deposition. Scarring from ablative lasers can result from excessively large zones of ablation.[7] As discussed, lasers used in vascular lesions, pigmented lesions, and laser hair removal can burn surrounding tissue and lead to scarring as well. In addition, cutaneous post-op infections can also lead to scarring.

Dyspigmentation

Dyspigmentation in laser surgery can result in both hypopigmentation and hyperpigmentation. Risk of dyspigmentation is highest in darker skinned (Fitz Patrick type III-VI) or excessively tan individuals, and may be reduce with the use of a fractionated laser delivery system. Hypopigmentation is a very rare complication and can be a delayed finding. The cause of delayed hypopigmentation has yet to be elucidated.[7][8] [9] Hypopigmentation is due to damage to epidermal melanocytes. The hyperpigmentation can be due to extracellular melanin from the destruction of melanocytes or a reactionary increased production of melanin.[10]

Burns

Burns occur from overheating of the tissue through excessive heat generation or by a failure of the cooling techniques. Proper patient selection and conservative settings or test spots administered by a skilled laser surgeon reduce the risk of laser complications.[2][11] An indication of excessive fluences and, therefore, excessive heating is graying of the tissue intra-operatively. If graying occurs, the procedure should be stopped and the settings and cooling systems should be evaluated. Hemorrhagic crusts and ulcerations may be seen several days post-op and can be warning signs for further complications, including scarring and dyspigmentation.[9] Patients may also report excessive pain, especially when compared to prior treatments. The degree of presentation is highly variable and can range from prolonged erythema to ulcerations and necrosis. Even though the presentation of cutaneous burns can range from erythema to ulceration, the differential can be quickly narrowed due to a history of laser surgery in the area. Ulcerations from cutaneous burns can look similar to post-op infections specifically herpes infections. Depending on the device being used, “foot-prints” matching the tip of the laser can be readily distinguished. An example of this would be circular patches of hypopigmentation seen after laser hair removal with an alexandrite laser.[8][12][8] Too much overlap of treatment zones can lead to burns and dyspigmentation, but too much intervening space in between zones of treatment can lead to noticeable areas of untreated skin.[2] Scarring and dyspigmentation can be noted weeks to months after treatment.Proper technique and conservative setting are essential in reducing the risk of burns. Various cooling technologies, when used correctly, can reduce the risk of cutaneous burns. Burns following laser therapy can be treated with immediate cooling and subsequently treated with bland emollients and topical steroids to promote re-epithelization.[8] 

Infections

Infections are one of the most common complications following laser surgery, especially ablative procedures since they disrupt the barrier function of the skin. Because of the risk for infection, an active infection is a contraindication to ablative resurfacing. Cutaneous infections can present atypically after ablative resurfacing, so clinicians should have a low threshold for culture and empiric treatment.[2][9] Herpes simplex virus (HSV) reactivation is commonly seen after laser treatments, particularly resurfacing of the perioral skin, suggesting that antiviral prophylaxis is indicated.[13] HSV reactivation presents as localized or diffuse painful erosions with or without vesiculation, typically within a week of treatment. Common pathogens as bacterial infections include Staphylococcus aureus, Pseudomonas, and Escherichia coli. Bacterial infections will present as purulent non-healing erosions. Superficial candidal infections can present as prolonged erythema or intense pruritus and can be seen up to 2 months after treatment.[7][9][8]

The diagnosis is usually readily apparent to clinicians, as the patient will report a recent history of laser surgery. Postoperative infections, on the other hand, can be very difficult to diagnose as the typical appearance of the skin is disrupted from baseline. This presents the clinician with the difficult task of differentiating cutaneous infection from burns or normal wound healing. A culture can be very helpful in the diagnosis.

Due to the higher risk of HSV reactivation in resurfacing procedures, HSV prophylaxis is indicated for 7 to 10 days after surgery. If there is HSV reactivation despite adequate prophylaxis, the patient may need intravenous antivirals. HSV infections following laser procedures should be treated aggressively due to the risk of scarring.[2][7][8][9]

Bacterial prophylaxis in laser surgery is controversial. Clinicians should have a low threshold for culture if an infection is suspected. Due to the risk of scarring, antibiotic therapy should be initiated if a bacterial infection is suspected. Antibiotics, likely ciprofloxacin, should be considered to cover pseudomonas infections.[7]

Scars

All patients should be counseled on the potential risk of scars, especially those receiving ablative therapy. Caution should be exercised in ablative procedures of the neck as there is a significantly increased risk of scarring, likely from the reduced number of pilosebaceous glands compared to the face.[7] Patients who have undergone radiation may also be at increased risk of scarring. Recent isotretinoin use is not a contraindication for laser surgery and does not have an increased risk of scarring.[14][15] Patients should be advised to return if there is evidence of scarring to allow for early treatment. Scars from laser therapy can be treated the same as other types of scars with topical or intralesional steroids among other therapies.

Ocular Injury

Ocular injuries can happen to any party in the room and result from ocular exposure to the laser beam. Injuries can be seen with direct exposure of the beam into the eye or reflection of the beam. Depending on the target chromophore of the laser, several different structures can be damaged. Ablative lasers target water and can, therefore, damage the cornea, whereas vascular and pigment lasers target melanin and lead to retinal damage. Warning signage outside the laser room should be used. The risk of ocular injury can be mitigated by opaque eyewear for the patient and wavelength specific filtering glasses for all parties present in the room.[5] Laser eye injuries can induce permanent blindness and are considered an ocular emergency. Permanent blindness is more likely in cases of direct ocular exposure. These patients require an emergency ophthalmology referral.[16] Symptoms of a laser injury include a bright flash of colored light and, occasionally, a popping sound coinciding with the firing of the laser. This is typically followed by decreased visual acuity and possibly floaters in the visual field. 

Laser devices are powerful tools that have an expanding array of indications, but these devices also have the power to permanently harm patients. The practice of laser surgery should be done by trained medical professionals in a medical setting, providing higher quality care with a lower risk of adverse effects and litigation.[17] Physicians and other providers should be aware of the risks of complications and knowledgeable about the complications of laser surgery. Early recognition and treatment of complications can help decrease the sequela of side effects. All healthcare personnel are responsible for the safety of the patient and their colleagues. Everyone in the procedure room should use proper eye protection and personal protective equipment.[2][5] (Level V)

Laser procedures should be performed with great caution as a majority of laser procedures are for cosmetic purposes. Surgeons should be selective in their choice of candidates for laser therapy and should not perform elective procedures on patients who are excessively tan. Darker skin patients are at increased risk of dyspigmentation and should be counseled on this risk. Nd:YAG lasers have been shown to have the lowest risk of dyspigmentation for dark-skinned patients but should only be used in the hands of experienced laser surgeons.[18] Strict sun avoidance for 2 weeks before treatment can reduce the risk of post-inflammatory hyperpigmentation. Hyperpigmentation typically resolves over several months but can be treated with strict sun avoidance, superficial chemical peels, and hydroquinone. Hypopigmentation may resolve with time and can be covered with makeup, or melanin production may be stimulated with fractionated CO2 laser or narrow band ultraviolet light treatments.[2][8][19][20]