Phototherapy is a medical treatment that utilizes specific wavelengths of light, either natural or artificial, to address various health conditions. This therapeutic approach is based on the principle that light energy can be absorbed by biological molecules to trigger a beneficial cellular response. Light therapy is a non-invasive method used across a spectrum of medical fields, ranging from targeted skin treatments to managing systemic biological functions.
Core Mechanisms of Biological Interaction
The therapeutic effect of light begins at the molecular level with chromophores, which are specialized molecules within cells that absorb photons of a specific wavelength. Examples of these light-absorbing structures include melanin in skin, bilirubin in the blood, and DNA within the cell nucleus. The absorption of light energy by these chromophores initiates a cascade of photochemical and photobiological reactions.
In the case of ultraviolet B (UVB) phototherapy for inflammatory skin conditions, a primary mechanism involves the immune system. The UVB photons are absorbed by immune cells, particularly the T-lymphocytes that drive the inflammatory process in conditions like psoriasis. This absorption leads to the induction of apoptosis, or programmed cell death, in the overactive T-cells present in the skin lesions.
The targeted destruction of these inflammatory T-cells helps resolve skin plaques. Furthermore, light exposure modulates the production of inflammatory signaling molecules, known as cytokines. By decreasing pro-inflammatory types and promoting anti-inflammatory responses, this action suppresses localized, immune-mediated inflammation.
Major Applications in Dermatology
Phototherapy is a long-standing and effective treatment for many dermatological disorders, mainly utilizing the ultraviolet spectrum of light. The treatment of Psoriasis, an autoimmune condition causing rapid skin cell buildup, is one of the most common applications. Narrowband UVB (NB-UVB), which emits light at a specific wavelength around 311 to 313 nanometers, is the preferred method because it offers a balance of efficacy and safety.
NB-UVB is more effective than older Broadband UVB, which uses a wider range of UV light, because the narrowband wavelength is optimized for therapeutic effect while minimizing the burning potential. This light penetrates the epidermis and superficial dermis to target and suppress the hyperactive T-cells responsible for the disease. Eczema (Atopic Dermatitis) and Vitiligo, a condition causing loss of skin pigment, are also routinely treated with NB-UVB due to its immunomodulatory effects.
Psoralen plus UVA (PUVA) combines the photosensitizing drug Psoralen with long-wave ultraviolet A (UVA) light. Psoralen is administered either orally or topically and increases the skin’s sensitivity to the UVA light. This combination therapy is potent, often reserved for more severe or treatment-resistant cases of Psoriasis or Vitiligo.
Systemic and Non-Skin Applications
Beyond treating skin disorders, light therapy has established applications in addressing systemic conditions, most notably in newborns and for certain mood disorders. Neonatal jaundice, a common condition in newborns, is treated using bright, visible blue light. This light, optimally in the 460 to 490 nanometer range, is absorbed by the bilirubin molecules deposited in the infant’s skin.
The absorption of the blue light triggers a photochemical reaction called photoisomerization, transforming the toxic, water-insoluble bilirubin into structural isomers, such as Z-lumirubin. These new forms are water-soluble and can be excreted directly by the infant through the urine and bile without requiring the liver to process them. This non-invasive intervention prevents the buildup of bilirubin, which can be harmful to the developing brain.
Bright light therapy also serves as a treatment for Seasonal Affective Disorder (SAD), a type of depression linked to the reduced daylight exposure in fall and winter. This therapy involves daily exposure to a light box that emits a high intensity of visible white light, typically 10,000 lux, aimed at the eyes. The light does not act on the skin but on the retina, where it stimulates specialized photoreceptors.
This retinal stimulation sends signals to the brain’s suprachiasmatic nucleus, the body’s master clock, helping to reset the disrupted circadian rhythm. Light therapy regulates the production and timing of key hormones, such as melatonin and serotonin. The goal is to suppress the nocturnal release of melatonin earlier and support serotonin signaling to elevate mood and energy levels.
Safety Protocols and Potential Risks
While phototherapy is an effective medical intervention, it requires careful administration due to potential side effects and long-term risks. Acute side effects are generally mild and include temporary skin redness (erythema), itching, and dryness, similar to a sunburn reaction. These reactions are typically managed by adjusting the light dosage and using moisturizing creams.
The long-term safety profile is a major consideration, particularly with older or more aggressive forms of treatment. PUVA, due to the combination of UVA and Psoralen, carries a higher potential for long-term complications, including accelerated skin aging, known as photoaging. Chronic exposure to PUVA is also associated with an increased risk of non-melanoma skin cancers, such as squamous cell carcinoma.
Strict safety protocols are in place to mitigate these risks, including the mandatory use of specialized eye protection to prevent damage to the lens and retina during treatment. Patients must also be screened for pre-existing conditions and medications that increase photosensitivity or a history of melanoma. Proper dosing, regular skin checks, and adherence to protective measures outside of the clinic, such as wearing sunscreen and protective clothing, are essential.