Ultraviolet and infrared radiation therapy. Physical characteristics, biological and therapeutic effects. Application methods, dosage, indications, contraindications, and safety considerations

01Physical characteristics

Phototherapy is treatment with optical radiation: ultraviolet, visible light and infrared, the part of the electromagnetic spectrum between X-rays and radio waves. Light behaves both as a wave and as a stream of packets of energy called photons.

The energy of one photon follows a simple rule: it rises with frequency and falls with wavelength (E = hν = hc/λ). Think of photons as balls thrown at a molecule. Infrared balls are soft: they only make the molecule shake, and shaking is heat. Ultraviolet balls are hard: they can knock an electron loose and change the molecule.

The borders between bands are conventions (the CIE's are used here); different books shift them by a few nanometres.

How deep it goes

Light that enters skin is reflected, scattered or absorbed by molecules called chromophores: DNA and proteins for ultraviolet, melanin and haemoglobin for visible light, water for most of infrared. Where a chromophore absorbs strongly, the light stops early.

• Ultraviolet stops in the epidermis and upper dermis, less than about 1 mm (UVB about 0.1 mm, UVA a little deeper). Its effects are therefore effects on skin, and on the blood passing through the skin.
• Red and near-infrared light (roughly 650 to 1000 nm) reach deepest of all optical radiation, a few millimetres and in part up to about a centimetre, because neither water nor haemoglobin absorbs them well: the "optical window".
• Mid and far infrared is soaked up by water in the outer skin, so it heats the surface.

The four rules of dosing light

1. 1

   Only absorbed light acts

   The Grotthuss–Draper law: radiation that passes through or is reflected does nothing. This is why ultraviolet acts in skin and why a photosensitiser, by absorbing more, makes a normal dose dangerous.
2. 2

   Dose is intensity times time

   The Bunsen–Roscoe law: within limits, a weak lamp for long and a strong lamp for short give the same effect. Dose in J/cm² equals irradiance in W/cm² times seconds.
3. 3

   Distance counts squared

   The inverse square law: double the distance and the intensity falls to a quarter. Distance is always fixed and written down.
4. 4

   Angle counts

   The cosine law: rays striking skin at a slant spread over more area and deliver less. A beam at a right angle gives the full dose.

02Biological and therapeutic effects

Infrared: a heat source you do not touch

Infrared is radiant heat: dry, without contact. Its effects are those of any superficial heat, described in the book as the same as conductive heat.

• Vessels open. Warmth relaxes the small arteries, so the skin reddens at once in a blotchy, poorly bordered pattern: a caloric erythema. More blood brings oxygen and carries away waste and pain chemicals.
• Pain eases. Warmth excites skin thermoreceptors, which damp pain signals at the spinal cord (the gate idea), and it breaks the pain–spasm–pain cycle.
• Muscle relaxes and collagen stretches. Warm muscle loses tone, and warm connective tissue lengthens more easily, which is why heat is given before stretching or exercise.
• Metabolism speeds up. Chemical reactions run faster as temperature rises, which helps healing in chronic but not in fresh, swollen injuries.

Near infrared can also act without heat, through photobiomodulation of mitochondria; that is the subject of laser therapy.

Ultraviolet: chemistry in the skin

An ultraviolet photon has enough energy to lift an electron to a reactive state or knock it away, creating radicals that start chemical reactions (the book calls this the photoelectric effect; physicists would say photochemical). The main results:

Light reaching the eye also signals the brain: bright light suppresses melatonin and shifts the body clock. That is visible light, not ultraviolet, and it underlies bright light therapy for seasonal depression.

Two kinds of red skin

The difference between heat erythema and ultraviolet erythema is a classic exam question, and the logic makes it easy: heat opens vessels directly, while ultraviolet first has to injure cells and make them release mediators.

03Application methods

Infrared sources

• Luminous lamps (incandescent or halogen bulbs, the Sollux type) give near infrared with some visible light, so the patient sees a glow. Near infrared reaches deeper.
• Non-luminous radiators (metal coils or ceramic elements) give mid and far infrared, heat the surface, and glow little or not at all.
• Light baths are boxes lined with bulbs placed over a limb or the trunk.

The patient lies comfortably with the area bare, dry and free of creams. The lamp is set to one side of the area and aimed at a slant, not hung directly over the patient, so that a breaking bulb or falling lamp cannot land on them; this also softens the intensity at the centre. Ultraviolet lamps, by contrast, are aimed perpendicular to the skin to deliver an exact dose.

Ultraviolet sources

Treatment is either local, a marked field with the rest covered, or general, the whole body in a cabin. Everyone in the room wears ultraviolet-blocking goggles.

04Dosage

Infrared: by feel

Infrared is dosed subjectively, by the patient's sensation of warmth, which is why intact sensation is essential.

Typical settings: the lamp at a fixed, recorded distance that depends on its power and the maker's instructions (the professor's slides give about 20 to 50 cm, the book about 100 cm for a strong lamp; a stronger lamp is set further away), about 10 to 20 minutes, daily or several times a week, a course of about 5 to 20 sessions. The patient's feeling of warmth, not the number, has the last word. Where an area has lost sensation, the book suggests judging by an intact area, or a mother's hand for a small child; in practice, an area without sensation is better not heated at all.

Ultraviolet: the minimal erythema dose

Ultraviolet cannot be dosed by feel: the skin feels nothing during exposure, and the damage shows only hours later. So the dose is measured against the patient's own skin. People differ widely: fair skin that always burns (skin type I) reacts at a fraction of the dose dark skin needs.

1. 1

   Expose test windows

   A biodosimeter, a plate with about six windows, sits on untreated skin (inner forearm, abdomen or buttock). The windows are uncovered one after another so that each square gets a longer exposure, at a fixed distance from the lamp.
2. 2

   Read the next day

   After about 24 hours the faintest square with an even, sharply bordered redness marks one minimal erythema dose (MED, or one biodose). Older texts read it after a few hours; the 24-hour reading is the standard now.
3. 3

   Prescribe in multiples

   Doses are written as fractions or multiples of the MED: 1/4 MED, 3 MED.

The old school of physiotherapy classifies doses by the reaction they aim for:

The logic of local and general treatment:

• Local irradiation of a small field can use erythemal doses, because a small burn on a limited area is tolerated. The field is treated again only after the redness has faded, every 2 to 3 days, and fields are taken in turn (the book's example of several spots on alternate days).
• General irradiation of the whole body uses suberythemal doses, because a sunburn over the whole body is a systemic injury, with fever and malaise. Sessions are daily or every other day, rising step by step. By the skin's sensitivity, the old schemes start at 1/4 MED (normal), 1/2 MED (accelerated, for low sensitivity) or 1/8 MED (slow, for high sensitivity).

Modern dermatological phototherapy follows the same logic with different numbers. Narrowband UVB is given 2 to 3 times a week, starting at about half to two thirds of the MED (or a dose set by skin type) and rising by about 10 to 20 per cent per session while no redness appears; psoriasis usually clears in about 20 to 30 sessions. In PUVA, oral 8-methoxypsoralen is taken about 2 hours before UVA, 2 to 3 times a week; the dose is set by the minimal phototoxic dose, read at 72 to 96 hours, because psoralen redness comes later than sunburn.

05Indications

Each indication follows from an effect.

Honest about evidence: superficial heat gives a small short-term reduction in pain and disability in acute and subacute low back pain, mostly shown with heat wraps; infrared lamps themselves are little studied. Dermatological phototherapy is well proven. Ultraviolet for wounds rests on small, old studies. Vitamin D is now given by mouth rather than by lamp, because a tablet gives a known dose without skin cancer risk.

06Contraindications

The contraindications are reasoned from what each radiation does.

The long list of photosensitisers in the book (dyes, perfumes, coal tar, gold, phenothiazines) has one logic: any substance that absorbs ultraviolet or visible light and passes the energy on to tissue turns a safe dose into a burn.

07Safety

• Eyes. Ultraviolet causes photokeratitis, a painful "sand in the eyes" inflammation of the cornea that appears 6 to 12 hours after exposure (welder's flash, snow blindness), and conjunctivitis; over years, cataract. Patient and staff wear ultraviolet-blocking goggles. After psoralen, eyes are protected with UVA-blocking glasses for the rest of the day. Bright infrared sources can also harm the lens and retina over time, so patients do not stare into them.
• Burns. Overdose of ultraviolet gives pain, swelling, blisters and, over the whole body, fever, chills and in extreme cases shock. Overheating with infrared gives a thermal burn. Long repeated infrared exposure causes erythema ab igne, a net-like brown pigmentation, which is a sign to stop.
• Cumulative ultraviolet harm. Every dose adds to lifetime exposure: photoageing and skin cancer. The risk of squamous cell carcinoma rises clearly after about 150 to 200 PUVA treatments, so courses are counted and recorded.
• Equipment. Lamps are calibrated, distances fixed, timers used, and the patient never falls asleep under a lamp. Ultraviolet lamps are switched on a few minutes before use to reach steady output, then the patient is uncovered.