Photobiology and Photosensitive Dermatoses Flashcards
UVC
200-280nm
-light absorbed by ozone layer
UVB
280-320nm
- readily absorbed at and just below surface of skin
- causes cutaneous erythema and sunburns
- suppresses DNA synthesis and mediates anti-inflamm effects of skin
- 1000x more erythemogenic than UVA
- erythea peaks 6-24 hrs after exposure
- phototherapy every other day
UVA
320-400nm
- wavelengths penetrate deeper into skin
- destroys collagen and elastic fiber
- wrinkles, loss of elasticity and pigmentary changes
Minimal erythema dose (MED)
minimal amount of particular wavelength of light capable of causing redness of skin
Chromophore
molecule in skin that absorbs light energy
peak absorption may be specific or broad range
-important because it may dictate how to protect against specific wavelength of light and may help direct choice of UV-based therapies for inflamm skin conditions
Laser
Light Amplification by Stimulated Emission of Radiation
- elecromag radiation to generate energy of broad/specific wavelength
- filtered to target specific chromophores (melanin, hemoglobin, water)
- don’t disturb skin barrier
- treat dyschromia/pigmentation, telangiectasias, vascular neoplasms, warts
Fitzpatrick skin phototype
diff types respond diff to UV light I - always burns, never tans II - often burns, sometimes tans III - sometimes burns, always tans IV - never burns, always tans V - moderately pigmented skin VI - darkly pigmented skin
Tanning
tanning from increase in #melanocytes, increased melanin synthesis and increased association of melanosomes
Immediate tanning rxn
- immediately following IVA exposure
- arise from oxidation of melanin in skin
Delayed tanning
-results from UVB and UVA exposure and peaks after 3 days
UV immunosuppressive effects
treats inflammatory skin disease (psoriasis, AD)
- mediated by depletion of epidermal LCs, cytokine activation (IL-1, IL-6) and T cell activation
- increases risk of nonmelanoma skin cancers and reactivation of herpes labialis
Phototoxic Reactions
photosensitivity that occurs based on amount of allergen person was exposed to
- redness, swelling, burning or stinging in sun-exposed sites
- bullae may occur if reaction is severe
- onset within hrs of exposure and peaks by 24 hrs
-medications: aminodarone, doxycycline, furosemide, hydrocholorthiozide, methotrexate and naproxen may result in phototoxic drug eruptions
Photodermatitis
occurs following exposure to caustic plant material coupled with UV exposure
- patchy, linear erythema of uncovered skin
- postinflamm hyperpigmentation common as acute phase resolves
- plants can contain furocoumarins –> photosensitizers
- wild parsnip, parsley, celery, line, lemon and fig
Photoallergic Reactions
occurs in photoexposed sites but delayed onset
- immunologically mediated disease
- onset in 24-72 hours after exposure
- requires sensitizing episode (delayed hypersensitivity)
- redness and itching
- lesions papular or papulovesicular
- causative doses can be small
- sulfa medications most common cause of drug eruption
Polymorphous Light Eruptions (PMLE)
most common in temperate climates (15% N. U.S.)
- onset usually 2nd, 3rd decades, but can occur @ any age
- UV action spectra varies among patients, including UVA and UVB light
- delayed type hypersensitivity
- outbreaks in spring, gets better over summer
- itching and burning may arise min-hrs after UV exposure
- cutaneous lesions develop within hours to days
- light through window/tanning bed can also cause flare
- itching, edema, pink papules coalescing into plaques and vesicles of sun-exposed skin
- typical sites: upper cheeks, nose, chin, back, lateral neck, V of chest, outer arms, forearms, shins, dorsal hands, feet
- extension to photo-protected sites can occur
- treated by reducing UV, sunblock, photo-protective clothing, topical corticosteroids
- low-dose, progressive phototherapy may induce skin “hardening”
Xeroderma Pigmentosum (XP)
extreme photosensitivity and high risk of melanoma and nonmelanoma skin cancers
- inherited AR
- impaired DNA nucleotide excision repair of UV-induced damage
- easy sunburns, vesiculations at early age
- freckles (lentigines) and xerosis
- basal cell carcinoma, squamous cell carcinomas before 10
- 30% neurologic abnormalities
- 10-20x internal malignancies (brain, lung, GI, renal, blood)
- Strict UV avoidance crucial
Phototherapy
use UV light immunosuppressant effects
-exposure to incrementally increasing UVA/UVB doses
-narrow band UVB @311nm good for psoriasis
>action spectrum of psoriasis = 311-313nm
-topical and oral psoralen increases absorption of UV light, usually combined with UVA
-topical psoralen risk of inconsistent absorption
-increased incidence of basal cell carcinoma, squamous cell carcinoma and melanoma
Photoprotection
physical blockers contain particles that block UV radiation by reflection
- zinc oxide, titanium dioxide
- white cast
chemical blockers contain agents that absorb UV light and disperse it as heat
-may be specific to UVA or UVB
UVA blockers
Benzophenones (oxybenzone) - possible photoallergy
Avobenzone (parsol 1789) - photolabile
Titanium dioxide/zinc oxide
Mexoryl - newest with better photostability
UVB blockers
Para-aminobenzoid acid (PABA) - lost of ACD
Padimate O - doesn’t penetrate corneum, less sensitizing
Salicylates (homosylate) - improve stability of agents
Cinnamates (octyl methoxycinnate) - easy removed
Titanium dioxide/zinc oxide
SPF
Sun Protective Factor ***only UVB measure
-ratio of time tot develop minimal erythema dose compared to onset of erythema in unprotected skin
=MED protected/MED unprotected
-broad spectrum covers UVA and UVB
apply 30 min before exposure
-use 1 oz and reapply every 2 hrs at least
