Photodermatoses

Figure 3.60 Electromagnetic spectrum
Figure 3.60 Electromagnetic spectrum
EM SPECTRUM AND BIOLOGICAL EFFECTS
Electromagnetic (EM) Spectrum (Figure 3.60)
  • Classification based on wave frequency and includes (starting with lowest frequency/energy) radio waves, microwaves, infrared radiation (IR), visible light (VL), ultraviolet light (UVL), X-rays and gamma rays (latter two known as ionizing radiation)
  • Energy of photon is proportional to frequency of radiation and inversely proportional to wavelength; thus, energy increases as wavelength decreases and frequency increases
  • Chromophores of epidermis include nucleic acid, protein, urocanic acid and melanin; chromophores of dermis include hemoglobin and porphyrins
  • Absorption spectrum: portion of EM spectrum absorbed by particular light-absorbing molecule or chromophore (i.e. wavelengths absorbed by porphyrins)
  • Action spectrum: portion of EM spectrum producing particular biologic effect (i.e. wavelength of radiation causing erythema)
  • Minimal erythema dose (MED): minimal amount of particular wavelength of light capable of inducing erythema
  • Ultraviolet light (UVL): (Table 3-32)
    • Non-ionizing type of radiation; invisible to human eye; classified into UVA (315–400 nm), UVB (290–315 nm), and UVC (200–290 nm)
    • Depth of penetration of UVL depends on wavelength: longer wavelength with deeper penetration (UVA penetrates deeper than UVB); practically all of UVC absorbed by ozone layer


   
 
Table 3-32 Comparison of UVA and UVB
FactorsUVAUVB
 
Wavelength
 
315–400 nm 290–315 nm
(UVA1: 340–400, UVA2: 315–340)
 
290–315 nm
 
Solar erythema
(sunburn)
Minor role; immediate erythema and distinct delayed erythema (6–24 h after exposure)
Major role; 6–24 h after exposure; UVB 1000× more erythemogenic than UVA; produces apoptotic ‘sunburn’ cell
 
Skin penetration
Epidermis through deep dermis (epidermal/dermal chromophores)
Epidermis only (fraction reaches upper dermis); causes epidermal thickening
 
Darkening
Immediate pigment darkening (hrs after exposure; due to oxid ation of pre-existing melanin, redistribution of melanosomes)
Delayed melanogenesis (48–72 h after exposure) due to ↑ # melanocytes, ↑ #/size melanosomes, ↑ synthesis/transfer melanin; provides photoprotection
 
Drug-induced
photosensitivity
 
Major contributor
 
Minor role
 
Carcinogenesis
Minor role; ROS production
Major role: mutations in keratinocyte DNA (CPDs) and immunosuppression
 
Vit D3 production
 
No
 
Yes
 
Glass penetration
Yes (penetrates window glass)
No
 
Miscellaneous
 
95% of UVR reaching earth’s surface, phytophotodermatitis
 
NBUVB 313 nm; Wood’s light ~365 nm (nickel oxide doped glass)
      
      
   
     
  
UVA: Drug-induced photosensitivity, photoaging, immediate pigment darkening, erythema
  
     
 
     
  
UVB: Photocarcinogensis, solar erythema, delayed pigment darkening, vitamin D3 synthesis