What causes blepharitis?

What causes blepharitis?
Blepharitis occurs in two forms:
"Anterior blepharitis" and "Posterior blepharitis"

Anterior blepharitis

Anterior blepharitis affects the outside front of the eyelid, where the eyelashes are attached. It is most commonly caused by bacteria (Staphylococcus) and scalp dandruff.

• Staphylococcal blepharitis
  This type of blepharitis is thought to be caused by a bacterium (germ) called staphylococcus. The staphylococcus bacterium commonly lives in low numbers on the skin without doing any harm. However, in some people, it seems that this bacteria causes a localised infection of the eyelids to cause blepharitis. Why this may occur in some people and not most people is not clear.
  
• Seborrhoeic blepharitis
  This type is closely associated with a skin condition called seborrhoeic dermatitis. In seborrhoiec dermatitis, the affected skin becomes more oily and can become scaly. Seborrhoeic dermatitis typically causes bad dandruff and sometimes a rash, commonly on the face and upper body. The underlying cause of seborrhoeic dermatitis is not clear. A yeast germ called Malassezia furfur is involved. However, it is not just a simple skin infection and it is not contagious (you cannot 'catch' this condition from others). This yeast germ lives in the sebum (oil) of human skin in most adults. In most people it does no harm. But some people seem to 'react' to this germ in some way which causes inflammation.
  
  Posterior blepharitis
  Posterior blepharitis affects the inner eyelid and is caused by problems with the oil glands. It can be associated with acne rosacea.
  
• Meibomian blepharitis — (often called meibomian gland dysfunction.)
  The tiny meibomian glands in the eyelids lie just behind the eyelashes. You have about 50 glands on the upper eyelids and 25 on the lower eyelids. They make a small amount of oily fluid (oily secretions) which comes out on the back of the eyelids next to the eye. This oily fluid forms the outer layer of the tear film which lubricates the front of the eye. People with meibomian blepharitis are thought to have a slight problem with their meibomian glands and the fluid they produce which may lead to eyelid inflammation. (This also explains why people with meibomian blepharitis often have dry eyes as the fluid they make may not be adequate to lubricate the eye.)
  
• Acne rosacea, which leads to red and inflamed skin
  
  Combinations of the above
  In practice, it is often not possible to tell the difference between the above causes. Indeed, seborrhoeic blepharitis commonly occurs together with meibomian blepharitis. And, skin which is inflamed anyway is more likely to become infected by the staphylococcus. So, all three causes may contribute to many cases with the main cause varying from case to case.
  
  A vicious circle may play a part
  Blepharitis tends to 'flare-up' for a time and then ease off in severity. If you have blepharitis, you are likely to always have a tendency to have it as each of the above causes tends to be a long-term problem. Sometimes a 'vicious cycle' may play a part. For example, your eyelids may become slightly inflamed and itchy, but not too bad. You may then rub them - which makes the inflammation worse. This in turn can lead to your eyelids becoming sore. You may then keep rubbing them further, which leads to more inflammation, and so on.

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