meibomian orifices and production of abnormal secretion upon pressure over the glands. Mechanism The mechanism by which the bacteria causes symptoms of blepharitis is not fully understood and may include direct irritation of bacterial toxins and/or enhanced cell-mediated immunity to S. aureus. Staphylococcal blepharitis is caused by an infection of the anterior portion of the eyelid by Staphylococcal bacteria. In a study of ocular flora, 46% to 51% of those diagnosed with staphylococcal blepharitis had cultures positive for Staphylococcus aureus in comparison to 8% of normal patients. Staphylococcal blepharitis may start in childhood and continue into adulthood. It is commonly recurrent and it requires special medical care. The prevalence of Staphylococcus aureus in the conjunctival sac and on the lid margin varies among countries, likely due to differences in climate and environment. Seborrheic blepharitis is characterized by less inflammation than Staphylococcal blepharitis; however, it causes more excess oil or greasy scaling. Meibomian gland dysfunction is a result of abnormalities of the meibomian glands and altered secretion of meibum, which plays an imperative role in lagging the evaporation of tear films and smoothing of the tear film to produce an even optical surface. Posterior blepharitis is an inflammation of the eyelids, secondary to dysfunction of the meibomian glands. Like anterior blepharitis, it is a bilateral chronic condition and may be associated with skin rosacea. There is growing evidence that, in some cases, it is caused by demodex mites. Diagnosis Diagnosis of the condition is done via a physical examination under a slit lamp. Cultures of debris are occasionally collected for bacterial or fungal testing. Examination In all forms of blepharitis, optometrists or ophthalmologists examine the tear film, which is the most efficient method in determining instability. The most frequently used method is to measure tear production via tear break-up time (TBUT), which calculates the duration interval between complete blinks. This serves as a primary indication of regional dryness in the pre-corneal tear film after fluorescein injections. If TBUT is shorter than 10 seconds, then this suggests instability. Staphylococcal blepharitis is diagnosed by examining erythema and edema of the eyelid margin. Patients may exhibit alopecia areata of eyelashes and/or growth misdirection, trichiasis. Other signs may include telangiectasia on the anterior eyelid, collarettes encircling the lash base, and corneal changes. Seborrheic blepharitis is distinguished by less erythema, edema, and telangiectasia of the eyelid margins. Posterior blepharitis and Meibomian gland dysfunction are frequently associated