Meibomian Gland Disease

Excerpt

The meibomian gland dysfunction (MGD) international workshop describes MGD as "chronic diffuse abnormality of the meibomian glands, characterized by terminal duct obstruction along with qualitative or quantitative changes in the glandular secretion." The obstruction of the terminal duct occurs because of hyperkeratinization of the duct epithelium and the viscous nature of the meibum. These can result in atrophy and drop out of the meibomian glands, thus decreasing secretion. The sequelae of MGD can be observed in the forms of tear film abnormalities, ocular surface irritation, inflammation, or ocular surface disease.

These meibomian glands are a type of sebaceous glands that are located in the eyelids. These are named after a German physician and anatomist 'Heinrich Meibom.' These glands lie parallelly in a single row within the tarsal plates of the upper and lower lids. The proximal ends of the meibomian glands extend towards the proximal margin of the tarsal plates. The distal end of the tarsus receives the secretion, i.e., meibum, through the excretory duct into the lid margin. It is estimated that the upper lid contains approximately 20 to 30 separate glands, and the lower lid has about 40 to 50 glands.

Meibomian glands produce lipids, which are the major component of the superficial lipid layer of the tear film. This lipid layer is known to protect the excessive evaporation of the tear film's aqueous layer. It also helps by lowering the surface tension and thus stabilizing the tear film. It acts as a lubricant during blinking and provides an outer barrier that prevents bacteria from entering the tear film. The functional disorders caused by the meibomian gland dysfunction resulting in alterations in the meibomian gland secretions are also referred to as posterior blepharitis. MGD can broadly be classified based on onset into congenital and neoplastic. Based on the duration, it can be classified into acute or chronic.

Based on underlying etiology, it can be divided into:

1. Low delivery

2. High delivery

The low delivery type can be hyposecretory (meibomian sicca) and obstructive (cicatricial or non-cicatricial). Meibomian sicca results from atrophy of the meibomian glands or secondary to medications. MG atrophy results in an overall reduction in functional meibomian glands. The most common form of MGD is the obstructive type. This results from hypertrophy and keratinization of the ductal epithelium. This is further classified into cicatricial or non-cicatricial. The high delivery type is also called hypersecretory (meibomian seborrhea) and results from excessive secretion of the lipids. Meibomian seborrhea occurs secondary to underlying systemic conditions such as seborrheic or atopic dermatitis and acne rosacea.