Low delivery of meibomian gland secretions is further classified into two major categories: hyposecretion and obstructive conditions.
Meibomian gland hyposecretion is characterized by decreased meibomian lipid secretion without gland obstruction. Although there is no published and verified evidence of
primary hyposecretion, this disorder is associated clinically with gland atrophy. A decrease in the number of functional meibomian glands is associated with contact lens wear, and this decrease appears to be proportional to the duration of contact lens wear.
35
The other category under low-delivery states is
meibomian gland obstruction. This is probably the most common form of MGD.
3,7,32 Histopathologic changes include hypertrophy of the duct epithelium and keratinization of the orifice epithelium. Low delivery is caused by glandular obstruction due to either terminal duct obstruction or altered secretion. The disorder is seen in older subjects or after the use of retinoids for acne treatment.
36 Androgen insufficiency or lack of androgen receptors is also associated with keratinization, obstruction, and alteration of meibomian gland secretions (see the Anatomy, Physiology, and Pathophysiology report).
37 Obstructive causes may be further classified as cicatricial and noncicatricial. In
noncicatricial obstructive MGD the ducts and orifices remain in their normal anatomic position; in
cicatricial obstructive MGD the ducts and orifices are dragged posteriorly into the mucosa. Causes of cicatricial obstructive MGD include trachoma, ocular cicatricial pemphigoid, erythema multiforme, and atopic eye disease. Noncicatricial obstructive MGD may be seen in Sjögren's syndrome, seborrheic dermatitis, acne rosacea, atopy, and psoriasis. Inflammation in adjacent tissues is commonly seen in conjunctivitis and anterior blepharitis, for example. Although inflammation is frequently associated with meibomian gland obstruction (the term
meibomitis has been used as a synonym), whether the inflammation is a cause or a result of meibomian gland obstruction remains unclear.