LIPID DISORDERS OF THE TEAR FILM
lipid components, which form the outermost layer of the
tear film, may cause several clinical disorders. Due to
some new clinical tests we have learned to analyze the
basis of these disorders, which originally have been
misunderstood as alterations of the aqueous phase of the
tears. The importance of the lipid phase for the
pathogenesis of dry eye disorders may be emphasized by
the fact that the outer tear film layer represents the
primary contact between tear film and environment.
Lipid disturbances of the tears are seen in the Meibomian dysfunction, in the blepharitis, in the office eye syndrome and in some contact lens induced alterations.
Meibomian dysfunction is not yet a well-defined clinical entity, but there are some prominent clinical signs. Meibomian dysfunction is characterized by numerous irregular Meibomian glands of the upper or lower lid. Pathologically altered parts of the lids may coincide with a normal lid-configuration. Upon slight pressure these glands excrete milky thick secretion, which may otherwise be seen in chalazia. Patients with Meibomian dysfunction complain about thickened lid margins, occasionally associated with redness due to papillary hypertrophy. In clinical practice the combination of al these signs is noticed very frequently. Some authors have found this alteration in 40% of otherwise healthy volonteers. This implies that Meibomian dysfunction belongs to the most important lipid disorders and has been underdiagnosed until recently.
With respect to the lid margin the blepharitis gains importance. The various aspects of blepharitis in clinical practice are reflected by McCulley's classification. He contrasts staphylococcal induced blepharitis to the seborrheic form, but there is also one form, which combines both of them. Additionally there is a seborrheic blepharitis associated with Meibomian seborrhea. Some glands may be inflamed due to secondary Meibomitis. lf all the glands are equally affected, we classify as primary Meibomitis. The most severe form is the ulcerative necrotizing blepharitis. The application of this complex system in clinical practice occasionally is difficult. We therefore prefer the earlier classification. If there is a primary disorder of the skin, we classify it as anterior blepharitis. Some authors use the name blepharitis sicca. If the primary disorder affects the Meibomian glands, we diagnose a posterior blepharitis. It is often named blepharitis seborrheica. Either of these two forms may lead to ulcerative, necrotizing changes of the whole lid margin. A blepharitis may also be seen secondary to mollusca or papillomata.
The interaction between eye and skin is important not only for the classification, but also for the therapy. Among the most frequently diagnosed skin abnormalities is the seborrheic dermatitis, which affects young men at the age of 20 to 30 years. Akne, another important dermatologic disorder, is seen in several age groups. Akne rosacea may induce the posterior blepharitis in women at the age of 50 to 60 years. Neurodermitis in its severe rnanifestation may cause the ulcerative blepharitis.
The office eye syndrome includes all the environmentally induced tear film disorders. They are meanwhile known as typical lipid disorders. But there are still no data available on the incidence of the office eye syndrome. The symptoms are numerous, but unfortunately not very specific. The patients complain of itching, burning and foreign body sensation, which are the typical dry eye symptoms. The skin appears dry, the nose is partially obstructed. Generally, the patients show fatigue, lethargy and lack of concentration. All these signs are reversible, when the patient leaves the office or any exposed area. Despite of several sophisticated clinical tests this still is the most important information, which we derive from history for the classification.
Lipid disorders ma be associated with contact lens wearing, but its occurence is rather low. Nevertheless, there are lipid deposits on contact lenses, which show a characteristic finger print pattern. Silicone containing lenses appear to be more lipophilic than all the other materials, thus providing more deposits. The visible pattern of lipid induced interference colours over contact lenses demonstrates various thickness of this tear film component.
The diagnostic system for lipid disorders of the tear film is not the typical battery of tests, which we use for dry eye patients.
The most important diagnostic procedure is the exprimation of the Meibomian glands of the upper and lower lid. We use locally anaesthetizing drops to insert a lid-plate, which facilitates the digital expression of the glands. Secretion, which does not appear like filiform toothpaste, is pathologically altered and demonstrates a lipid disorder.
Besides of this most important test there are some other methods available. We can visualize the Meibomian glands and their ducts using a retroillumination. The evaporation might be determined using specially fit goggles under laboratory conditions. The typical evaporation rate of 4 x 10^-7 g x cm^-2 x s^-1 will increase under pathological conditions. The osmolarity of the tears can be measured using a tear osmometer and, again, the value will increase over 300 mOsm/l under pathological conditions.
A typical lipid disorder does not show a pathological Schirmer's test. The performance of Schirmer's test is only indicated if we suppose an aqueous disorder to be associated. The typical diagnostic procedure of lipid disorders of the tear film does not include Schirmer's test.
The therapy of lipid disorders shows several characteristic differences of the therapeutic steps of dry eye patients.
In patients with lipid disorders of the tear film cleaning and massage of the lid margin is the most essential therapeutic step. It can either be performed by the doctor like described under diagnostic procedures, or it can be done by the patient using a Q-tip for tender rubbing the outside of the lid margin. Its regular application 2 to 3 times per day seems to be important. Warm compresses of 37 to 38ºC could solubilize the otherwise stiff secretion of the Meibomian glands. The use of baby shampoo is beneficial for cleaning the lid margin without irritation. Further therapeutic steps include antibiotics, which should only be used after resistogram. There is no indication for tear substitutes unless an aqueous tear deficiency is diagnosed. Patients with lipid disorders, who are resistent to local therapy, should be selected for long-term systemic antibiotic therapy. It is not only the antibiotic effect, which is supposed to be advantageous, but also the presumed inhibitory action on the bacterial lipase. We use tetracyclins for 50 days, occasionally in more severe cases up to 6 months.
Nearly all the lipid disorders of the tear film represent a therapeutic challenge, since due to the chronic course of the disorder the therapy must contain several individual components for a longer period of time. The patient's cooperation is essential for the long-term success.