KELLY KINNEY NICHOLS, OD, MS
Learn more about the etiology, diagnosis and treatment
of elusive dry eye syndrome.
Dry eye has been a hot topic over the past several years. New tests, drugs,
KELLY KINNEY NICHOLS, OD, MS
Learn more about the etiology, diagnosis and treatment
of elusive dry eye syndrome.
Dry eye has been a hot topic over the past several years. New tests, drugs, contact lenses and products have been introduced to help diagnose and manage patients with ever-elusive dry eye. This article overviews current etiological theory, diagnosis and treatment.
Dry eye syndrome is a commonly diagnosed clinical disorder of the anterior ocular surface characterized by dry, irritated, stinging eyes. While the mild to moderate case of dry eye is rarely sight-limiting, the disease affects a large portion of the population. Current estimates indicate that over 10 million people in the United States suffer from dry eye syndrome. Recent studies have shown that the prevalence of patients reporting dry eye symptoms is approximately 15 to 25 percent, while severe dry eye symptoms are reported less frequently. Dry eye syndrome is more prevalent in women and in the elderly. Recently, Schein et al. reported that 10.5 percent of adults over the age of 65 years routinely purchase and use lubricant eye drops to relieve dry eye symptoms. This finding indicates that a large percentage of the population voluntarily purchases over-the-counter treatments to alleviate ocular discomfort and pinpoints a group that would benefit from a greater understanding of the disorder. With the aging United States baby boomer generation, it is expected that the prevalence of dry eye will increase as we move into the next millennium, and the eye care professional will be faced with increasing numbers of patients with dry eye syndrome. Dry eye is one of the most common anterior segment disorders encountered by eye care professionals throughout the world.
Reaching a definitive dry eye diagnosis is difficult due to our lack of understanding dry eye etiology. Researchers have made significant headway in recent years; however, we have yet to fully understand the disorder. To aid diagnosis, the National Eye Institute (NEI)/Industry report on dry eye defined dry eye as follows (Lemp, 1995):
Dry eye is a disorder of the tear film due to tear deficiency or excessive tear evaporation which causes damage to the interpalpebral ocular surface and is associated with symptoms of ocular discomfort.
While this global definition is an improvement, the report also states that ocular surface damage or symptoms may be absent in some cases of dry eye. To date, definitions of dry eye have focused on describing dry eye without emphasizing the mechanisms underlying the disease. The difficulties researchers have with the definition may be inherent in the disease process itself or due to our lack of instrumentation to adequately measure signs and symptoms. In addition, symptoms do not correlate well with objective diagnostic tests. This may be due to the poor repeatability of clinical diagnostic tests. Researchers are still in the early phases of understanding what objective findings are classically present in dry eye syndrome, as well as which symptoms correlate with the objective findings.
Masquerading disorders can often confound a dry eye diagnosis. Meibomian gland disease, blepharitis, epithelial basement dystrophy and others can mimic the signs and symptoms of dry eye. A thorough evaluation of the anterior segment is necessary, as dry eye is often a diagnosis of exclusion. The NEI/ Industry Workshop on Clinical Trials in Dry Eye (Lemp, 1995) sets forth classification of dry eye states. In the classification, evaporative dry eye is differentiated from tear-deficient dry eye. In the oil-deficient category, absent meibomian glands, obstructive meibomian gland disease and anterior and posterior blepharitis are listed as associated disorders. In this diagnostic scheme, dry eye and meibomian gland disease can be diagnosed at the same time.
In recent years, various components of dry eye have been studied extensively, including many of the diagnostic tests for dry eye. What is missing from the literature, is a "gold standard" dry eye diagnostic test to which all other tests can be compared. In the design of clinical trials, choosing appropriate outcome measures is paramount in good study design. If a "gold standard" test does not exist, defining suitable outcome measures becomes a challenge. Foremost in identifying an adequate clinical test is determining the repeatability and validity of the test.
The Tear Film
The primary function of the tear film is to provide a smooth refractive surface for the eye. The tear film also traps debris and helps remove sloughed epithelial cells, provides oxygen to the cornea, has antibiotic properties (i.e. lysozyme and IgA) and acts as a protective barrier to underlying layers. In order to maintain uniformity, the tear film must be continuous and distributed evenly across the ocular surface. Uniform tear film distribution is provided by the action of the lids, and the frequency and quality of the blinking pattern can affect the lids' ability to "spread" the tears over the ocular surface.
Acute red eye due to soft lens
Composition Holly (1977) first described the pre-ocular tear film as three distinct layers: the mucin