**2. Epidemiology, forms, severity, symptoms, signs, and risk factors of DED**

DED remains one of the global health problems characterized by the significant impact on the quality of life of patients. It has a global prevalence ranging from 20 to 50%. Data on the prevalence of DED reported over the last 10 years vary widely, which is related to, among others, different standardization of study groups, the lack of uniform diagnostic criteria, the selection of subjective tests (questionnaires of vision quality), and ocular surface examinations to confirm the DED diagnosis. The results of studies based on subjective symptoms indicate the prevalence of DED in the range from 5 to 50%, while studies based on ocular tests indicate the prevalence of up to 75% of the population. International epidemiological studies have estimated the prevalence of DED from 5% to 30% in the population over 50 years of age. The disease is more common in women (1.3–1.5 times more prevalent than in men) and Asians, and its prevalence increases with age [4].

There are two major forms of DED: evaporative DED (EDE) and aqueous deficient dry eye (ADDE). EDE is a predominant form of DE responsible for about 70% of cases. MGD is considered to be the main cause of EDE. MGD is at the center of the vicious cycle of DED. International workshop on MGD defines MGD as a chronic

**Figure 1.**

*The simplified vicious cycle of DED based on [1–3].*

#### *Dry Eye Disease: Chronic Ocular Surface Inflammation DOI: http://dx.doi.org/10.5772/intechopen.114118*

diffuse abnormality of MG that is commonly characterized by terminal duct obstruction or qualitative or quantitative changes in glandular secretion [5]. Key pathophysiological features of MGD are gland blockade due to hyperkeratinization, ductal stenosis, and chronic stagnation of the meibum. That ultimately leads to gland atrophy and alternations in the lipid tear film layer. MGD may be considered a key trigger of tear film instability, inflammation, apoptosis, and neurosensory abnormalities.

In terms of severity, the predominant forms of DED are mild and moderate. Severe cases are mostly related to systemic, autoimmune diseases (such as rheumatoid arthritis, polyarteritis nodosa, systemic sclerosis), Sjogren syndrome (Sjogren syndrome dry eye; SSDE), and graft-versus-host disease (GVHD).

The impact of DED on the quality of patients' life is significant. This disease has been shown to have a negative impact on patient's daily activities. Due to DED, affected patients may experience decreased productivity as a result of irritating and chronic symptoms. There are several studies underlining the relationship between DED and sleep disorders or depression [6, 7]. Major complaints include pain, eye irritation, foreign body sensation, blurred vision, burning, dryness or watery eyes, fluctuating vision, and photophobia.

If symptoms are accompanied by ocular signs, namely homeostasis markers including decreased tear breakup time (TBUT), increased hyperosmolarity, and positive ocular surface staining the diagnosis of DED may be made. Further, division based on ocular signs includes evaporative and aqueous deficiency DED, as presented in **Figure 2**.

Risk factors of DED were established based on studies with different evidence levels. The epidemiology committee of TFOS DEWS II gathered all risk factors and

#### **Figure 2.**

*Diagnostic algorithm of a patient with DED. OSDI—ocular surface disease index; NIBUT—non-invasive tear break up time; MGD—meibomian gland dysfunction; TMH—tear meniscus height; the following sequence of diagnostic tests is recommended: NIBUT, osmolarity test, FBUT with fluorescein (fluorescein tear break up time), ocular surface staining. The diagnostic algorithm is based on the TFOS DEWS II methodology recommendation [8].*


#### **Table 1.**

*DED risk factors established by TFOS DEWS II [4].*

divided them into mostly consistent, probable, or inconclusive factors. Factors were also stratified into non-modifiable and modifiable risk factors [4]. The risk factors of DED were presented in **Table 1**.

DED is a disease of many etiological factors, multiple forms, and different severity; therefore, the various management and therapeutic options according to disease form and severity should be considered. Also, regarding the strong association with indoor and outdoor environmental factors, the management and therapy report committee recommends the following strategies, as a part of the initial management of the disease, education regarding the condition, its management, treatment, and prognosis, modification of the local environment, education regarding potential dietary modifications (including oral essential fatty acid supplementation), and identification and potential modification/elimination of offending systemic and topical medications [9].
