# AS-OCT and Ocular Hygrometer as Innovative Tools in Dry Eye Disease Diagnosis
## 1 Introduction  
Dry eye disease (DED) is one of the most common ophthalmic conditions, affecting millions of people worldwide with a prevalence of between 5% and 50%, based on the diagnostic procedures used and study populations [1, 2, 3, 4, 5]. Although symptoms arising from DED have been reported as the most common reason to visit an ophthalmologist, it is considered one of the most underdiagnosed and undertreated syndromes in ophthalmology [2, 6].  
The definition of a dry eye, according to the Tear Film and Ocular Surface Society Dry Eye Workshop II (TFOS DEWS II), is: \"a multifactorial disease of the ocular surface characterized by a loss of homeostasis of the tear film, and accompanied by ocular symptoms, in which tear film instability and hyperosmolarity, ocular surface inflammation and damage, and neurosensory abnormalities play etiologic roles\" [1]. DED is classified into two subtypes defined by the underlying mechanism of the disease: aqueous deficiency DED, with inadequate tear production from the lacimal gland, and evaporative DED, where the function of the meibomian glands is typically compromised [1, 7, 8]. Dry eye disease is induced by many factors, including age, gender, levels of certain hormones, autoimmune disorders, local environmental conditions, contact lens wear and exposure to some medications [2, 9, 10, 11].  
Based on the TFOS DEWS II, there are several factors that lead to the loss of tear film homeostasis with the development of abnormalities affecting the ocular surface [7]. The pathophysiology of DED comes from evaporation-induced tear hyperosmolarity, which leads to the inflammatory damage of the ocular surface and associated ocular symptoms [7; 12; 13]. The symptoms of DED include eye irritation, photophobia, ocular discomfort and fluctuating vision disturbance [14]; however, chronic ocular pain is the most commonly reported symptom among DED patients [15]. The condition can be painful and may result in lasting damage to the cornea and long-term damage to the corneal nerves through the irritation and inflammation of the ocular surface [8; 13]. DED symptoms can significantly impact patients' work productivity and quality of life and have various effects on different everyday activities, including a delayed reaction time while driving and a decline in sustained reading performance. DED has also been associated with depression and anxiety, especially when the patient is affected by Sjogren syndrome [2; 16].  
Despite recent technological advances and research targeting DED that have led to new approaches to DED diagnosis, the clinical results collected from all available diagnostic tests often do not correlate with the severity of clinical symptoms reported by the patient. As a result, there is currently no single clinical test that can be considered definitive in the diagnosis of DED [17; 18]. Therefore, several tests are typically used in sequence, combining the results obtained with information collected from questionnaires regarding patient symptoms. These tests demand significant resources and are time-consuming. Tests for determining the physical parameters of tears include the tear break-up time (TBUT), the Schirmer's test, tear osmolarity and tear meniscus height [19]. Other tests used for the diagnosis of DED include ocular surface staining, corneal sensibility, interblink frequency, corneal surface topography, interferometry, aberrometry and imaging techniques, such as meibography, anterior segment optical coherence tomography (AS-OCT) and in vivo confocal microscopy (IVCM).