Acidity and buffering system of ocular surface (according to study of conjunctival sac)

ANNALS OF OPHTHALMOLOGY 5, 2014 The acid–base balance is a part of human homeostasis. Cells and tissues of the human body are susceptible to pH change: stability of molecular structures and enzyme activity can be maintained only in the tolerable range of pH. The maintenance of the acid–base balance is vital biochemical parameter required for normal physiology, and its change is associated with a wide variety of human diseases [1]. The ocular surface is a widespread medical term used for the group of anatomical structures (the cornea, the conjunctiva, the lacrimal apparatus and eyelids) interconnected as a single anatomo–functional system. The conjunctiva, as well as any other mucous membrane, has its own specific pH value required for maintaining normal functioning of the ocular surface. According to published data, human tear pH varies from 6.5 to 7.8, and mean value received in some studies varies from 6.93 to 7.45 [2–7]. The ocular surface comfort zone of pH is relatively considered as a range of 6.6–7.8 [8]. The acidity overrange results in eye discomfort common for numerous eye diseases [6, 9, 10]. Physiological pH levels should be taken into account in order to avoid irritation, discomfort or abnormal changes of the ocular surface, when pharmaceuticals, contact lens solution and cosmetics are produced. Acidity alterations were found in contact lenses wearers, in lacrimal stenosis, keratitis, keratoconjunctivitis sicca , and postoperatively after corneal transplantation [6, 9]. Taking into consideration pH changes seen in different eye diseases, acidity can be regarded as an associated clinical manifestation or a diagnostic criterion. The tear pH is considered to be a component of keratoconus pathogenesis. Tentative data received during the pilot study, allows making a link between the tendency towards tear alkalization in keratoconic eyes and abnormal allocation of trace elements in keratoconic corneas. Thus, a fundamentally new pathogenetic scheme of keratoconus development was suggested [10]. Determination of the ocular surface pH with a glass microelectrode prevails in medical trials for measuring tear acidity [2–6]. The conjunctival sac contains 6–7 μl of fluid and the tear film thickness is about 6–12 μm – thus, it is really difficult to carry out pH measurement with a glass microelectrode in the immediate region of the tear film, as there is a high probability of a glass microelectrode application onto the conjunctiva. At that, the conjunctival epithelium pH is likely to be measured rather than pH of the free tear fluid. There are few studies on the subject of the tear composition influence on the tear pH. The bicarbonate Acidity and buffering system of ocular surface (according to study of conjunctival sac)