Tear osmolarity is a very reliable index to assess the damage caused by medical treatment of glaucoma on the ocular surface, especially in those patients suffering from Dry Eye.
Treatment of glaucoma and tears
Medical treatment is the first option in the open angle glaucoma (80% of the glaucoma), however we know that its prolonged use is the cause of multiple alterations in the ocular surface (OSD), especially when they are drugs with preservatives (1) and even more so when it comes to Benzalkonium Chloride (BAK) (2)
Epidemiological studies show the relationship between antiglaucoma treatment and ocular surface involvement (3-8), with the presence of symptoms in 60% of cases (5), increasing this percentage proportionally to the number of drugs used, being maximum when the treatment reaches three drugs (8) and when it is prolonged over time (9).
Osmolarity and treatment of glaucoma
In most cases with alterations of the ocular surface, hypersomolarity of the tear is the most frequent and most relevant sign, regardless of the cause that originated the condition. Tear hyperosmolarity induces apoptosis and an inflammatory process of the ocular surface, with the release of cytokines and other inflammatory mediators (10-14).
The literature review (15-19), shows that in medically treated glaucomatous patients, 60% presented alterations compatible with ocular surface disease (Figure 1), 67.5% presented alterations in the stability of the tear film (reduced BUT) and 47.5% have a tear osmolarity greater than 308 mOsm/L. normal limit value.
Osmolarity and ocular surface
In a recent study (20), the statistical analyzes show a high correlation between the osmolarity values and the OSDI index, which measures the symptoms of damage to the ocular surface: r: 0.486; P = 0.002, as well as the BUT, which measures the stability of the tear film: r = −0.49; P = 0.009. There is also a high correlation between osmolarity and the number of drugs used (r = 0.409; P = 0.009), the number of instillations (r = 0.405; P = 0.01), and the number of instillations of drugs with preservatives (r = 0.629; P, 0.0001). No correlation was observed between osmolarity and Schirmer test nor with vital dyes.
It is evident that the degree of severity and symptomatology of the involvement of the ocular surface is strongly correlated with tear osmolarity. (21-28), the greater the discomfort, the higher the osmolarity (P=0.016) (20).
Glaucoma droplets, ocular surface and osmolarity
The discomforts that accompany the treatment of glaucoma are one of the main factors that explain poor compliance with or abandonment of treatment (26). This fact makes it especially important to evaluate the ocular surface, before starting treatment and during it, to reveal possible changes that should be treated.
It is important to know that the tests used in the evaluation of the Dry Eye it has a different value in these patients. The Schirmer test and the vital dyes do not have a high correlation with the OSDI values in glaucomatous patients, since it is not a true dry eye, it is a condition due to the toxic effect of the drugs, even more so when they use preservatives .
Treatment of glaucoma and dry eye
The presence of BAK in drugs causes an increase in osmolarity due to its detergent effect on tears (quaternary ammonium). It reduces the stability of the tear film and favors its evaporation, causing an evaporative-type dry eye syndrome, increasing osmolarity, a fact that is reflected in BUT, with a very high correlation between both, increased osmolarity and reduced BUT. As it is not a dry eye due to a deficit in tear secretion, there is no correlation with the Schirmer test, which remains with normal values.
In view of the aforementioned data, we think that when faced with a patient with glaucoma, whether we have just diagnosed it and are considering medical treatment, or in cases that are already being treated, it is essential to assess the state of the ocular surface since , as we have said, there may be a relationship between the treatment of glaucoma and dry eye. This is motivated because if there is any disorder, it is basic to attach a complementary treatment aimed at its improvement or even propose the alternative. glaucoma surgery, so as not to further damage the ocular surface and avoid possible non-compliance with the treatment.
Control of the ocular surface in patients with glaucoma
To assess the state of the ocular surface, especially in patients who are already being treated medically, the tear osmolarity test appears to be the most significant factor for making decisions. It is a painless test, simple to perform and very precise, especially now that we have new systems such as TearLab, based on detection with a chip that requires only 50 nL. (21-22)
We believe that the analysis of tear osmolarity is a very valuable test to assess the medical treatment of glaucomatous patients and its possible harmful effect on the ocular surface, causing or aggravating an evaporative dry eye syndrome.
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