The characteristics of the ocular surface of the paediatric population are not yet as well known as those of the ocular surface of adults, although, even that of children is subject to numerous risk factors such as climate change, pollution, and myopia treatments (e.g. contact lenses and orthokeratology). In addition, factors such as uneven eyelid closure, inadequately corrected refractive problems and poor sleep quality can affect the tear film function of healthy children.
The importance of the tear film for the ocular surface
The tear film plays an essential role in maintaining homeostasis of the ocular surface. Indeed, the stability of the tear film and its adequate secretion promote the integrity of the ocular surface by ensuring cleansing, lubrication, nourishment and immune protection. Deficiency of tear film components, such as the lipid and aqueous layer, can lead to its instability and reduced tear secretion. These deficiencies often manifest themselves clinically through a significantly shorter tear film break-up time (BUT) and/or reduced marginal meniscus height during standard clinical tests.
The persistence of an unrecognised tear film deficiency condition, or a delay in diagnosis, can cause epithelial or corneal scarring and other structural damage to the ocular surface that may impair visionsometimes irreversibly. This is a major issue, as the children report less symptoms of ocular surface disease compared to adults, although the incidence of dry eye is similar in the two populations (10-44% in children and 5-50% in adults).
Understanding the normal characteristics of the ocular surface and the function of the tear film in children is important to improve diagnosis and identify the variation of normal parameters in dry eye and in ocular surface diseasesas most of the recommended normal values refer, in fact, to results of studies on adults.
Ocular surface variables in children
Recently, it was published in The Ocular Surface a review that summarises all current knowledge on ocular surface variables in healthy childrenfocusing on the stability and secretion of the tear film in order to determine the combined rate of BUT and tear secretion and examine its association with predictive factors such as age, gender and ethnicity.
The review examined 23 studies with a total of 5,291 participants (infants 0-29 days, infants 1 month - 1 year, and children 1-18 years) and a meta-analysis that included 15 eligible studies with a total of 1,077 participants. The results showed that the tear film break-up time in children was 14.64 seconds for fluorescein sodium (FBUT) and 21.76 seconds for non-invasive (NIBUT), the latter rising to 32.5 seconds in infants. The value found for NIBUT in the healthy paediatric population was higher than that reported for adults; The reduction of TBUT with increasing age is attributable to increased secretion of the Meibomian glands and a thicker lipid layer of the tear film evidenced in children, and/or the increased tear evaporation rates that occur with increasing age. However, the BUT in this review showed no positive correlation with the average age difference. Instead, a significant difference between the BUT of Asian children and that of children from other regions, the former showed a reduction of 7.4 s (p = 0.04); similar results have also been shown in other studies in the adult population.
The tear secretion measured by the Schirmer I test was in children 16.26 mm/5 min with anaesthesia and 29.30 mm/5 min without; whereas in infants the same measurements were 9.36 mm/5 min and 17.63 mm/5. This result is rather consistent, as tear secretion in children should be equal to or greater than that of adults, and become equivalent to normal adult values within 2-3 weeks after birthin infants is lower than in children because the lacrimal gland is not yet fully developed.
Other ocular surface variables, including corneal fluorescein staining, corneal sensitivity, tear film osmolarity, tear meniscus height, calico cell density and bacterial activity, are poorly reported in children. It is therefore difficult to establish whether there is a clear difference between the few reported values of these variables in healthy children compared to healthy adults. For example, corneal sensitivity values reported in infants and children are similar or reduced compared to values reported in normal adults, but differences in methodology make direct comparison difficult. The osmolarity value of the tear film reported in children ranges from 284 to 336 mOsmol/L, is higher than the values reported in healthy adults and there appears to be no statistical association between tear film osmolarity and race, age or gender. The calico cell density in adults is highly variable, with a wide range from 4 to 2226 cells/ mm2 for average values. In the healthy human conjunctiva there should be a high density of calyciform cells, with a tendency towards higher values for samples taken from normally covered portions (lower and upper bulbar conjunctiva). Studies in children have examined samples from the superior and inferior bulbar conjunctiva, but the values found are not as high as reported for adults.
In adults the meibomian gland contributes to more than 60% of the symptoms of ocular surface disease, such as in dry eye. Changes have been reported in children and adolescents in this gland but no association with ocular surface diseases such as dry eye has been reported. Interestingly, there is no difference in the morphology of the Meibomian gland between children and adults, however, its assessment in children is at risk of underestimation due to the difficulty in turning over and observing the eyelid with the instruments currently in use.
The conclusions of the review
This review allows us to make some important considerations: the tear film stability in children appears to be only slightly higher than in adultsMoreover, for the most of the ocular surface variables analysed there is no clear difference between children and adults. Yet, exposure to risk factors identified as causes of ocular surface disease in adults is also rapidly increasing among children. For this reason and because of the substantial heterogeneity revealed by the study, it is necessary to standardise the protocols for assessing tear function in children in order to limit the level of inconsistencies in measurements and to arrive at a greater understanding of ocular surface characteristics in healthy children, which is essential for improving diagnosis. Given the reduced availability of data, there is also a clear need for further studies focusing on the paediatric population.
Source:
The ocular surface in children: A review of current knowledge and meta-analysis of tear film stability and tear secretion in children. Ngozi Charity Chidi-Egboka, Nancy E. Briggs, Isabelle Jalbert, Blanka Golebiowski. The Ocular Surface, Volume 17, Issue 1, January 2019, Pages 28-39.
Dr. Carmelo Chines
Direttore responsabile
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