Relation between dental fluorosis and intelligence quotient in school children of Bagalkot district.

Introduction

Water is an essential valuable natural resource for sustaining life and environment which we have always thought to be available in abundance and free gift of nature. However, chemical composition of surface water or subsurface, geothermal or non-thermal, is one of the prime factors on which the suitability of the water for domestic, industrial or agriculture purpose depends.[¹] In India, high groundwater fluoride concentrations associated with igneous and metamorphic rocks such as granites and gneisses have been reported.[²] As a result, people of 19 states in India are drinking fluoride contaminated water above WHO’s maximum allowed concentration of 1.5 mg/l.[³] Fluoride has beneficial effects on teeth at low concentrations in drinking water, but excessive exposure to fluoride in drinking water, or in combination with exposure to fluoride from other sources, can give rise to a number of adverse effects. These range from mild dental fluorosis to crippling skeletal fluorosis as the level and period of exposure increases.[²] Apart from these, there are reports that excess fluoride consumption promotes cancer, hip fracture, leads to still births or birth defects and has detrimental neurological effects.[⁴] Alerted by such reports we conducted a study aimed at investigating if there is a link between the intellectual ability of child and exposure of the child to excess fluoride by looking at the Intelligence Quotient (IQ) scores and dental fluorosis status of the child.

Materials and Methods

Our study was conducted on school-going children between the age group of 7 and 11 years, in the Bagalkot district, Karnataka, which is known to have groundwater fluoride levels to range from less than 0.5 ppm to greater than 5.5 ppm in various regions.[⁵] Two regions one from each, Bagalkot taluk and Hungund taluk were selected, with groundwater fluoride concentrations of less than 0.5 ppm and from 2.5 to 3.5 ppm, respectively. Using random sampling method 120 children were examined and 80 children without dental fluorosis were included in the study from Bagalkot region. Similarly, 150 children were examined and 80 of them with dental fluorosis were included in the study from Hungund taluk region. The children included in our study had a normal birth history, were permanent residents in the region of examination and they had no history of trauma to the head, neither did they have a history of chronic illness nor were they on any medication. All of this was confirmed with a short interview with the parents of each child. Apart from these criteria, by every other comparison i.e., the cultural level, standard of living, lifestyle habits, were essentially the same for the two regions so that the IQ measurement could objectively reflect the effects of fluoride on child’s mental ability. For all the children the dental fluorosis status was noted according to the Dean’s index, 1945.[⁶] Questionable cases of dental fluorosis were not considered for the purpose of this study. IQ measurement was done for every child using the Coloured Progressive Matrices component of Raven’s educational test. The testing was administered individually to each child, in compliance with the guidelines and directions of the Raven’s progressive matrices manual. Retesting was done on 10 children after a day so as to check the reliability of the testing protocol.

Statistical analysis of the data was done using ?² test and Tukey’s multiple post hoc procedure using the STATA 9.0 version software.

Results

It was observed that the mean IQ score of children with dental fluorosis, which was 66.62, was significantly lower than the score for children without dental fluorosis, which was 76.36. [Table 1]. It was also noticed that mean IQ scores for children with mild, moderate and severe dental fluorosis which were 66.72, 66.31 and 66.40, respectively, were not significantly different but the difference between mean IQ for children with mild dental fluorosis and no dental fluorosis was statistically significant. [Table 2]

Comparisons drawn to see the proportion of children in various IQ categories revealed that 72.5% of children with dental fluorosis had extremely low or borderline IQ scores whereas 47.5% of children without fluorosis had extremely low or borderline IQ scores. Then, only 16.25% of children with dental fluorosis had average or high average IQ scores whereas 28.75% children without dental fluorosis had average or high average IQ scores. [Table 3]

Intraexaminer variability was checked by paired t-test and no significant difference in the values obtained was found (P-value=0.6910) and correlation reliability between test and retest scores by correlation coefficient method was calculated to be, r=0.9919, which was highly significant.

Discussion

Safe drinking water in India is a constitutional mandate.[⁷] Still, 30% of urban and 90% of rural population of India is dependent on untreated water source, of which 80% is groundwater.[⁷] This groundwater consumed for drinking has various toxic effects like it causes increased hepatic cell size, nephrosis, myocardial mineralization and degeneration of seminipherous tubules in testes, all of which have been noticed in controlled animal studies.[²] Apart from these, excess fluoride has been found to be neurotoxic. Studies have shown that when mice were exposed to drinking water with high concentration of fluoride there is deterioration of learning and memory ability, decreased thickness of postsynaptic density, increased width of synaptic cleft and higher cholinesterase activity in brain tissue.[⁸⁹] Also there is an increase brain cell apoptosis in offspring rats seen with high fluoride exposure.[¹⁰] Fluoride has also shown to induce S-phase cell cycle arrest and DNA damage in primary cultured rat hippocampal neurons.[¹¹]

Based on these facts one could legitimately question the applicability of these findings on humans, but dental and skeletal fluorosis is no longer a mysterious phenomenon and is a unanimously accepted toxic effect of overdose of fluoride on humans. As a matter of fact, the very first thing that led to the historical research work done on fluoride was the curiosity about the so called “Colorado stain”, which was nothing but the dental manifestation of exposure to excess groundwater fluoride.[¹²] Sadly, the harmful effects of fluoride not just alter the aesthetic appearance of teeth but also affects other body organs. There are reports of neurological manifestation of exposure to excess fluoride in humans causing headache, insomnia and lethargy.[¹³]

In aborted human fetuses collected from endemic fluorosis area it was found that accumulation of fluoride in brain tissue causes disruption of certain neurotransmitters and receptors in nerve cells and also increased numerical density of volume of neurons and undifferentiated neuroblasts.[¹⁴¹⁵] There is also evidence of increased activity of alkaline phosphatase in femur and kidney and there is a reduction in number of mitochondria, rough endoplasmic reticulum and free ribosomes in neurons of cerebral cortex.[¹⁶]

These neurotoxic effects of fluoride also presents as impaired development of intelligence in children.[¹⁷] IQ has been used to measure intellectual ability for many years and there are many standards for measuring IQ. We used the Raven’s progressive matrices because the results obtained by it are not affected by the cultural, ethical, linguistic or racial background of the children.[¹⁸] The results of our research show that exposure to high levels of fluoride, as also determined by the dental fluorosis status of the child, does have detrimental effect on the mental ability of the child. Although the effect on the IQ of the child does not seem to be affected by the degree of fluorosis, nevertheless there is a correlation between dental fluorosis and the intellectual ability. In a meta-analysis performed in China to review studies on fluoride and IQ between 1988 and 2008, a consistent and a strong association between exposure to fluoride and low IQ was found and it was also noted that children who live in fluorosis area have five times higher odds of developing a low IQ than those who live in nonfluorosis area.[¹⁹] Similar results have been found in other studies done separately by Y Lu, Shulin Liu, QW Xiang, Seraj B and Guojian Wang as well.[^2021222324]

Conclusions

R. Srikanth, in 2009, pointed out that in India about 62 million people are suffering from various levels of fluorosis, of which 6 million are children below the age of 14.[⁷] It should not come as a surprise that excess fluoride is one of the three major hurdles according to tenth 5-year plan in India.[⁷] Although the biomechanism of fluoride in reducing IQ is not clear but on the basis of our study and previous studies, it is evident that exposure to high levels of fluoride is certainly harmful. Considering these facts and findings it is suggestive that defluoridation of groundwater with excess fluoride in it is an issue which is in a dire need for attention.