Another challenging finding to explain is that the only significant result was a decreased prevalence of high myopia in relation to organophosphate exposure. By contrast, seminal works in Japan in the 1960's describing the so-called Saku disease suggested that high exposure to organophosphates increased myopia.
7 Because these pesticides are known to increase levels of ACh, this effect is thought to be related to alterations of the muscarinic and nicotinic processes in the eye.
17 In support of this hypothesis, several experimental studies have clearly demonstrated the crucial role of the muscarinic and nicotinic pathways in the process of eye elongation and therefore myopia.
20–22 Moreover, it has been clearly demonstrated that the muscarinic antagonist atropine is effective at preventing myopia in children.
23–26 Despite being in contradiction with the notion that organophosphates may increase myopia, our results are nevertheless in agreement with some experimental research in chicks. In a study using chlorpyrifos, an organophosphate pesticide widely used in the United States, deprivation-induced myopic animals showed shorter vitreal chambers and were thus less myopic after exposure (sex was not specified in that study).
53 Decreased myopia was also found in a similar experimental model following exposure to diisopropylfluorophosphate, an indirect cholinomimetic.
54 The mechanisms underlying these atypical (reverse) results are unclear. First, it has been shown that the effect of some myopigenic compounds might differ depending on dose. For instance, intravitreal drug exposure to reserpine in chicks can make the eye more myopic at high doses (>1000 nmol) but less myopic at low doses (<100 nmol).
55 Second, the over-stimulation of the muscarinic pathway can lead to its desensitization because of a decrease in the number of muscarinic receptors.
56 Third, a possible mechanism to account for a decrease of high myopia as a function of exposure to organophosphates is through thyroid dysfunction. Indeed, it is known that these pesticides can alter thyroid function by decreasing the metabolic concentration of T3.
57–59 In parallel, some studies in patients with hyperthyreosis (excessive production of thyroid hormone) reported the presence of high myopia.
60–62