2. The Role of Atropine in Preventing Myopia Progression
2.1. Antimuscarinic Eye Drops: Overview of the Market
Antimuscarinic drops are used all over the world as cycloplegics, mydriatics, and for the penalization of the healthy eye in the treatment of amblyopia 
. In the last decades, we have witnessed an increase of evidence proving the efficacy of antimuscarinic drugs in preventing myopia progression in children all over the world. However, their ocular topical use is still off-label in many countries. In fact, no pharmaceutical agent has been approved by the US FDA for preventing myopia progression, although atropine is already used in Asia to control myopia in children 
. The effectiveness in preventing myopia progression in children has been demonstrated for different concentrations of atropine, cyclopentolate 1%, and for pirenzepine 2% 
. Several randomized trials and meta-analyses have explored both the efficacy and the side effects of different atropine concentrations. The impact on AL and SED was highest for 1% atropine and lowest for 0.1% [35,36,37]
. However, higher concentrations were also less tolerable and had a higher incidence of rebound effect after treatment discontinuation 
2.2. Atropine: Introduction to the Compound
The chemical formula of atropine is the following: C17
Topical atropine has a partition coefficient of 1.83 and a pKa of 9.43 at 7.4 pH, which means that it is ionized on the ocular surface [41,42]
. A recent pharmacokinetic study on rabbits revealed that after 5 h from topical administration, the highest concentration of atropine was detected in the conjunctiva, with a concentration gradient established anteriorly to posteriorly. Moreover, the concentrations in the cornea and sclera were similar. Therefore, the authors concluded that atropine reaches the anterior and posterior chambers by simple diffusion via the conjunctival, scleral and uveal routes 
. After 24 h, preferential binding of atropine to posterior ocular tissues was found. Atropine showed a good ocular bioavailability with concentrations of two magnitudes higher than its binding affinity in most tissues after 3 days 
. It has been also reported that atropine binds melanin, both in vitro and in rabbits 
In humans, the systemic absorption of topically applied atropine is generally low, but systemic side effects can indeed occur, especially in children, likely due to their smaller body volumes. In one study the reported systemic bioavailability of atropine in healthy individuals ranged from 19% to 95% 
. The largest amount of the drug is metabolized by enzymatic hydrolysis, particularly in the liver and 13–50% of the molecule is excreted unmodified in the urine [46,47]
Pharmacokinetic studies highlighted that the pharmacological effect begins after 48–120 min from its administration and lasts until 7–14 days 
Atropine is a nonselective reversible muscarinic antagonist. It binds to all five subtypes of muscarinic receptors (mAchR, MR1–MR5), preventing acetylcholine from interacting with them. On the other hand, pirenzepine is M1-selective [49,50]
. These receptors are coupled with G-proteins (GPCR) and have been found in the human iris, ciliary body 
, lens epithelium 
, retinal amacrine 
and pigment epithelial cells [54,55]
, and scleral fibroblasts 
The pathological mechanism of myopia and the pathways involved in the antimyopic effects of atropine are still largely unknown. However, some evidence on the matter does exist.
For example, Lind et al. studied chick fibroblasts in vitro and reported that the antimyopic effect of antimuscarinics may be at least partially mediated by the inhibition of M1 receptors in the sclera 
. A study in chicks demonstrated the inefficacy of M2 antagonists in opposing form deprivation myopia 
. However, M2 receptors have been implicated in myopia development in an in vivo study on mice 
. It was also reported that a M4-antagonist can prevent myopia progression in chicks 
. Therefore, one of the pathways for the antimyopic effect of antimuscarinics is the interaction with ocular mAchR.
In this regard, a protective role of dopamine (DA) has long been described 
, but a comprehensive theory for its mechanism of action is still lacking 
. In the human retina, dopamine is produced by amacrine and interplexiform cells 
. Dopamine has five GPCRs (D1–D5), some of which have been identified in animal retinal cells and RPE [65,66,67]
. In mice, the activation of D1 inhibited the development of myopia [68,69]
. In tree shrews, the activation of D2 and D4 receptors was reported to have an antimyopic effect 
, but in guinea pigs and D2 knock-out mice the activation of the D2 receptor seemed to do the opposite [71,72]
. Moreover, nonselective D-agonists can inhibit myopia progression in animal models [73,74]
. This leads us to the conclusion that atropine and DA could act in parallel biochemical pathways that would later converge on a common effector 
In particular, in preclinical models, atropine has been shown to stimulate the release of DA into the extracellular space and the vitreous, which may inhibit a retinal signaling process that is supposed to be involved in axial elongation, and thus myopia progression 
2.5. Clinical Efficacy
In the last few decades, low concentrations of atropine have gained considerable attention for their efficacy in slowing myopia progression in children. This was initially reported in non-randomized studies [86,87,88]
Subsequently, the Atropine for the Treatment of Childhood Myopia study (ATOM 1), which was a two-year long, randomized, placebo-controlled, double-masked trial, demonstrated that the progression of myopia was substantially slower in the group treated with atropine 1% than in the one that received a placebo. Specifically, 65.7% of atropine-treated eyes had a progression of less than 0.50 D, and 13.9% of them progressed more than 1.00 D. In contrast, 16.1% and 63.9% of placebo-treated eyes showed a progression of less than 0.50 D and more than 1.00 D, respectively 
. Moreover, after two years, the treated group had a significantly slower myopia progression, with a difference of −0.92 D in SED (95% confidence interval (CI): −1.10 to −0.77 D; p
< 0.001) and 0.40 mm in AL (95% CI: 0.35–0.45 mm; p
< 0.001) compared with the placebo.
Subsequently, the Atropine for the Treatment of Childhood Myopia 2 (ATOM 2) study was published, which had the aim to assess whether lower concentrations of atropine could be effective in reducing myopia progression, with potentially fewer side effects. The study comprised a two year-long treatment phase, followed by a one-year-long washout period.
Patients were randomized into three treatment groups (atropine 0.5%, atropine 0.1% and atropine 0.01%). After two years, myopia progression was lowest in the group treated with atropine 0.5% and highest in the 0.01% one. However, the progression rate of patients treated with atropine 0.01% was deemed clinically not very different from that seen at higher concentrations, and the ocular side effect profile was significantly better. Indeed, 50% of the 0.01% group progressed by less than 0.5 D, while these rates were 58% and 63% in the 0.1% and 0.5% groups, respectively. Among all three groups, around 18% of subjects progressed by 1.0 D or more 
3.6. Safety and Tolerability
Topical atropine is associated with some ocular AEs, including mydriasis, photophobia, and reduced accommodation, with symptoms of glare and blur during near-work activities. Although usually mild, side effects can hinder school and outdoor activities, and therefore represent a significant cause of treatment interruption.
The most problematic AEs reported with atropine eye drops are the rebound phenomenon and ocular side effects, which occur in around 5% of patients. The former is defined as the rapid increase in axial elongation shortly after treatment discontinuation. Other relevant side effects are an ocular allergic reaction and photophobia and blurriness of near vision, which are due to myosis and cycloplegia [35,37,94,98,99]
. The ocular side effects are more common and marked at higher concentrations of the molecule. This is especially true for the rebound phenomenon, which seems to be dose-related. In addition to this, low concentrations of atropine are often available in formulations that use benzalkonium chloride (BAK) as a preservative, which is known to be toxic to the corneal epithelium and it has been associated with dry eye syndrome. Some studies have even highlighted its toxic effects to the retinal tissue [101,102]
The ATOM 1 study reported only ocular side effects, which were allergic reactions (4.5%), glare (1.5%) and blurred near vision (1%) 
. During the ATOM 2 study, the rebound phenomenon appeared to be dose-related 
Atropine has been long known for its properties as a mydriatic and cycloplegic. Recently, a new side to this molecule has emerged, which is its potential in preventing myopia progression in children and adolescents [35,36,37]
. However, the exact mechanism of action of this drug and the pathophysiology of myopia onset and progression remain to be clarified.
In any case, atropine has shown promise in preventing the progression of myopia in children, and we are constantly transitioning to a concentration that has the highest efficacy and fewest side effects.
Indeed, despite being more effective, higher concentrations (1%, 0.5%) are associated with an increased risk of a rebound phenomenon and ocular side effects [35,36]
. On the other hand, lower concentrations (in particular 0.05%) seem to maintain a clinically satisfying efficacy, with a much lower incidence of ocular side effects [9,37]
. As a consequence, low concentrations of atropine appear to have a better clinical profile and represent a valid treatment strategy to slow the progression of myopia in children and adolescents. However, further randomized clinical trials with larger follow-up periods are needed.