In contrast, a gradual loss of tension on the zonules with age is central to the Schachar

The mechanism of accommodation and its relevance to presbyopia has been debated since Victorian times, and the arguments are still surprisingly vehement.Are new data bringing us any closer to a resolution? Sir Arthur Conan Doyle, author of the Sherlock Holmes books, also wrote a book called The Lost World, dominated by a brilliant and insufferably arrogant scientist, Prof Challenger. After an expedition to the lost world – a remote and inaccessible plateau in South America, where dinosaurs still roam – Challenger returns to London to present his findings to a public meeting of the Royal Society. A ferocious argument erupts, which is only resolved when the professor lets loose a pterodactyl, which flaps slowly around the Natural History Museum, before escaping through a window. Science doesn’t usually arouse such high passions these days, though we still rely on data – if a pterodactyl can be regarded as a datum – to support any conclusions. The Lost World was written in 1912, more than half a century after Helmholtz started another long-running and at times equally acrimonious debate, now 150 years old, on the mechanism of accommodation. Despite the release of a whole pack of pterodactyls, or at least data, the debate still retains a Victorian sense of melodrama and personality, and indeed some protagonists do occasionally come alarmingly close to blows. In essence, Helmholtz argued that constriction of the ciliary muscle releases the tension on the zonules, leading to a relaxation and rounding of the lens, so increasing its power. Some 40 years later, Tscherning argued more or less the opposite: that ciliary-muscle contraction increases the tension on the zonules, thereby altering the shape of the lens without changing its thickness.Another prominent theory, originally postulated by Mueller around 1854, and given modern form by Coleman, is that accommodation is achieved through anterior movement of the lens, under pressure from the vitreous. But it was the resurrection of Tscherning’s conception by Schachar in the 1990s that re-injected the sense of Victorian melodrama into the debate. “People get very resolute in their beliefs,” Dr Spencer Thornton MD, University of Tennessee Health Science Center, Memphis, told EuroTimes.“There’s been a big resistance to anything beyond Helmholtz. I don’t doubt that increased lens sphericity contributes to accommodation, but in my view there are at least six mechanisms that all contribute partially including anterior lens movement, the belt action of the iris, and indeed gestalt, the brain’s ability to interpret.” The flip side of the coin is presbyopia.The Helmholtzian view posits sclerosis of the lens, by which he is usually taken to have meant a loss of elasticity, preventing the rounding necessary for accommodation.The hypothesis doesn’t predict changes in the zonules, the ciliary muscle, lens movement, vitreous phase or lens growth, all of which are modified to some extent as we age, though not necessarily at the same rate as the progression of presbyopia. In contrast, a gradual loss of tension on the zonules with age is central to the Schachar theory of presbyopia. Continuous lens equatorial growth throughout life is posited to lower zonular tension, thereby rendering the zonules unable to pull the lens into the correct shape to accommodate.To its great credit, the theory makes a number of testable predictions, many of which are the polar opposites of the Helmholtzian view. In particular, the equatorial diameter of the lens should increase with age, lowering zonular tension.Anything that can restore zonular tension, such as scleral expansion/spacing, should in principle restore accommodation (assuming that the lens has not become too sclerotic in the interim). In contrast, if presbyopia is caused by hardening alone there is no reason whatsoever why scleral spacing should work. If it does work, then Helmholtz must have been wrong about the cause of presbyopia, and by extension, about the mechanism of accommodation too.