The Mechanism of Accommodation and Presbyopia

The human eye is a remarkable optical device. In less than a second, a young human eye can accommodate from infinity to closer than 10 cm. Accommodation occurs with minimal effort and can be rapidly repeated with no apparent evidence of fatigue. Unfortunately, maximum accommodation decreases throughout life and by the fifth decade leads to presbyopia, the inability to read at a normal working distance. Interestingly, the mechanism by which the human eye is able to adjust focus has been debated for over 300 years. No previous theory has been put forth that can account for all the physical changes known to occur during accommodation.Accommodation occurs as a consequence of a change in shape of the lens of the eye. The key changes during accommodation are central surface steepening combined with peripheral surface flattening, a shape that I refer to as the "steep profile". In this treatise, I employ mathematics, physics, mechanics, physiology, and anatomy to fully explain the mechanism of accommodation.The book begins with a short introduction to accommodation and its present theories. Readers are then introduced to the mathematical analysis of an ellipse and the changes that occur in its shape when its major axis is increased under constraints that simulate the human lens during accommodation. This approach is taken to demonstrate that the shape of the accommodated lens is just one manifestation of a fundamental and universal phenomenon, the steep profile. This basic understanding is then used to predict the amplitude of accommodation of different vertebrates. Next, the anatomy and physiology basic to the accommodative mechanism are investigated and the experimental evidences supporting and contradicting the steep profile mechanism are evaluated. Then, sophisticated mathematical and finite element analyses are applied to fully describe the major parameters that affect accommodation and its age-related decline, which results in the universal manifestation of presbyopia in the fifth decade of life. This is followed by a discussion of the experimental controls required to observe the accommodative mechanism. In an example of the universality of the steep profile phenomenon, the implications of steep profile formation on lenticular galaxy evolution are presented. The book concludes with a summary of findings that must be observed during accommodative experiments. Included in the appendices is a MATLAB software program for demonstrating a steep profile in an ellipse and complete copies of pertinent published papers.