A normal healthy cornea is smooth and has the same thickness throughout. The shape of the cornea resembles a ball, with with the same amount of curvature in all directions.
The cornea is more curved in one direction than the other – in other words, the cornea is shaped more like an egg than a ball. This shape distorts vision, but can also easily be corrected with glasses because the distortion is symmetric.
This cornea is diseased and needs further examination and treatment. In the case shown in the image below, the cornea is flat in the upper part and markedly bulges out in the lower part (which is what’s known as the “cone”). Vision is distorted (people see double, triple and even quadruple images, and in many cases, glasses cannot restore decent visual acuity. Further, keratoconus is progressive and can lead to severe visual impairment, especially in children and adolescents.
Astigmatism should, put simply, remain the same throughout your life. Small changes in astigmatism are possible, but it it raises great concern when something like corneal curvature increases from 0.75 diopters to 2 diopters. An example in the table below, shows an increase in astigmatism (“cyl”) over period of several years, although moderate, but no intervention was performed. If you have an astigmatism/ corneal curvature that increases over the years, it’s definitely worth visiting a corneal specialist.