Axial length is the distance from the very front to the very back of the eye, typically expressed in millimeters. This measurement is a direct indicator of the eye’s physical size and shape. Small variations in this length dramatically affect how light is processed, making it a powerful diagnostic tool for eye care professionals.
Defining Axial Length
Axial length is the precise measurement spanning the eye from its anterior surface, the cornea, to the retina or fovea at the back. This distance is a factor in determining the eye’s overall focusing power because it sets the final destination for incoming light rays. A typical axial length for a healthy adult eye falls between 23 and 25 millimeters. The eye undergoes rapid growth during infancy and early childhood, stabilizing as a person enters adolescence.
Axial Length and Refractive Errors
The relationship between the eye’s length and its focusing ability explains common vision problems, known as refractive errors. An eye that is too long causes light to focus in front of the retina, resulting in myopia (nearsightedness) where distant objects appear blurry. Conversely, a shorter axial length causes light rays to focus theoretically behind the retina. This condition is hyperopia (farsightedness), which makes near objects difficult to focus on clearly. A change of just one millimeter in axial length can alter the eye’s refractive power by approximately three diopters.
Methods for Measuring Axial Length
Eye care specialists use sophisticated instruments in a process known as biometry to measure axial length with high precision. The traditional method is A-scan ultrasound biometry, which uses high-frequency sound waves. However, this technique requires direct contact with the eye, which can lead to slight corneal compression and less accurate readings. The current standard of care is optical biometry, a non-contact technique that uses low-coherence light. Optical biometry is significantly more accurate and repeatable, often achieving a precision of less than a tenth of a millimeter.
The Role of Monitoring Axial Length
Tracking changes in axial length is important in modern eye care, especially for myopia control and surgical planning. For children developing myopia, regular measurements track the progression of eye elongation, which is a more objective marker than monitoring prescription changes. The goal of myopia control treatments is to slow this axial growth, ideally keeping the length below 26 millimeters, a threshold associated with a higher risk of serious eye diseases. In surgical settings, axial length measurement is required for calculating the correct power of an intraocular lens (IOL). The precise axial length is used in complex formulas to ensure the implanted IOL focuses light correctly onto the retina, aiming for the best possible post-surgical vision.