Optometry Skills: Baseline Measurements & Keratometry

The difference between the steepest and flattest meridians of the cornea.

The difference between the corneal astigmatism measured by keratometry and the astigmatism prescribed in the patient's prescription.

The vertical diameter of the visible iris. It is a crucial parameter in determining the appropriate contact lens diameter for a patient.

The distance between the eyelid margin and the corneal apex, which affects the contact lens diameter.

The curvature of the sclera compared to the cornea. A flatter sclera requires a flatter contact lens.

The number of times an eye blinks per minute.

A high blink rate can be a symptom of dry eye.

A low blink rate can lead to more deposits building up on contact lenses.

The amount of tension in the eyelids.

Too high lid tension can prevent someone from wearing contact lenses, especially rigid gas permeable (RGP) lenses.

The shape, position, and height of the eyelids.

A small palpebral aperture (both lids covering parts of the cornea) is not suitable for RGP lenses.

A high lower lid position (where the lower lid covers a significant portion of the cornea) is unsuitable for RGP lenses.

Indicates the meridian with the highest optical power, representing the steeper curvature.

Indicates the meridian with the lowest optical power, representing the flatter curvature.

Represents the difference between the steepest and flattest meridians of the cornea, and is measured in diopters (D).

The combined refractive error of corneal astigmatism and internal astigmatism.

The refractive error related to the lens and posterior corneal surface, distinct from corneal astigmatism.

A type of contact lens designed to correct high levels of astigmatism, especially when the cornea has a significant contribution to the total astigmatism.

A type of soft contact lens typically used for low to moderate astigmatism, or when the internal astigmatism is the major contributor to the total astigmatism.

The shape of the cornea, formed by the junction between the cornea and sclera. It helps determine the lens fit.

Measurements like keratometry, anterior eye assessment, and the patient's history and symptoms, are all essential for determining which contact lens is the best fit for the individual.

A technique used to measure the curvature of the cornea.

In keratometry, two mires are projected onto the cornea, and their reflected images are used to calculate the corneal radius. The distance between the device and the cornea also plays a role in the calculation.

Multiple types of devices, including Zeiss-Bombe, Zeiss CL-150, Rodenstock C-Mess, Topcon OM-1, and Topcon OM-4, are available for keratometry.

A type of keratometer that was commonly used in the past.

The Javal-Shiotz keratometer requires the user to adjust the focus and re-center the mires throughout the procedure, ensuring accurate measurements.

During keratometry using the Bausch & Lomb keratometer, it's important to adjust the focus and re-center the reticle (a pattern of lines) for accurate readings.

The distance between the centers of the pupils of both eyes, measured in millimeters. It's essential for determining the proper interpupillary distance (IPD) for contact lens and spectacles prescriptions.

The diameter of the pupil, typically measured in millimeters, and varies under different lighting conditions:

• Photopic (bright light): 3-4 mm
• Mesopic (dim light): 4-5 mm
  Pupil diameter significantly impacts contact lens selection, especially for rigid gas permeable (RGP) lenses. Smaller pupils require smaller lens diameters, while larger pupils may necessitate larger lens diameters.

A diagnostic procedure that measures the curvature of the cornea, expressed in diopters. It's crucial for contact lens fitting, especially for RGP lenses, to ensure proper fit and comfort.

A set of measurements, such as HVID, VPA, and pupil diameter, taken to assess various aspects of the eye. These measurements are essential for determining the most appropriate contact lens fit for individual eye characteristics.

The largest diameter a contact lens can have while still providing adequate clearance to the eyelids. This is especially important for larger palpebral apertures to prevent the lens from touching and irritating the eyelids.

The distance between the front surface of the cornea and the back surface of the contact lens. It's a crucial parameter for lens fitting, particularly for RGP lenses, to ensure optimal lens fit and oxygen permeability to the cornea.

The difference between the central corneal radii, measured in millimeters (∆r = r flat – r steep). It indicates the toricity of the cornea, which is the curvature difference between the cornea's steepest and flattest meridians. This difference results in corneal astigmatism (CA).

A corneal astigmatism where the steepest meridian is aligned with the horizontal meridian, resulting in the cornea being more curved in the horizontal meridian.

A corneal astigmatism where the steepest meridian is aligned with the vertical meridian, resulting in the cornea being more curved in the vertical meridian.

A corneal astigmatism where the steepest meridian is oriented at an angle between 120 and 150 degrees or 30 and 60 degrees.

The curvature of the cornea measured along the meridian that has the steepest curvature.

The curvature of the cornea measured along the meridian that has the flattest curvature.

A measurement that indicates the degree of astigmatism in an eye. It is the difference between the steep and flat corneal radii (∆r = r flat – r steep).