It may be necessary to figure the segment inset for each eye separately for persons with:

The need to calculate the segment inset for each eye separately often arises in cases of facial dissymmetry. Facial dissymmetry refers to the unequal shape or structure of the left and right sides of the face. When fitting multifocal or bifocal lenses, precise alignment of the optical points of the lens segment with the patient's visual axis becomes crucial for optimal vision.

In the presence of facial asymmetry, one eye may require a different inset measurement compared to the other eye to ensure that the segment of the lens sits properly in front of the pupil when the wearer is looking straight ahead. If this adjustment is not adequately made, it can lead to visual discomfort or difficulty in using the lenses effectively.

In contrast, while the other conditions mentioned may affect vision or lens performance, they do not necessarily dictate the need for different segment insets for each eye. Corneal astigmatism and refractive astigmatism pertain to the shape of the eye and how light is focused but typically involve adjustments to the overall prescription rather than separate segment positioning. Facial neuralgia, which pertains to nerve pain, does not inherently affect how spectacles should be fit concerning segment placement.