Late-Day Asthenopia After High-Minus Glazing
A 34-year-old software engineer has worn single-vision glasses full time since age 12. Her new pair was made from a prescription written elsewhere: OD -6.00 -1.00 x 180, OS -5.50 -0.75 x 180. Her monocular pupillary distances, measured twice, are 32 mm right and 30 mm left. She chose a rectangular acetate frame marked 52-18, with a 145 mm temple. The laboratory edged both lenses with their optical centers at the frame's geometric centers, and the optician verified this on the lensmeter before dispensing. Two weeks later she reports late-afternoon brow ache, eye strain, and a sensation that the computer screen is 'pulling' on her eyes. Distance acuity is 20/20 in each eye, and the glasses are otherwise comfortable. She works 9-hour days at a 60 cm monitor and does not wear contact lenses. The optician suspects the glazing rather than the prescription.
1. Using Prentice's rule, calculate the horizontal prism induced by the glazing at each eye and in total. State the direction and explain how it produces her symptoms.
Read the worked answer
Prentice's rule: P = c x F, where c is decentration in centimetres. The right pupil is 3 mm (0.3 cm) nasal to its optical centre; the left is 5 mm (0.5 cm) nasal.
- Right: 0.3 x 6.00 = 1.80 Δ
- Left: 0.5 x 5.50 = 2.75 Δ
- Total: about 4.55 Δ
Because the eye looks through a point nasal to the optical centre of a minus lens, the thicker edge is nasal, so the prism base is nasal (base-in) in each eye. Base-in prism increases convergence demand: to keep a 60 cm target single, the patient must converge roughly 4.5 Δ more than the target requires. That extra load on the medial recti fits the late-day brow ache and strain, and the temporal image displacement can feel like the screen is pulling. Acuity remains 20/20 because the problem is prism, not power.
2. Calculate the decentration, in millimetres and direction, that each lens requires so that the optical centers align with her monocular PDs. What practical glazing checks would you make before ordering new lenses?
Read the worked answer
Each lens is 52 mm wide with an 18 mm bridge, so the geometric centre of the right lens is 26 mm from the right edge and the midline is 61 mm from that edge. The patient's right pupil is 32 mm from the midline, i.e. 61 - 32 = 29 mm from the right edge. The right optical centre must move from 26 mm to 29 mm: 3 mm nasal. The left lens centre is at 96 mm; the left pupil is at 61 + 30 = 91 mm, so the left optical centre must move 5 mm nasal.
Before ordering, check blank size. A rough minimum blank size is ED + 2(decentration) + 2 mm. With an ED of about 56 mm, the right needs about 64 mm and the left about 68 mm; if the original blanks were 65 mm, the left cannot be re-edged from the existing lens and a new lens is needed. Also check nasal edge thickness: a -5.50 lens decentred 5 mm nasally becomes thin at the nasal edge and may not hold in the frame groove. Confirm optical centre height at or just above the pupil.
3. Describe how you would verify the finished glasses on the lensmeter and what you would tell the patient about the cause of her symptoms.
Read the worked answer
Mount the lens on the lensmeter with the front surface against the stop, focus the eyepiece, and move the lens until the target cross is centred; that point is the optical centre. Dot it with the marking device, then measure from the dot to the pupil position with a PD ruler or the frame's boxing system. A correctly made lens will read zero prism at the dot and will place the dot at the patient's monocular PD. Check sphere, cylinder, and axis as well.
Explain that the first pair was edged with the optical centres too far apart, creating about 4.5 Δ of unwanted base-in prism; the new pair will move the centres 3 mm and 5 mm nasal. Most patients adapt within a few days; if symptoms persist, recheck fitting height and the prescription.