Bifocal lens E-line centering for patient with accommodative strabismus

Introduction

Bifocal lenses called E-line are used in children with accommodative strabismus. Children with accommodative strabismus have convergence excess during accommodation. It usually leads to diplopia and monocular suppression and finally to amblyopia.

We can decrease the convergence excess with decreasing of accommodation. We prescribe bifocal lenses with addition 2.5 to 3 D (Rutrle 2001). We have to use appropriate type of bifocal lens with appropriate decentration. Half of the spectacle eye should be filled with addition. For this purpose can be used Franklin bifocal which do not respect hygienic and aesthetic conditions. That is why we use so called E-line bifocals. E-line bifocals are centered on the pupil center during natural distance gaze. Optical centers are placed vertically in one line.

Special type of bifocal lens called E-line
Picture 7.1: Special type of bifocal lens called E-line (VD – centration distance point, ON – near optical point, decentration in millimeters, Rutrle 2001).

Goals

  • Center special bifocal lens E-line
  • Calculate vertical decentration on both eyes
  • Calculate horizontal decentration on both eyes
  • Verify adequate pupil position during near working

Equipment

Bifocal special lens E-line, spectacle frame, hand PD ruler, special lens for demo-foil and mirror with fixation point.

Methods

Center special bifocal lens E-line

Put on spectacle frame on the patient’s face. Patient looks naturally at infinity distant object. Mark pupil position on spectacle demo-foil. Note distance x which means horizontal distance between pupil and spectacle frame eye periphery. Further note distance x which means vertical distance between pupil and spectacle frame eye periphery. We also need to know basic parameters of the spectacle frame. Use boxing system.

Calculate vertical decentration on both eyes

According to boxing system calculate vertical decentration of the important point V for the right and left eye (vR , vL).

Calculate horizontal decentration on both eyes

According to boxing system calculate horizontal decentration of the important point V for the right and left eye (hR , hL).

Schematic calculation of the E-line horizontal and vertical decentration
Picture 7.2: Schematic calculation of the E-line horizontal and vertical decentration (xR – horizontal distance from pupil to nasal part of the rim, vR,L – vertical decentration, hR,L – horizontal decentration, b – eye frame height, VD – distance important point, yR – distance from the pupil to the rim bottom, Rutrle 2001).

Verify adequate pupil position during near working

Use mirror to check the position of the pupil during near working.

Results

vR =

vL =

hR =

hL =

vR = yR – ½ b [mm]     (7)

vL = yL – ½ b

y … height of the pupil, v … vertical decentration, b … vertical size of the rim

hR = ½ FS –PDR [mm]     (8)

hL = ½ FS – PDL

FS … frame size, PD … pupillary distance, h … horizontal decentration

Discussion

We centered bifocal lens E-line directly on the pupil’s center during the natural distance gaze. This lens helps to patients with accommodative strabismus. In horizontal direction we centered on pupils. With mirror method we can check lens centration.

Conclusion, notes, comments

What is the edge height in E-line centration?

Which mechanism eliminates convergence strabismus?

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