Dispensing case study: Lens selection and the ‘jack-in-the-box’ effect

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Figure 4. Clear set

By Jamin Krine, Specsavers Strathpine, QLD

Background

Px:

  • 63-year-old retired male
  • New patient
  • Was wearing 22mm round-seg bifocal blended lenticular with multicoating
  • Is aphakic (has no crystalline lens) due to congenital cataracts
  • Had never had sunglasses before but really wanted them
  • Stated that he would only accept a round segment bifocal. He had ventured away from them previously as he was not able to achieve clear vision at near and experienced asthenopia (eye strain) despite the prescription being the same or very close to his current prescription.

Outside Rx from specialist:
R: +13.75/+2.25 x 180
L: +11.00/+1.75 x 5
Near add: +2.50

Transposed:
R: +16.00/-2.25 x 90
L: +12.75/-1.75 x 95
Near add: +2.50

Concern: Patient was concerned with the weight of his current spectacles and clarity of vision. He ideally wanted his next spectacles to be lighter and clearer than his current glasses.

Discussion

  • A high-powered spherical lens will cause a large ring scotoma (Figures 1 to 3). This will cause the patient to experience a ‘jack-in-the-box’ effect where objects viewed in the periphery will disappear as they enter this ring scotoma and then seem to suddenly reappear as they exit the scotoma. Reduction in BVD and appropriate lens selection are vital in reducing or, ideally, eliminating this negative effect.
  • As the prescription was from an external specialist, the amount of information available was quite limited. To ensure that what was dispensed would work and meet the patient’s needs and expectations (which were discussed during the pre-dispense), various measurements and observations of the patient’s existing spectacles were required. Of key importance was BVD, face form angle, pantoscopic tilt and lens design. Neutralised the patient’s current spectacles and marked them up, checking their centration.
  • Finding a lab that would manufacture the required lens and prescription presented a challenge. Phoned various labs, both local and interstate, to see what options were available, then researched the options online to determine which one was most suitable.

Solution

  • An Omega 1.50 22mm round segment bifocal was selected. This lens uses a continuous polynomial aspheric front surface to give the effect of a blended lenticular. This design eliminates the ring scotoma and jack-in-the-box effect common in high-plus lenses (Keirl & Payne, 2008).
  • Top rim of round segment measured was set at lower limbus as per the patient’s previous spectacles.
  • Selected a frame with geometric centres aligned as closely as reasonably possible with the patient’s distance centration. Was unable to find a frame that perfectly fit both the patient’s head and matched his distance centration in both eyes. Selected TH 73 (52 eye size, 17 DBL, 37.4 depth) for both clear (Figures 4 and 5) and sunglasses (Figure 6). Distance OCs measured as R: 31 and L: 29 with distance heights of 21, meaning decentration of 3.5mm out and 2.3mm up for the RE and 5.5mm in and 2.3mm up in the LE. Fortunately, the decentration in the LE did not adversely affect lens selection.
  • BVD, face-form angle, and pantoscopic tilt of display frame were adjusted to closely match the patient’s current spectacles and align with manufacturer guidelines for the selected lens prior to measuring with VDTM. Monocular OCs were used.
  • Anti-reflective multicoating was added to the clear set to increase light transmission and reduce reflections.
  • Brown grade 2 tint was recommended as opposed to grey. The ability of a brown lens to improve contrast was of benefit as the patient had poor vision naturally (Tunnacliffe, 2007, pp. 97-116) (Naidu, et al., 2003).
  • The patient was very pleased with his new glasses. He was especially thankful that he had prescription sunglasses for the first time. He commented that his vision was even better in his new glasses than he had expected and found them preferable to his old set. He was particularly impressed with the time and care taken in finding the ideal solution for him and said he would be a customer for life.

Learning Outcomes

Sometimes a dispenser is required to think outside the box, especially when faced with a high prescription and a patient with specific requirements. A dispenser should be well-informed on lenses that are available, in order to be able to provide a solution to complex visual issues.


REFERENCES
Keirl, A. & Payne, R., 2008. Dispensing II: Complex lens dispensing. [Online] Available at: https://cvt123.files.wordpress.com/2012/06/complex-dispensing-article.pdf
[Accessed December 2018].

Naidu, S. et al., 2003. The effect of variably tinted spectacle lenses on visual performance in cataract subjects. Eye & Contact Lens: Science & Clinical Practice, 29(1), pp. 17-20.

Tunnacliffe, A., 2007. Essentials of Dispensing. 3rd – 2007 Revised ed. London: Association of British Dispensing Opticians.

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