Lateral Decentration: Doug Benoit, OD, FAAO
Patient TP presented for evaluation of discomfort with his existing GP lenses. The lenses were about 1 ½ years old and had grown increasingly uncomfortable over the previous 6-8 months. He noted that his vision was off and the lenses were sometimes very hard to remove. On further questioning, he revealed that he had very blurry vision with his glasses after lens removal. This sometimes lasted for several hours.
Test Procedures, Fitting/Refitting, Design and Ordering
Visual acuity with his current lenses was 20/25 OD, OS, OU. Over-refraction showed no improvement. With slit lamp evaluation, both lenses were clean. The right lens positioned temporally and inferiorly, while the OS was nasally and inferiorly located. Movement was adequate but each lens dropped after the blink. Fluorescein evaluation showed alignment in the vertical meridian with slightly excessive peripheral clearance laterally OU, with the blink. Each cornea had mild 3 & 9 staining.
Current Lens Parameters:
|Base Curve Radius (BCR)||7.75mm||7.80mm|
|Overall Diameter (OAD)||9.00mm||9.00mm|
OD: -2.00 -0.75 x 095
OS: -2.25 -1.25 x 090
OD: 44.25 @ 005; 43.50 @ 095
OS: 44.50 @ 005; 43.25 @ 095
Mires were slightly blurry OU.
The palpebral fissure height was 12mm OU. The upper lids overlapped the superior limbus by 2mm. The lower lids were tangent to the lower limbus.
This patient appeared to be a good candidate for an aspheric GP lens design. My local CLMA laboratory has a bi-aspheric design that is quite successful. Diagnostic lenses were applied having the following specifications:
|Overall Diameter (OAD)||9.60mm||9.60mm|
The lenses were inserted and comfort was good, with minimal tearing.
OD: 20/40+ with Over-Refraction +1.25DS 20/20
OS: 20/30+ with Over-Refraction +0.75DS 20/20
Slit Lamp Evaluation:
Each lens positioned superior centrally with good lid attachment. Movement was good in all positions of gaze. Fluorescein showed alignment-to-slight apical clearance OU with good paracentral touch and good peripheral clearance.
The Following Lenses Were Ordered:
|Material||Boston EO OU|
|Design||Bi-aspheric with minus carrier lenticular OU|
Patient Consultation and Education
TP was advised to decrease his wear time while waiting for the new lenses. This should lessen the problem with spectacle blur that he has experienced. The need for possible lens parameter changes to enhance the lens-to-cornea fitting relationship, vision and/or comfort was discussed. This may be necessary if the blur to the keratometric mires was due to corneal molding.
The new lenses were dispensed after being inspected to ensure the parameters were as specified. His comfort was great soon after insertion. His visual acuity was 20/20 + OD, OS, OU. No change was found with the over-refraction. Each lens positioned in a superior-central, lid attachment manner. Neither lens dropped after the blink. Movement was good in all positions of gaze. Fluorescein evaluation showed central alignment OD and slight apical clearance OS. Both lenses exhibited trace paracentral touch with good peripheral clearance. Proper care was reviewed and fresh solutions dispensed. TP was advised to gradually increase his wearing time.
Follow-Up Care/Final Outcome
At the 10 day progress evaluation, TP reported good comfort all day long and consistently good vision. He had no problem with lens removal and experienced no spectacle blur. His entering vision was 20/20 + OD, OS, OU. Slit lamp examination showed the same lens-to-cornea fitting relationship, etc., as previously described. TP has continued to do well.
Discussion/Alternative Management Options
This patient represents one of the challenges practitioners are confronted with when fitting GP lenses. The against-the-rule astigmatic patient generally needs more care in parameter selection and follow-up than the with-the-rule patient. Initially, a GP lens can appear to fit and move well, but this can change as the lens is worn for a longer time and slight corneal molding occurs. It is best to see these patients at various times of day for progress evaluations so that any physical findings can be detected. Also, the patient must be questioned about his or her observations relative to comfort and vision.
In these patients a thin, aspheric design can be quite successful because they allow the lens to rest more on the paracentral cornea which can help prevent lateral decentration. Also, thinner designs have a center of gravity that is more posterior which helps keep the lens from dropping. A larger lens diameter can be important, as observed in this case. The larger lens allows for better lid attachment which not only helps prevent decentration, but aids comfort, decreases 3 & 9 o’clock staining and helps minimize lens adhesion. In this patient a minus carrier lenticular also helped with lid attachment.
A soft toric lens could also be successful with this type of patient. Lens stability is generally good due to the larger diameter, and vision can be almost as good as with a GP lens. Unfortunately, if the lens does rotate, vision can suffer. Also, the soft lens is more fragile which can lead to increased costs.
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