Spectacle Blur and PMMA-Induced Corneal Hypoxia

BY JEFFERY LAFERLA, O.D.

Patient L.L., a 29-year-old veterinarian, wanted to replace her 10-year-old eyeglasses. She was wearing a four-year-old pair of PMMA contact lenses 14 hours a day with no complaints. She had worn hard lenses for nearly 15 years.

DIAGNOSTIC DATA

Distance acuities were 20/20- OD and 20/20 OS with the patient's habitual lenses -- 7.73mm (43.62D) base curve, -4.50D power, 8.8mm OAD (OD); 7.62mm (44.25D) base curve, -4.50D power, 8.8mm OAD (OS). Both lenses moved well with upper lid attachment and a diffuse fluorescein pattern. At the time of the appointment, she had been wearing her contact lenses for about eight hours.

Subjective refraction immediately following contact lens removal was
-4.25-0.25x80 OD (20/25+) and
-4.25-0.75x75 OS (20/20-). The pupils were equal, round and reactive to light with no afferent defect. Slit lamp examination was normal. I didn't evaluate for central corneal clouding because I believed L.L. was wearing RGPs. All other findings were normal. I prescribed the following spectacle Rx: -4.25-0.25x80 (OD); -4.25-0.50x75 (OS).

Three weeks later, she returned complaining of distance blur with her new eyeglasses. Following 12 hours of wear that day, she had removed her PMMA lenses one hour before her appointment. Spectacle corrected acuities were 20/40- for each eye. Subjective refraction was -3.25-1.50x10 OD (20/30) and -3.50 -1.00x30 OS (20/40). Keratometry measured 43.00/43.50 WTR OD (clear mires) and 42.75/43.50 WTR OS (mild mire distortion).

REFRACTIONS AID DIAGNOSIS

Because of the marked difference in the two subjective refractions, I diagnosed spectacle blur secondary to PMMA-induced corneal hypoxia, OU. I explained to L.L. that to be able to transition from contact lenses to eyeglasses without spectacle blur, I'd have to refit her with rigid gas permeable contact lenses to minimize corneal swelling.

I saw her again four days later at 10:00 a.m. As directed, she had not worn her contact lenses since 8:00 the previous evening. Subjective refraction was -2.25-2.00x180 OD (20/25-) and -2.25-1.50x15 OS (20/30-). Keratometry measured 41.87/43.37 WTR OU with clear mires OD and mild mire distortion OS. Due to the large refractive and keratometric fluctuation from the visit four days previously, I asked her to return in eight hours. At that visit, the refraction was -4.75-1.00x35 OD (20/30) and -4.75-1.50x165 OS (20/30). Keratometry measured 42.62/44.25 WTR OD (clear mires) and 42.50/43.50 WTR OS (mild mire distortion).

Since L.L. requires crisp visual acuity to practice veterinary medicine, I prescribed the following lenses even though her corneas were still changing: FluoroPerm 30, 7.80mm (43.25D) base curve, -5.00D power, 9.2mm OAD (OD); 7.87mm (42.87D) base curve, -4.75D power, 9.2mm OAD (OS).

RGPs TO THE RESCUE

Three weeks later, L.L. had no vision complaints with the RGPs and achieved 20/20 acuity in each eye. She experienced spectacle blur for up to one hour after contact lens removal. Overrefraction was +1.00DS OD and +0.50DS OS. I ordered the following new Fluoro-Perm 30 lenses: 7.71mm (43.75) base curve, -4.50D power, 9.2mm OAD (OD); 7.78mm (43.37D) base curve, -4.75 power, 9.2mm OAD (OS).

Five weeks later, L.L. reported only 15 to 20 minutes of mild blurriness when transitioning to her eyeglasses. Distance acuities were again 20/20 and both lenses were comfortable. Refraction was -3.25 -0.50x82 OD (20/20); -3.75-0.50x70 OS (20/20). Keratometry measured 43.00/43.50 WTR OD and 43.00/43.25 WTR OS (clear mires OU). I changed the spectacle prescription to: -3.50-0.50x82 (OD);
-4.00-0.50x72 (OS). One month later, she was satisfied with both her eyeglasses and her contact lenses, without experiencing significant spectacle blur.

The effects of corneal hypoxia have made RGP lenses the hard lens of choice. For this patient, switching to RGPs was the only way she could alternate between contact lenses and spectacles without compromising visual acuity. This case underscores the importance of a thorough history and reminds us that not all patients wearing new hard lenses are wearing RGPs. CLS

References are available. To receive them via fax, call (800) 239-4684 and request document #31.

Dr. LaFerla is in group practice in Kansas City, Mo.