OCULAR PHYSIOLOGY/
EFFECTS OF EXTENDED WEAR
Objectives of this lecture:
1. To understand corneal physiological requirements for extended wear and the potential complications that can result from hypoxia
I. USAGE
A. Estimated Worldwide Mode of Wear
EW (6
nights) = 5%
EW (1-2 nights) = 10%
DW
= 85%
B. Patient Preferences for Mode of Wear
DW= 3%
Occasional EW= 21%
EW (6 nights)= 35%
Continuous Wear= 41%
II. CORNEAL PHYSIOLOGICAL CHANGE
A. During Sleep
B. Corneal Response
a. RGP EW reduces corneal
stromal pH with a direct nonlinear relationship between pH and Dk/L
b. pH (upon awakening) varied from 7.01 (PMMA) to 7.26 (fluoropolymer); No Lens
= 7.38
a. Disruption occurs in the form
of:
Polymegethism - change in cell size
Pleomorphism - change in cell shape
Blebs
b. Caused by hypoxia and acidosis
c. Polymegethism may be irreversible and complicate IOL surgery
C. Upon Awakening
a. Dk/L of CL
b. Tear Pum
III. OXYGEN - HOW MUCH IS ENOUGH
A. Quantifying Oxygen Delivery
B. Evolution of Closed Eye Critical Oxygen Levels
Current Standard (Holden & Mertz; 1984)
Category |
EOP |
Dk/L |
Safe DW |
9.9% |
24.1 |
EW: No Residual Swelling |
12.1% |
34.3 |
Ideal |
17.9% |
87.0 |
Patients vary greatly in corneal oxygen need.
C. Oxygen Transmission in Extended Wear: Rigid Versus Hydrogel Lenses
1. Paragon Optical Data - Oxygen Transmissibility:
Fluoroperm 92= 57.5
Fluoroperm 60= 42.8
Acuvue= 40.0
SeeQuence= 24.2
CSI - T= 24.0
Permalens= 18.1
2. Health Learning System Data - Oxygen Transmissibility
Material |
Dk |
Thickness (L) |
Dk/L |
38% H2O |
10x10-11 |
0.05mm |
20x10-9 |
55% H2O |
20x10-11 |
0.08mm |
25x10-9 |
70% H2O |
30x10-11 |
0.12mm |
25x10-9 |
S/A RGP |
50x10-11 |
0.15mm |
33x10-9 |
FS/A RGP |
70x10-11 |
0.15mm |
47x10-9 |
Fluoropolymer |
100x10-11 |
0.20mm |
50x10-9 |
3. LaHood, Sweeney, Holden Study (1988): Overnight Corneal Edema with
Hydrogel, RGP and Silicone Contact Lenses
a. Methods:
b. Results:
4. Adapted Wearers - Nelson et al.:
5. Low Dk RGP Overnight Swelling
6. Corneal Deswelling
IV. EFFECTS OF EXTENDED WEAR
A. Corneal Ulcers
1. Pathogenesis of Pseudomonas Keratitis in CL Wearers:
Contact Lens
Inserted onto eye >Becomes coated
with Mucin > Pseudomonas adheres to lens > Pseudomonas transfer/adhere
Protien > Initiation of Corneal Infection to injured epithelium
Cornea
Epithelium
Injured by Trauma, Defect, Hypoxia, Toxins
2. New England Ulcerative Keratitis Study
a. Relative Risk of Ulcerative Keratitis Among Users of Daily Wear Soft Contact Lenses (Schein et al., 1989)
(1) Methods
(2) Results
Lens Use and Risk
(a) EW - Each consecutive day of wear before removal for cleaning increases
risk by at least 5%
(b) DW - Each additional night of wear per 2 week period increases risk by at
least 46%
b. The Incidence of Ulcerative Keratitis Among Users of DW and EW Soft Contact Lenses (1989)
(1) Methods
c. Estimated Annual Rate of Ulcerative Keratitis in U.S.”
Mode of Wear Cases (per 10,000) Total Cases
Daily Wear 4000/9 million DW
Extended Wear 8000/4 million EW
d. Incidence of Ulcerative Keratitis per 10,000 CL Users/Year
Soft DW: 4
Soft EW: 21
Hard: 2
RGP: 4
e. Summary of Findings:
(1) Relative Risk of Ulcerative Keratitis
(2) Incidence of Ulcerative Keratitis
(3) Factors Not Associated with Ulcerative Keratitis
f. New England Journal Editorial (same issue, Sept., 1989)
“Since wearing a soft contact lens continuously for even a week substantially increases the risk of ulcerative keratitis and since patients are likely to push beyond any suggested guidelines, there is concern that this change does not go far enough in warning patients of the risk.”
B. Microcystic Edema
C. Polymegethism
1. Coefficient of Variation of Different Populations
· No Lenses, Young Adult 0.31 ± 0.04
· Long-term PMMA wear 31-82%
· After 5 years EW soft lens wear 22-27%
· Silicone Elastomer Lenses No Increase
2. Polse (1986): “We seem to be prematurely aging the corneas with soft contact lenses. We are producing an endothelial mosaic that looks like a 60 y/o instead of a 25 y/o in 3 to 4 years of extended wear.”
3. Significance:
D. Summary - Chronic Hypoxia
Consequences After 5 Years of Hydrogel EW:
Epithelium 15% in O2 Uptake
85% Microcysts (Mean # = 15)
6% Thinning Stroma 2% Thinning Endothelium 95% in Polymegethism (Mean = 22%; equivalent to 80 y/o)