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

1. Oxygen Reduced (from 155mm Hg to 55mm Hg partial pressure or 21% to 9%)
2. Carbon Dioxide Accumulation
3. Flushing of Debris Impeded (Adherence)

B. Corneal Response

1. Reduced Epithelial Metabolic Activity
2. Reduced Corneal Sensitivity
3. Corneal Swelling
4. Decreased Stromal pH
   Effect of Rigid CL O2 Transmissibility on stromal pH in the Living Human Eye (Bonanno, Polse, Ophthalmol, Oct. 1987)
   Conclusions:

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

5. Endothelial Response to Chronic Hypoxia

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

6. Corneal Exhaustion Syndrome

C. Upon Awakening

1. Presence of Contact Lens Inhibits Recovery
2. Dependent Upon:

a. Dk/L of CL
b. Tear Pum

III. OXYGEN - HOW MUCH IS ENOUGH

A. Quantifying Oxygen Delivery

1. Dk = Oxygen Permeability (diffusion x solubility)
2. Dk/L = Oxygen Transmission (Dk/thickness)
3. EOP = Equivalent Oxygen Percentage (how much of 21% atmospheric oxygen is predicted to reach anterior cornea)

B. Evolution of Closed Eye Critical Oxygen Levels

1. Polse and Mandell (1970) 11-19mm Hg O2 tension, 2% EOP, Dk/L = 10 - 20 x 10-9
2. Mandell and Farrell (1980) 23-37mm Hg O2 tension, 4% EOP, Dk/L = 20-30 x 10-9

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:

• N = 104 Unadapted Subjects
• Materials:
  14 Hydrogel
  14 RGP
  3 Silicone Elastomer

b. Results:

• Hydrogel  = 10-14.4% swelling
• Silicone  = Similar to “No Lens”
• RGP
  Fluoropolmer =  6% swelling (100)
  Fluoroperm 92 = 7.6% swelling (92)
  Equalens = 10.2% swelling (55-70)
  Paraperm EW = 11.0% swelling (56)
  Boston IV = 12.9% (28)

4. Adapted Wearers - Nelson et al.:

• Approximately 7.5% swelling with Boston Equalens, Paraperm EW
• No Residual Swelling

5. Low Dk RGP Overnight Swelling

• 15-16% Swelling
• Long-term Effects

6. Corneal Deswelling

1. Rapid Deswell with RGP EW lenses (even low Dk)
2. Soft EW = average of 3.5% residual swelling
3. Considering Dk and tear pump, the RGP lens has the potential to deliver 2-3 times more oxygen than soft

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

• N = 99
• Age = 12 or older
• CL wearers (all types)
• Corneal Stromal Infiltrate with Overlying Ulceration
• Ab for presumed corneal keratitis

(2) Results

• Clinical Features:
  (a) Culture positive = 63%
  (b) Visual Axis = 51%
  (c) Ulcer Size                         2 mm = 56%
                                                  3-5 mm = 32%
                                                  6 mm = 12%

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

• 5 New England States
• 4 Month Period

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

• No ON use 2-3 soft lens wearers
• Some ON use 19-25

Extended Wear 8000/4 million EW

• No ON use 3-8 soft lens wearers
• Regular ON    use 22-34

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

• EW vs DW: 4 to 1, regardless of use
• EW: ON use vs. strict DW = 1-15 to 1
• DW: Some ON use vs. strict DW = 9 to 1

(2) Incidence of Ulcerative Keratitis

• DW: 4/10,000
• EW: 21/10,000
• Rate ratio of EW vs DW: 5 to 1

(3) Factors Not Associated with Ulcerative Keratitis

• Age, sex, race
• New lens, lens age
• Time since last visit to eye care professional
• Type of disinfection
• OD versus MD
• Pre-existing ocular disease

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

1. Epithelial vacuoles caused by hypoxia
2. Discussed under “Hydrogel Extended Wear”

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:

• Affected corneas swell more rapidly
• Deswelling occurs more slowly
• There is less reserve to stress/surgery
• There is no significant reversal

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)