6 - ocular risk factors and biomechanics

What is the normal IOP range?

10-21 mmHg

what is the source of elevated IOP in POAG and ocular HTN?

diminished TM outflow

What is the relationship between glaucoma and IOP?

1. As IOP increases → the number of eyes with POAG increases

2. If IOP is >22 → 1.5x more likely to go blind

3. As IOP increases → the proportion of patients blind from the disease increases

Why is IOP highest during sleep?

1. supine position

2. increased corneal edema

3. peaks and troughs in AH production with circadian rhythm

clinically, when is the highest IOP measured?

in the morning

What are factors of IOP fluctuations?

1. diurnal fluctuations

2. IOP pulses with respiration and heartbeat

3. blink, eye movements, eye rubbing, smartphone use

How does respiration and heartbeat affect IOP?

ocular pulse amplitude → IOP fluctuations range from 0.9-7.2mmHg

patients with high IOP variability are more likely to?

lose VF stability and RNFL thickness compared to patients with stable IOPs

How can optometrists capture the most accurate IOP/Tmax for our glaucoma patients?

measure multiple IOPs at various times of day:

-alternate morning + afternoon appointments

-obtain at least one IOP within 1-2 hours of waking up

-serial tonometry

What is serial tonometry?

taking multiple IOP readings (at least 3x) on same patient with same tonometer on given day

what is the relationship between IOP and rate of progression?

*for every 1mmHg lowering of IOP → there is a 10% decrease in the rate of progression*

What is the gold standard IOP measurement?

Goldmann tonometry → form of applanation tonometry

What are the downsides to goldmann tonometry?

assumes a 520nm CCT

-if pt has thin cornea → IOP will be underestimated

-if pt has thick cornea → IOP will be overestimated

large inter-observer variability ±1.5mmHg

repeated measurements decrease IOP d/t mechanical force of enhancing outflow

What causes an overestimation of goldmann tonometry IOP?

1. thick mires

2. pressure on globe → examiner, blepharospasm

3. valsava

4. superior gaze > 15º

5. >3D ATR

6. thick cornea

What causes an underestimation of goldmann tonometry IOP?

1. thin mires

2. repeat measurements

3. >3D WTR

4. thin cornea

What is average CCT?

545 microns

How does race affect CCT?

Black patients on average have thinner CCT (529 microns)

What factors influence CCT?

1. race

2. older age

3. RE (more myopic → thinner)

should we correct Goldmann IOP values based on CCT?

NO → while trend is true, scale is not linear and correcting for CCT did not improve accuracy for predicting glaucoma

What was the purpose of the OHTS study?

determine the percentage and the risk factors associated with patient having high IOP to convert to glaucoma in 5 years

What was the study population of the OHTS study?

patients with IOPs of 24-32 with no glaucomatous damage

What were the results of the OHTS study in terms of progression?

9.5% of observed patients progressed to POAG

4.5% of treated patients progressed to POAG

therefore ~20 'at risk patients' must be treated to prevent 1 at risk patient from developing glaucoma in 5 years

what were the risk factors for POAG according to OHTS?

IOP > 25.75

CCT ≤ 555

What were the results of the OHTS study in terms of CCT?

those who developed POAG has an average CCT of 553, those who didn't had an average CCT of 574

patients with thin corneas (<555microns) were 3x more likely to develop glaucoma than those with thick corneas (>588 microns)

according to OHTS, what CCT is a major risk?

CCT <530 is a major risk

What were the results of the 20 year OHTS follow up?

time adjusted cumulative incidence of POAG is 45.6% → not a big difference between treated (42%) and not treated (49%)

black people had highest incidence (55%)

incidence of VF loss was 25% across ALL

What did the early manifest glaucoma trial tell us about CCT?

CCT predicted the progression of VF loss in patients with early glaucoma

this association was strongest in patients with high IOP

What is the correlation between CCT and lamina cribrosa?

there is NONE

what kind of material is the cornea?

elastic and viscous → regains original form but does not regain its original shape

what is corneal hysteresis?

corneas ability to dampen or dissipate external forces applied to it

what does the Ocular Response Analyzer (ORA) measure?

1. IOPg

2. IOPcc

3. corneal hysteresis

what does a higher value (mmHg) of corneal hysteresis on ORA indicate?

the better the cornea is at dampening energy

what structures contribute to the dampening affect?

sclera + cornea → whole-eye measurement

what is IOPg?

IOP value that approximates GAT

what is IOPcc?

derived from both IOP and biomechanical factors

less influenced by corneal properties than Goldmann

what is the average corneal hysteresis (CH) value?

~10 mmHg

will corneal hysteresis be a constant value?

NO → dynamic value that will change over years

how does corneal hysteresis change depending on race?

black patients have lower values than white patients

what is the relationship between corneal hysteresis and CCT?

positively associated → ↑ CCT → ↑ CH

what is the relationship between corneal hysteresis and age?

negatively associated → ↑ age → ↓ CH

what is the relationship between corneal hysteresis and IOP?

negatively associated → ↑ IOP → ↓ CH

what does a corneal hysteresis value of <9 indicate?

risk factor for conversion/progression

what does a corneal hysteresis value of 9-9.5 indicate?

suspicious for conversion/progression

what does a corneal hysteresis value of 10 indicate?

normal/avg value

what does a corneal hysteresis value of >11 indicate?

potentially protective against conversion/progression

how does corneal hysteresis and POAG correlate?

CH values in pts who developed POAG were significantly lower than those who did not

no difference in CCT or IOP btwn those who converted to POAG and those who did not

each 1 mmHg lower CH was associated with what percentage risk of developing glaucoma?

a 21% increased risk of developing glaucoma

how does corneal hysteresis compare in glaucoma vs control subjects?

CH is significantly lower in glaucoma subjects → 8.5 vs 9.5 mmHg

what was found in asymmetric glaucoma?

in worse eye → low mean CH + high IOPcc

how does corneal hysteresis and NFL thickness correlate?

low CH is associated with fast rate of NFL thinning over 4 years

CCT is not associated with NFL thinning

each 1 mmHg decrease in corneal hysteresis is associated with how much NFL thinning?

loss of 0.13 microns/year

how does corneal hysteresis correlate with VF loss?

each 1 mmHg lower corneal hysteresis is associated with a 0.25%/year faster rate of VFI loss over time

this effect is larger for pts with higher baseline IOP (>30 mmHg, 0.89%/year faster) than lower baseline IOP (0.11%/year faster)

how does CH vs CCT affect VF loss?

CH explains 17.4% of VF loss

CCT explains only 5.2%

what can increase corneal hysteresis?

1. filtering surgery → trabeculectomy

2. laser surgery → SLT

3. topical PGA therapy

how does corneal hysteresis predict IOP response to prostaglandins?

pts with low baseline hysteresis tend to have a larger response to PGAs than pts with high baseline hysteresis

why is corneal hysteresis important?

lamina cribosa and peripapillary sclera in eyes with high CH may better compensate for elevated IOP by dampening the force exerted on RGC axons

how does a Tonopen measure IOP?

based on the force necessary to displace a plunger on the tip of the instrument

what are the benefits of Tonopen? disadvantanges?

advantage → portable, can be used in supine position

disadvantage → less accurate than Goldmann, esp outside physiological range

what is iCare? how does it measure IOP?

rebound tonometer, no air puff

small probe makes momentary corneal contact → analysis of probe rebound off the cornea leads to IOP calculation

what are the advantages of iCare?

1. no anesthetic required

2. can be used at home