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Understanding Ocular Hypertension

Accreditation Number: ODO 002/001/06/2019

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INTRODUCTION

Ocular hypertension (OHTN) refers to raised intra-ocular pressure (IOP) in the presence of normal optic nerve head (ONH) appearance and with no visual acuity (VA) or visual field (VF) loss. Nevertheless, the patient is monitored closely as a glaucoma suspect because high IOP is a risk factor for glaucoma.  The intention of monitoring OHTN patients is to consider initiating treatment for individuals who are at risk for developing primary open angle glaucoma (POAG).

WHAT IS THE CUT-OFF PRESSURE THAT DEFINES OHTN?

An IOP measurement of ≥21 mmHg signifies OHTN. There is no conclusive difference between men and women regarding this IOP threshold.

THE RELATIONSHIP BETWEEN CENTRAL CORNEAL THICKNESS AND IOP

The cornea is a viscoelastic material that has the ability to bend, flex, and absorb stress from applied pressure depending on its rigidity and biomechanical properties. Corneal hysteresis (CH) may be defined as the biomechanical property of the cornea when a force is exerted on it and how it exerts a corresponding reactive force. Studies show that increased CH (>9mmHg) allows the cornea to absorb intraocular stresses caused by diurnal IOP fluctuations. When CH is >9mmHg, the mechanical stress is reduced at the level of the lamina cribrosa and the optic nerve, and less glaucomatous neuropathy is seen in these patients. In contrast, when CH is <9mmHg, there is an increased risk for developing glaucomatous optic neuropathy. Factors that contribute to low CH (<9mmHg) include thin central corneal thickness (CCT), age, elevated IOP, and increased HbA1C. Also, studies reveal that low CH is common in eyes with glaucoma regardless of the glaucoma subtype. Research has found that CCT of different ethnicities can be variable, with people of African descent having the thinnest CCT on average, followed by those of Japanese descent, while Caucasians, Hispanics, Chinese and Filipinos having approximately equal CCT.  Often, a thick CCT leads to artificially high IOP measurements, and a thin CCT leads to artificially low IOP measurements.

WHAT DID WE LEARN FROM OCULAR HYPERTENSION TREATMENT STUDY?

The Ocular Hypertension Treatment Study (OHTS) was formulated to answer two important questions:

(1) Does reducing IOP in ocular hypertensive eyes reduce their risk of developing POAG?

(2) Are there identifiable features in ocular hypertensives that predict who is at an increased risk of developing POAG?

 Important findings of OHTS were:

  • Patients with OHTN develop POAG at a rate of about 1% per year.
  • Baseline age, IOP, CCT, vertical cup-to-disc ratio, and pattern standard deviation assists in stratifying the level of risk of OHTN patients.
  • Glaucoma treatment produces approximately a 50% decrease in the incidence of glaucoma.
  • Topical glaucoma medications are safe and effective in reducing the incidence of glaucoma in ocular hypertensive patients. Although the 50% reduction in the incidence of glaucoma occurs across the risk spectrum of ocular hypertensive patients, the absolute reduction is most significant in high-risk patients. This means that high-risk patients should be observed more carefully and may benefit from early treatment.
  • Based on thinner baseline CCT and larger baseline cup-to-disc ratio, black patients as a group are at higher risk for developing POAG.
  • There is a strong correlation between CCT in ocular hypertensive patients and conversion to open-angle glaucoma. Patients with thin CCT (<558µm) are three times more likely to convert to glaucoma than those with thick CCT (≥588µm). Therefore, patients with CCT <558µm are at the greatest risk of conversion and should be monitored closely.
  • The decision to treat ocular hypertensive patients should be based on their risk of developing POAG, taking into consideration the patient’s age, health status, life expectancy, and practitioner’s clinical judgement.

RISK FACTORS OF OHTN?

There are several risk factors of OHTN including age greater than 40 years, race with black patients affected the more, family history of glaucoma or OHTN, high myopia, patients on chronic steroid medications, diabetes mellitus, hypertension, pigment dispersion syndrome, pseudoexfoliation syndrome, previous eye injuries, and eye surgeries.

MANAGEMENT OF OHTN

A corneal pachymeter is a useful clinical tool in the management of OHTN.  Central corneal thickness measurement is necessary to adjust the IOP to achieve a more accurate IOP reading. Furthermore, CCT values are also required for monitoring the risk of progression during follow-up treatments. The correction factor table, such as that shown below, is an important resource.

Generally, OHTN management is based on the use of risk calculators. Risk calculators are predictive models that use factors that have been identified for the development or progression of the disease. Risk calculators provide an objective tool for the eye care practitioner in assessing the patient’s risk for developing OHTN. The patient’s age, baseline IOP, age, VF pattern standard deviation and CCT, vertical C/D ratio, and diabetes mellitus status are entered in a risk calculator. This determines the OHTN patient’s risk estimate percentage of susceptibility to developing glaucoma in five years. Once treatment for OHTN is indicated, the regimen followed is similar to that used for POAG management. 

Table showing the correction factor for IOP based on CCT measurements provided with Pachmate pachymeter

CCT(µm) IOP Adjustment(mmHg)
445 +7
455 +6
465 +6
475 +5
485 +4
495 +4
505 +3
515 +2
525 +1
535 +1
545 0
555 -1
565 -1
575 -2
585 -3
595 -4
605 -4
615 -5
625 -6
635 -6
645 -7

CLINICAL PEARLS

  • In essence, OHTN is raised IOP 21mm Hg, but no evidence of progressive optic nerve damage or visual field defects indicative of glaucoma is present.
  • Risk calculator Apps assist with OHTN management. However, context and clinical experience must guide the eye care practitioner’s decisions.
  • Intraocular pressure slowly rises with increasing age in OHTN.
  • Corneal thickness influences IOP measurement.
  • Women could be at a higher risk of ocular hypertension after menopause.

CONCLUSION

Glaucoma is a serious public health concern, owing to the: increasing prevalence, asymptomatic nature of the disease, and association with many variables for the risk of blindness. Eye care practitioners are concerned with elevated intraocular pressure in patients with ocular hypertension because it is one of the main risk factors for glaucoma.

REFERENCES

  1. Fechtner RD, Lama PJ. What Can We Learn From OHTS and EMGT? What Can We Learn From OHTS and EMGT? Glaucoma Today. January 2003.
  2. Kass MA. The Ocular Hypertension Treatment Study. Glaucoma Today, Summer 2011.
  3. Kass MA, Heuer DK, Higginbotham EJ, Johnson CA, Keltner JL, Miller JP, Parrish RK II, Wilson MR, Gordon MO. The Ocular Hypertension Treatment Study: a randomized trial determines that topical ocular hypotensive medication delays or prevents the onset of primary open-angle glaucoma. Arch Ophthalmol. 2002 Jun;120(6):701-13;829-30.
  4. Mansberger SL, Cioffi GA. The probability of glaucoma from ocular hypertension determined by ophthalmologists in comparison to a risk calculator. J Glaucoma.2006;15(5):426–31.
  5. Gordon MO, Beiser JA, Brandt JD, et al. The Ocular Hypertension Treatment Study: baseline factors that predict the onset of primary open-angle glaucoma. Arch Ophthalmol. 2002;120:714-720.
  6. Mansberger SL, Medeiros F, Gordon M. Diagnostic Tools for Calculation of Glaucoma Risk. Surv Ophthalmol. 2008 Nov; 53(SUPPL1): S11–S16.

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