Rate of Progression – variable

Studies have shown that glaucoma damage progresses for most patients, even if their intraocular pressure (IOP) levels are within the normal range. The rates are highly variable among patients, even in those with similar IOP levels.

Rate of progression: -0.9+ / -0.2% / year (95% confidence)
Slope significant at P < 0.1%
Rate of progression: -2.2+ / -0.7% / year (95% confidence)
Slope significant at P < 0.1%
Rate of progression: -5.0+ / -0.7% / year (95% confidence)
Slope significant at P < 0.1%
First chosen baseline test not used in order
to correct for marked learning effects.

Rate of progression (RoP) – a new standard

Assessing the RoP is a new standard recommended in several guidelines including those of the European Glaucoma Society (EGS).

The EGS guidelines recommend:

  • Routine assessment of the RoP in glaucoma patients
  • Three field tests per year to be performed in the first two years after a glaucoma diagnosis to rapidly achieve this purpose
Ref: Terminology and Guidelines for Glaucoma, 4th edition, 2014

Initial Target IOP

As it is not possible to predict the RoP at the time of diagnosis, the initial Target IOP is set using:

  • Level of damage
  • Life expectancy
  • Untreated IOP levels
  • Additional risk factors, e.g. exfoliation syndrome
* Consider central corneal thickness

A new Target IOP

After two to three years, enough visual field tests (minimum of five) should have been performed to calculate an initial estimate for the RoP. The EGS recommends performing three tests per year in the first two years after diagnosis. This helps to define the new Target IOP within a reasonable time after diagnosis.

* Consider central corneal thickness

SSY Engine

SSY Engine is intended as an aid to help determine a new Target IOP when modifying therapy to alter the observed RoP.

Rate of Progression: -5.0+ / -0.7% / year (95% confidence)
Slope significant at P < 0.1%
First chosen baseline test not used in order
to correct for marked learning effects.

Reproduced with permission from A.Heijl, V.M.Patella, B.Bengtsson, “Effective Perimetry”, Carl Zeiss Meditec Germany, Jena, 2012

Another less common application may be to calculate the effects of a slightly higher Target IOP in patients in whom treatment must be reduced, e.g., due to drug-induced side effects.

Intraocular pressure (IOP) and Rate of Progression (RoP)

The relationship between IOP and risk of glaucoma progression is found in several prospective treatment trials. The data (table) shows that even a modest lowering of IOP can lead to a considerable risk reduction, leading to an increase in time to progression and a reduction in the RoP.

Risk reduction is often calculated using Cox proportional hazard models which assume that the risk quotient between the two study groups (e.g. treated and controls) is constant over time. This was the case in Early Manifest Glaucoma Trial (EMGT).¹ Time to progression is directly related to perimetric slope (RoP), i.e. reducing slope by 50% will double the time to progression.

If the absolute risk is constant over time, the percentage of risk reduction equals to the percentage decrease in RoP. If the absolute risk increases with time, the slope reduction will be smaller than risk reduction. EMGT data indicated that risk reduction is reasonably constant at least during the first 12 years in the study post-diagnosis. This is in line with observations that progressions in manifest glaucoma often are linear.² A direct comparison of slopes showed that RoP was reduced by approximately 40% when risk was reduced by 50%.³ It is therefore advised to reduce the risk numbers somewhat when translating to slope reductions.

SSY Engine expects that the measured RoP will change by 12% for every mmHg in IOP change. This percentage seems realistic at many IOP levels that are common in treated glaucoma patients, for example 15-20 mmHg. These percentages represent means from many patients. Another important caveat is that it is reasonable to assume an interaction between IOP and IOP reduction; each mmHg reduction at lower IOP levels may reduce risk more than each mmHg reduction at higher IOP levels. Currently data from landmark trials have not confirmed this, but it is possible that future analyses may elucidate this point.

Observations supporting this notion include that the percentage given for the Canadian Glaucoma Study is higher than that of the EMGT, and that the Canadian Glaucoma Study had lower baseline IOP levels than EMGT. Likewise, the percentages are slightly lower for the Ocular Hypertension Treatment Study (OHTS) and the European Glaucoma Prevention Study (EGPS) – studies of patients with ocular hypertension and higher baseline IOP levels.4

That 12% per mmHg cannot be true for high IOP levels is illustrated by an example: reducing IOP from 30 to 22 mmHg cannot be expected to stop glaucoma progression almost entirely. Nevertheless, a 12% RoP change is reasonable at the pressure levels that are common in treated glaucoma patients.

This first version of SSY Engine has been based on the best identified current knowledge, while keeping the model for calculating Target IOP simple and comprehensible. With increasing knowledge in the future, e.g. on the interaction between IOP and IOP reduction, SSY Engine should be modified accordingly.


¹ Leske et al. Arch Ophthalmol 2003; 121(1): 48-56.

² Mikelberg et al. Am J Ophthalmol 1986:101:1-6.; Kwon et al. Am J Ophthalmol 2001; 132:47-56.; Pereira et al. Ophthalmology 2002; 109:2232-2240.; Bengtsson et al. Arch Ophthalmol 2009; 127:1610-1615.; Nouri-Mahdavi et al. Investigative Ophthalmol Vis Sci 2004; 45:4346-4351.; McNaught et al. Graefes Arch. Clin Exp Ophthalmol 1995; 233:750-755.; Azarbod et al. Investigative Ophthalmol Vis Sci 2012; 53:5403-5409.

³ Heijl et al. Arch Ophthalmol 2002; 120: 1268-1279.

4 Chauhan et al. Arch Ophthalmol 2008; 126: 1030-1036.; Leske et al. Arch Ophthalmol 2003; 121(1): 48-56.; Gordon et al. Arch Ophthalmol 2002; 120: 714-720.; Miglior et al. Ophthalmology 2007; 114: 3-9.

* Increased risk per mmHg of higher follow-up IOP
** Increased risk for every mmHg higher baseline IOP
† Decreased risk for every mmHg higher mean IOP decrease from baseline
Leske et al. Arch Ophthalmol 2003;121:48-56. Gordon et al. Arch Ophthalmol 2002;120:714-20.
Miglior et al. Arch Ophthalmol 2007;114:3-9. Chauhan et al. Arch Ophthalmol 2008;126:1030-6.

Calculating the new Target IOP

The clinician enters information regarding the:

  • age of the patient at the time of the first field test (baseline field)*
  • patient’s current age
  • current VFI value in the Humphrey perimeter (or the MD value in the Octopus perimeter)*
  • mean IOP during the follow-up

* For patients with visual field defects at the time of the first field test, the baseline is the first or the first two visual field tests (for Octopus, usually one visual field test is used as baseline).

For patients that initially had normal visual field tests but later developed visual field loss, the baseline is the latest normal and the first pathological visual field test (for Octopus, the latest normal visual field test is used as baseline).

The age/function diagram will display a solid trend line showing the current RoP and the estimated development as an extrapolated dotted portion of the same line. A grey dot will appear at the end of the line. This dot can be repositioned to simulate an altered RoP, and a new Target IOP will be calculated..

The result of the simulation is based on the assumptions described above which are averages from many patients. The result can provide an appreciation of the magnitude of necessary change in IOP to achieve the desired RoP.

For a more detailed description – see User Guide

Simulation view – Humphrey

Simulation view – Octopus