Learning to Interpret My Heidelberg Retina Tomograph (HRT-II) Results

My doctors are responsible for the real intrepretation of my Heidelberg Retina Tomograph (HRT-II) results. However, being a curious person, I like to understand the results myself. This blog post is my attempt to share what little I know about HRT results so far.

I have made my HRT-II results from 2006 and 2007 available online:

• My HRT-II results from 2006
• My HRT-II results from 2007

As I explained in the blog post about my HRT-II results from 2007, the contour line is used to obtain the stereometric parameters (such as disk area, cup area, etc.). I reported my stereometric parameters in the blog post I just referenced. (I'll also repeat them below.)

Here is what I have learned so far. The most important paramters are the disk, cup and rim areas, the cup and rim volumes, the maximum cup depth, the cup shape measure, and the mean RNFL thickness.

In general, the instrument manufacturer says the values measured by the HRT-II can vary about 5%. I am currently interpreting this to mean the reported values (shown on the test results) can vary 5% in either direction due to "errors" in measurement independent of any actual changes in the structure of the eye.

It is possible to consider individual parameter values. However, a more sophisticated analysis would consider multiple parameters at the same time.

When looking at individual parameter values, rim and cup volume, cup shape and retinal surface height variation along contour line (Height Variation Contour parameter) all change significantly with deteriorating visual field. So looking at these individual parameter values can be helpful. They can be very helpful in following the progression of glaucoma.

You want less cup volume and less cup depth in general. Increasing cup depth and/or increasing cup volume indicate increasing (glaucoma-related) damage to the optic nerve head. Also a wide, flat bottom cup shape is indicative of damage. A normal cup bottom slopes more gradually on the temporal side. Here is a comparison showing a normal cup shape on the left and a glaucomatous optic nerve head on the right. (Click the link for a full size image.)

When evaluating multiple parameters together, researchers have found that the combination of the three parameters (1) cup shape measure, (2) rim volume, and (3) height variation contour were the most important group of parameters for differentiating a normal disc from a glaucomatous disc. The importance of the individual parameters is in the order given. A discriminant function using these parameters was published by Mikelberg and coworkers in Ophthalmology 1997; 104: 545:548.

There is a second analysis approach that also uses multiple parameters. It is called ranked sector distribution (RSD) curves. Apparently my doctor doesn't use this. I'll ask more about it in the future.

Applying some of this information to my own HRT results for my right eye, as an example, we can notice several things. Below are my parameter values:

 

HRT	Left			Right
	2006	2007	Change	2006	2007	Change
Disk Area	2.011	2.032		1.763	1.755
Cup Area	0.876	0.905	3.3%	1.131	1.243	9.9%
Rim Area	1.135	1.127	-0.7%	0.632	0.512	-19.0%
Cup Volume	0.208	0.217	4.3%	0.371	0.444	19.7%
Rim Volume	0.212	0.233	9.9%	0.123	0.088	-28.5%
Cup/Disk Area Ratio	0.435	0.445	2.3%	0.641	0.708	10.5%
Linear Cup/Disk Ratio	0.66	0.667	1.1%	0.801	0.842	5.1%
Mean Cup Depth	0.261	0.287	10.0%	0.426	0.441	3.5%
Maximum Cup Depth	0.707	0.71	0.4%	0.801	0.813	1.5%
Cup Shape Measure	-0.213	-0.162	-23.9%	0.025	0.043	72.0%
Height Variation Contour	0.287	0.324	12.9%	0.332	0.3	-9.6%
Mean RNFL Thickness	0.19	0.187	-1.6%	0.221	0.169	-23.5%
RNFL Cross Sectional Area	0.959	0.947	-1.3%	1.041	0.792	-23.9%
Reference Height	0.333	0.412	23.7%	0.48	0.47	-2.1%
Topography Std. Dev.	13	10	-23.1%	11	19	72.7%

Comments About Individual Parameters: 

My Cup Area has increased almost 10%. It is not good to see an increasing cup area. Futhermore, the 10% change is greater than the 5% variability that is considered the margin of error.

Rim Area has decreased 19%. Again is is not good to see rim area decreasing.

Cup Volume has increased almost 20%. Increasing cup volume is also indicative of glaucomatous damage.

My Rim Volume has decreased a whopping 28.5%.

My Maximum Cup Depth has only increased 1.5%, which is within the margin of error.

My Cup Shape Measure has changed by 72%. Notice that my HRT results from 2006 show more of a gradual slope on the temporal side of my cup (in my right eye).

My Height Variation Contour has changed about 10%.

My Mean RNFL Thickness has decreased 24%.

My left eye parameter values are quite different. Cup area, rim area, cup volume, rim volume and mean RNFL thickness don't seem to show any significant changes. However, mean cup depth did increase 10%, cup shape measure changed 24% and height variation contour changed 13%.