Glaucoma
What Does Ocular Hypertension Treatment Study Mean
to Practicing Optometric Physicians?
Several major landmark studies in glaucoma cases have been
completed and published over the past several years. Each of
them have contributed to our clinical understanding of glaucoma,
but none has impacted glaucoma care like the Ocular Hypertension
Treatment Study (OHTS).
OHTS has demonstrated what most of us have long suspected;
reducing IOP protects the optic nerve from glaucomatous optic
neuropathy. Yet, the unanswerable question remains: Which patients
merit therapeutic intervention, and which patients are best
served by close observation and annual follow-up?
No study, no instrument, nor expert can definitively answer
this enduring question. The OHTS, however, has revealed a number
of key points that can help the informed clinician refine the
decision-making process.
Before we get into the details of this study, bear in mind
that almost all studies are conducted on a specific cohort
of persons with numerous inclusion and exclusion criteria which
may or may not be relevant to the individual patient in your
examination room. We will share the findings of OHTS as published;
however, we are going to season these results with our clinical
perspectives, and with knowledge gleaned from other studies
and publications.
Let’s first look at the key summary conclusion of the OHTS.
Compared to those in the observation group, the risk of the
treatment group progressing to glaucoma was reduced by 60%.
This is a substantial and significant risk reduction! The contraview
to this data shows that the 40% of the treatment group who
were not treated did not go on to develop glaucoma over the
time period of the OHTS. This puts a special decision-making
burden on the doctor to strategically determine which patient
is best served by observation, and which patient is best served
by therapy
.Distribution of Central Corneal Thicknesses in
OHTS
 Now, let’s dissect the study to obtain as much information
a possible to enhance our decision-making abilities. The key
clinical findings that placed patients at risk were:
- increased vertical C/D ratio greater than 0.5
- IOP above 24mm Hg as measured by Goldmann tonometry
- central corneal thickness below 555µm
While it is common knowledge that large vertical C/D ratios
and increased IOP are risk factors, the importance of central
corneal thickness (CCT) was a major new risk factor revealed
in this study. Indeed, of all the clinical parameters, CCT
was the factor most closely associated with glaucomatous conversion.
For all the doctors owning, or contemplating owning one of
the newer retinal scanning devices, we think you would enhance
your level of clinical care by first acquiring a corneal pachymeter.
We are convinced that a new standard-of-care in glaucoma is
set by OHTS and requires a CCT baseline on all glaucoma and
glaucoma suspect patients.
The concept of CCT needs to be evaluated and understood. It
is important to realize that thicker corneas cause higher than
true IOP readings on applanation tonometry, and conversely,
thinner corneas result in lower than true IOP readings.
In practical perspective, many patients with ocular hypertension
(i.e., normal optic nerves and other pertinent findings) may
be being treated, yet the sole reason for an elevated IOP reading
may be a thicker-than-normal cornea. Quite possibly, when all
of our current ocular hypertensives and all other “glaucoma
suspects” have their CCTs measured (hopefully before the end
of 2003), many may be able to discontinue their IOP-lowering
meds. Conversely, many patients who are suspect for, or have
diagnosed glaucoma, yet have “normal pressures,” may need to
be treated, or treated more aggressively, since their true
IOP is actually higher than the tonometer reads.
In order to gain a meaningful perspective on CCT, its distribution
within the population needs to be known. Different studies
yield different thicknesses, but on balance, normal CCT ranges
from 535µm to 555µm. As would be expected in a population of
ocular hypertensives, the distribution of the bell-shaped curve
is skewed slightly towards greater thickness, as can be seen
in the chart.
A conversion algorithm is not uniformly agreed upon, although
there are general guidelines (see “Perspective on CCT and IOP).
CCT is yet another potentially helpful piece of data. Its greatest
impact and guidance will be from the second and third standard
deviation measurements, since a normal CCT will represent more
of a neutral finding. In other words, a very thin or very thick
cornea can have the greatest clinical impact on our decision-making
process regarding the need to treat, or the aggressiveness
of our therapy. These same considerations can be applied regarding
LASIK alterations of central corneal thickness. (In our search
for the most cost-effective pachymetry, we have found the Sonagage
ultrasonic unit to meet our needs. We have this unit in our
offices, and highly recommend it. You can contact Sonogage
at 1-800-798-1119, or at www.sonogage.com.)
In the five-year, cross-sectional OHTS, the goal of therapy
was to reduce IOP by 20%, or to below 24mm Hg. The enrollment
IOPs had to be between 24 and 34mm Hg. Here is where studies
show their greatest weakness and require a great deal of discernment
in interpreting their results. For example, what if our patient’s
“ocular hypertension” status is considerably longer than five
years? In reality, many, if not most, of our ocular hypertensives/glaucoma
suspects will be followed for 10 or more years.
Also, what would be the ultimate conclusion from OHTS if IOP
were reduced by 30% (instead of 20%)? Would there be an 80%
or 90% reduction in glaucoma risk (versus the 60% risk found)?
As is so common, most studies tend to raise as many questions
as they answer.
Here’s the bottom line: You are still required to look at
all factors and think hard. There is no scanning device or
any other technology that is going to make the glaucoma diagnosis;
it is going to be the well-trained, and well-equipped physician
who will collect as much history and data as possible and then,
in partnership with the patient, formulate a care plan that
represents the greatest overall good. Such a plan should take
into consideration the following factors:
- age
- race
- family history
- general health
- presence of diabetes
- presence of moderate to high myopia
- status of optic nerve head
- central corneal thickness
- static threshold perimetry (SITA by Zeiss-Humphrey)
- patient’s willingness to use eyedrops as prescribed
- ability of the patient to use eyedrops
- effectiveness of eyedrops
- side effects of eyedrops
- cost of eyedrops
This seems like a lot to consider, and it is; this is why
you are called “doctor.” There is no simple answer, and certainly
not via the over-rated, overly promoted, scanning devices (although
they can occasionally provide potentially meaningful supplemental
data).
OHTS brings us back to the basics, and gives us CCT as another
important piece of diagnostic data OHTS further demonstrates
the validity of lowering IOP as the prime focus for protecting
the optic nerve from higher-than-healthy IOP.
As patients in the OHTS continue to be followed and analyzed,
it will be interesting to see what further insights are gleaned
over the next few years.
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