Excerpts
From: The International Glaucoma
Review,
Volume 11-2, Literature, April-June, 2009
Familial incidence of angle closure
Thirty percent of relatives of primary angle closure patients
had clinically significant angle closure. This study was
conducted in white British families. It offers a unique opportunity
to study the mechanism (anatomical features and genetic basis)
of angle closure in European people compared to East Asians
(S. Low et al.)
M & T:
This is another sound reason to recommend eye examinations
to genetically-linked family members – in addition to POAG.
Do IOP fluctuations matter?
From the eight reviewed papers [concerning IOP fluctuation/variation],
three concluded that IOP fluctuation was an independent predictor
of progression, and another three that it was not. The remaining
two papers presented mixed evidence.
M & T:
If a rational consensus on a specific topic cannot be reached,
it seems reasonable to conclude that this specific parameter
is not a key determinant in the risk assessment and decision-making
process.
Determining [glaucomatous] disease progression with GDx-VCC
Retinal nerve fiber layer (RNFL) imaging with scanning laser
polarimetry has been shown to be clinically useful for the
detection of RNFL damage in glaucoma, although its role in
monitoring progression has not been well-established.
With the availability of the GDx GPA software, RNFL progression
can now be analyzed using trend-based or event-based approaches
for individual patients. Measuring RNFL thickness with digital
instruments is likely to become a new paradigm for the objective
assessment of glaucoma progression.
Time-domain versus spectral-domain OCT
Spectral-domain (SD) optical coherence tomography (OCT) was
recently introduced, replacing the time-domain (TD) variety.
SD OCT offers distinct advantages, such as better scan resolutions
due to its quicker speed with up to nine different manufacturers
building devices, compared to one for TD-OCT. Unfortunately,
none of the new instruments are backwards compatible with
the old one. [Thus], one needs to baseline each person when
switching to a new instrument. Measurements taken on one
instrument cannot be used or compared to those taken from
a different instrument.
Blood flow: vascular imaging with 1 micron OCT
Povazay et al. explore the use of one micron optical coherence
tomography (OCT) for retinal, choroidal and vascular imaging.
A caveat to the clinician is that the technology used on
Povazay et al.’s work is complex and expensive; however,
it is likely that costs will fall and complexity will decrease
with potential future commercialization of one micron light
source OCT systems.
M & T:
It is amazing what technological advances we will one day have
in our offices!
Diurnal fluctuation of blood flow parameters
The authors report that most of the outcome variables showed
significantly larger fluctuations in patients with POAG (primary
open-angle glaucoma) compared to healthy controls. These
changes were not associated with OPP (ocular perfusion pressure)
or IOP (intraocular pressure). Changes over time correlated
among the different ocular hemodynamic outcome measures in
patients with POAG, but not in the control subjects. These
findings contribute to the growing body of evidence that
vascular dysregulation and fluctuation of ocular blood flow
occur in glaucoma patients.
M & T:
We look forward to the day when “blood flow analysis” is another
valid parameter we can use in the glaucoma assessment.
Risk factors: long axial length
An association between myopia and the susceptibility of the
optic nerve head (ONH) to developing a glaucomatous optic
neuropathy at all levels of intraocular pressure (IOP) is
emerging, but is not present in all studies. If the myopic
eye is more susceptible to normal and elevated levels of
IOP, what is the actual causal factor? This study is of interest
because in the patient population it establishes axial length
(not refractive error) as a risk factor, which only one previous
study has done. It is of interest that, while axial length
in NTG (normal tension glaucoma) and POAG patients was greater
than in normal eyes, there was no difference in axial length
between these two groups. This study supports the concept
that axial length separately influences ONH susceptibility
in glaucoma. The causal factor(s) underlying that influence
- i.e., large scleral shell, thin sclera, thin lamina, steeper
translaminar pressure gradient, large scleral canal opening,
abnormal connective tissue material properties, or abnormal
scleral and lamina cribrosa astrocyte molecular biology -
remains to be determined.
Glaucoma medication in pediatric population
Timolol is the first choice in pediatric glaucoma provided
there are no cardio-respiratory problems. Topical carbonic
anhydrase inhibitors are useful as second-line drugs or when
a beta blocker is contraindicated. The alpha-adrenergic agonists
(brimonidine) have potential serious adverse effects in children
and therefore, are contraindicated in children younger than
six years of age or weighing less than 20kg. Prostaglandin
analogs are intriguing drugs to be considered in treatment
of pediatric glaucoma. However, long-term studies are required
to establish the safety and efficacy of the use of prostaglandin
analogs in children. The potential side effects of topical
anti-glaucoma medication may be greater in children because
of small blood volume and immature metabolic enzyme systems
in neonates and young children. Therefore, punctal occlusion
after topical anti-glaucoma drops administration must be
explained to the caregivers and should be practiced routinely
in order to reduce the systemic absorption and side effects.
Failure to adhere to drug taking regimens
Fourteen subjects participated in two focus groups and the
findings from these were used to guide ten patient interviews.
- Most patients felt they had received too little education
about glaucoma and that this was typically done using handouts.
Physicians appeared too busy.
- Patients who frequently ‘forgot’ drops appeared less concerned
about disease worsening than those who stated they rarely
forgot.
In the non-judgmental environment created in these focus
groups and interviews, many patients admitted to suboptimal
adherence. The literature is consistent that a significant
minority does not take medications as prescribed. Patients
require more attention at the beginning and often do not
feel adequately educated about the disease or about how
to cope with taking medications. Greater emphasis is clearly
needed here. Finally, the same barriers seem to come up
using multiple methods; poor physician-patient communication,
lack of instruction, a perception that not taking drops
is safe, and difficulty taking drops when traveling or
altering one’s schedule, are all important causes of failure
to adhere to drug-taking regimens.
M & T:
Most certainly, doctors seeing 20-some patients each day
can (if they choose) do a much better job of educating their
patients than doctors seeing 30, 40, or 50 patients each
day. Do the math!
Ocular Blood Flow In Glaucoma
World Glaucoma Association: Consensus Series 6
Anatomy and Physiology (of Ocular Blood Flow)
- Blood supply to the prelaminar and laminar portion of the
optic nerve head comes from branches of the short posterior
ciliary arteries.
Comment: These often form an incomplete vascular ring around
the optic nerve head (‘Vascular ring of Zinn and Haller’),
before giving off branches into the tissue of the optic nerve
head located inside of the peripapillary scleral ring of
Elschnig. These vessels feature an anastomotic blood supply.
Clinical Measurement of Ocular Blood Flow
- At the present time, there is no single method for measuring
all aspects of ocular blood flow and its regulation in glaucoma.
Clinical Relevance of Ocular Blood Flow Measurements
- Lower ocular perfusion pressure (OPP = blood pressure – intraocular
pressure) is a risk factor for primary open-angle glaucoma.
Should Measurements of Ocular Blood Flow be Implemented into
Clinical Practice?
- Although there is an association between measurements of
ocular blood flow and glaucomatous progression, no casual relationship
has been established.
M & T:
As stated earlier, we believe the field of blood flow analysis
is evolving toward one day becoming another parameter which
we will use to further enhance our ability to more comprehensively
evaluate our glaucoma and glaucoma-suspect patients.
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