It is assumed that alterations in aqueous humor outflow account
for increased intraocular pressure in the majority of patients with
uveitis in whom secondary glaucoma develops. Intraocular pressure can
be defined by the equation IOP =
F/
C +
P v, where
F is aqueous
production,
C is outflow facility, and
P v is episcleral venous pressure.
Although aqueous hypersecretion would result in elevated intraocular
pressure if outflow facility were to remain unchanged, inflammation of
the ciliary body typically results in decreased aqueous
production,
14 a fact that has been demonstrated in
experimental models of uveitis.
15 16 When aqueous
hypersecretion is observed in forms of experimental uveitis, it is a
transient phenomenon that gives way to hyposecretion.
16 Also, the existence of aqueous hypersecretion as a mechanism for
secondary glaucoma in clinical situations has been
questioned.
3
There are a variety of potential mechanisms by which outflow facility
might be reduced in patients with uveitis. Mechanical obstruction of
the anterior chamber angle can result from pupillary block with iris
bombe, peripheral anterior synechiae, or forward rotation of the
ciliary body.
3 Other proposed mechanisms include blockage
of or damage to the trabecular meshwork by cells or
protein.
3 4 Whereas pupillary block and peripheral
anterior synechiae can be excluded by careful examination, obstruction
of the trabecular meshwork by cells and small molecules would be
difficult to identify clinically.
Evidence from animal studies has demonstrated that polymorphonuclear
leukocytes can infiltrate the trabecular meshwork and possibly lead to
obstruction.
17 The average pore size of the trabecular
meshwork is 2 μm
18 and a leukocyte is approximately 7 to
9 μm.
19 Thus, although leukocytes can become deformed,
an obstructive phenomenon may occur with a cellular reaction in the
anterior chamber. We found no relationship, however, between outflow
facility and cells, when graded with a semiquantitative system. We
cannot rule out the possibility that a relationship between cells and
outflow facility could be identified with more precise measurements,
but a practical technique for such precise measurements is not
available. Laser photometry cell counts are associated with wide SDs
when specific cells are counted
7 and are not considered to
be reliable for these types of precise comparisons. Nevertheless, our
study suggests that the role of cells in changing outflow facility may
be small or nonexistent, because patients with uveitis and low flare
had normal outflow facility, despite the presence of cells on clinical
examination. It is possible, however, that cells play a role only if
they adhere to and disrupt the trabecular meshwork. Case reports have
described patients with evidence of cellular precipitates on the
trabecular meshwork with elevated intraocular pressure, despite minimal
anterior chamber cellular reaction.
20 Thus, the amount
suspended within the aqueous humor may not be relevant. The
precipitates disappeared and the pressure decreased after treatment
with topical corticosteroids. The effect on outflow facility was not
investigated in these studies, however.
It has also been proposed that trabeculitis, or inflammation of the
tissues that form the trabecular meshwork, can lead to trabecular
dysfunction and obstruction to aqueous humor outflow in some patients
with uveitis. Hogan et al.
21 described an enucleated eye
from one patient with herpetic uveitis and glaucoma in which there were
thick edematous bands of tissue and inflammatory cells within the
trabecular meshwork. Animal studies of herpetic uveitis have
demonstrated inflammatory cells in the trabecular
meshwork.
22
We chose to study elevated aqueous humor protein as an alternative
explanation for reduced aqueous humor outflow. Laser flare photometry
is a noninvasive technique that can be used to quantify the amount of
protein within the aqueous humor.
7 Multiple studies have
demonstrated that protein concentration is linear in relation to laser
flare photometry readings, both in vitro and in vivo.
12 23 Thus, this technique allowed us to make precise comparisons of aqueous
humor protein levels with measurements of outflow facility in patients
with uveitis. Patients enrolled in this study demonstrated a wide range
of laser flare photometry readings, consistent with ranges reported for
other heterogeneous groups of patients with uveitis.
13 24 Our results demonstrate that there is a relationship between aqueous
humor protein concentration and outflow facility in patients with
uveitis. Furthermore, laser flare photometry readings do not seem to be
simply indirect markers of alterations in aqueous humor outflow by
other factors related to uveitis, because outflow facility in patients
with active uveitis and low laser flare photometry readings was similar
to that of control subjects, even though these patients had as many as
2+ cells detected in clinical examination. This observation adds
strength to the hypothesis that an increased protein level may play a
causal role in lower outflow facility.
Although we had insufficient statistical power to identify whether
there were small differences in outflow between normal control subjects
and patients with uveitis and low flare, we had sufficient statistical
power to state that outflow facility was not reduced by more than 24%
of normal levels. In previous studies of outflow facility in patients
with open-angle glaucoma, these levels were approximately 40% lower
than in age-matched control subjects.
10
Epstein et al.
4 support the hypothesis that elevated
aqueous humor protein may lead to changes in outflow facility. They
perfused enucleated eyes with various dilutions of serum and found
decreases in outflow facility. Serum diluted to 1.3 g protein/100
ml resulted in a 36% decrease in aqueous outflow facility. This
protein concentration is within the range observed in patients with
uveitis.
25 26 They also noted that the obstruction of flow
could not be altered by a saline washout and was not proportional to
the viscosity of the perfusate, which led them to believe that the
protein was either deposited on or adsorbed into the trabecular
meshwork. It has been suggested that the scavenging ability of the
endothelial cells may become overwhelmed by various materials, which
leads to disorganization of the trabecular meshwork and obstruction of
outflow.
27 Support for this hypothesis comes from
histologic samples from patients with pigmentary glaucoma, in which
pigment-laden endothelial cells did not maintain normal trabecular
architecture.
28 Perhaps high levels of protein result in
similar changes.
Laser flare photometry does not identify the nature of the protein
changes in aqueous humor. It is believed that an increasing proportion
of higher molecular weight proteins increases flare.
7 Thus, the higher flare readings may indicate the presence of larger
molecular weight proteins that may have a greater effect on outflow.
The relationship between outflow facility and protein may be more
complex than a simple bulk protein effect, however.
McEwen
29 has argued that the presence of proteins in
aqueous humor has no effect on its viscosity as it passes through the
trabecular meshwork. As an alternative mechanism, increased protein
levels may include an outpouring of regulatory proteins that have an
effect on the cytoskeletal regulations of aqueous humor outflow through
the trabecular meshwork.
30
Fautsch et al.
31 have shown that the product of the gene
MYOC, which can be found in the aqueous humor, increases
outflow resistance when infused into anterior segments, using a
perfusion organ-culture technique in human eyes. Other proteins, such
as recombinant β-galactosidase, bovine serum albumin, and fetal calf
serum did not increase outflow resistance in the same system. Also,
heat-denatured MYOC protein did not increase outflow resistance,
suggesting that outflow resistance is related to an active property of
the protein. It is likely that these proteins are not of sufficient
amount in clinical disease to alter photometry measurements themselves,
however. With regard to the MYOC protein, we found no references on a
computerized literature search (Medline, provided in the public domain
by the National Center for Biotechnology Information, Bethesda, MD, and
available at http://www.ncbi.nlm.nih.gov/PubMed/) for its
concentration in aqueous humor of patients with uveitis. Various
regulatory cytokines, however, are found in concentrations ranging from
5 to 500 × 10
−4 μg/100 ml within the
aqueous humor of patients with uveitis.
32 33 In contrast,
the total aqueous humor protein concentration in normal eyes ranges
from 10 to 20 mg/100 ml,
25 and in patients with uveitis,
it can increase to levels as high as 2300 mg/100 ml.
34 In
one study, mean total aqueous humor protein in a group of patients with
chronic uveitis ranged from 62 to 388 mg/100 ml (mean, >166.8 ±
132.8 mg/100 ml). Albumin and immunoglobulins accounted for more than
90% of the proteins present.
23 We also found, by
regression analysis, no evidence that topical corticosteroids had a
direct effect on outflow facility.
The spread of measurement values around the regression line in
Figure 1 may simply reflect the imprecision of photometry or Schiotz tonography
measurements or both. Alternatively, it may be that change in outflow
is a dynamic process occurring over time, as might be seen with protein
regulation of trabecular function. Thus, outflow facility may not
always reflect the aqueous humor protein levels at the same time point.
In support of this concept, we observed one of our patients with
HLA-B27–associated anterior uveitis serially and noted that
improvement in outflow facility lagged behind the declining aqueous
humor protein levels over several visits. We did not, however,
routinely measure changes in laser flare photometry results over time
in patients with active disease, and thus we do not know precisely the
temporal relationship between changes in aqueous humor protein levels
and decreased outflow facility or the duration of outflow changes.
Nevertheless, a relationship between laser flare photometry results and
outflow appears to be present both in patients with recent onset of
uveitis and in those with chronic disease.
Our groups were not matched for gender, but we found no evidence that
gender affected outflow facility. Previous studies have shown no effect
of gender on laser flare photometry results.
24
There was no correlation between outflow facility and intraocular
pressure, which may be explained by use of glaucoma medications, or, in
the absence of such drugs, by alterations in aqueous humor production
or other compensatory mechanisms, such as increased uveoscleral
outflow, that may occur in patients with uveitis. None of the patients
in this study had elevated intraocular pressure or glaucoma, although
six patients were being treated with aqueous humor suppressants.
Whether changes in aqueous humor outflow facility of the magnitude seen
in this study play a role in the pathogenesis of uveitic glaucoma
remains unknown. The regulation of intraocular pressure is
multifactorial, and the interaction of factors in patients with uveitic
glaucoma is undoubtedly complex, but this study suggests that increased
aqueous humor protein levels may play a role in some cases through its
effect on outflow facility. Aqueous humor protein should be included
among the factors investigated in future studies of uveitis and
glaucoma.
The authors thank Kowa Company, Ltd. Electronics and Optics
Division, Tokyo, Japan, for providing the Kowa FM-500 for use at the
Jules Stein Eye Institute during this study.