Effects of Selective Laser Trabeculoplasty on Aqueous Humor Dynamics

Describe the background of the study. Include a critical evaluation of existing knowledge,
and specifically identify the information gaps that the project is intended to fill.

Laser trabeculoplasty is currently a well established treatment option for lowering the
intraocular pressure in the management of open angle glaucoma1,2. The procedure involves
placement of laser spots over the trabecular meshwork, possibly through a variety of laser
types. ALT has been the most widely utilized option for laser trabeculoplasty over the past
few decades. Since its availability approximately a decade ago, SLT is currently an
additional option for doing laser trabeculoplasty. 3,4. Current evidence suggests that SLT
achieves its IOP lowering effect without thermal damage to the trabecular meshwork5. The
IOP lowering effect of SLT has been shown to be comparable to ALT over the long term6.

In physiological terms, ALT has been shown to lower the IOP primarily by increasing the
conventional outflow facility7. There are two putative mechanisms by which this may be
facilitated8. Thermal coagulation and shrinkage by ALT may mechanically open up the adjacent
trabecular meshwork and Schlemm's canal. Alternatively the delivery of laser energy may
trigger biological processes and activation of matrix metalloproteinases in the
microenvironment of trabecular meshwork that lead to a remodeling of tissues resulting in
improved outflow facility. ALT uses argon or another thermal laser to photocoagulate
tissues around the trabecular meshwork. The settings vary depending on the tissue response
and pigmentation. Most clinicians use 600-1500mW, 50-100 spots, 50 micron spot size and 0.1
sec duration.

Also, there is limited data on fluorophotometric and tonographic studies on ALT and none on
SLT to the best of our knowledge. All available reference known to the investigators have
been included in the list of references.

The effect of SLT on the aqueous humor dynamics is currently unknown. Given the lack of
demonstrable thermal damage on histopathologic examination5, it is unlikely that the effects
of SLT could be mechanically mediated. There has been some suggestion of a potential
interaction of IOP lowering effect when prostaglandins and SLT are used together9,10. This
suggests that there could be a possible sharing of mechanisms between SLT and
prostaglandins. The IOP lowering effect of SLT may be mediated in part by effects on
uveoscleral pathways in addition to the trabecular outflow pathway. This study will
determine the changes in aqueous humor dynamics produced by SLT that lead to its IOP
lowering effect. SLT is a Q switched frequency doubled Nd:YAG laser. The duration is 3
nanoseconds and spot size 500 microns. Power varies from 0.5-1.5 mJ and total number of
spots can be between 50-over 100. The mechanism of action may be disruption of melanosomes,
but this has not been unequivocally proven yet.

Inclusion Criteria:

- 40 years of age and older

- Eye consideration for SLT (intraocular pressure above target with current
medications, intolerance/allergy to topical medications, patient preference as
primary treatment)

- Open angles on gonioscopy

Exclusion Criteria:

- Any previous surgical or laser procedures

- Secondary glaucoma including pigmentary, exfoliative, uveitic, and traumatic
glaucomas

- Corneal opacities interfering with adequate fluorophotometry

- Any active ocular infection within the past 2 months

- Any corneal pathology increasing the likelihood of corneal abrasions

- Inability to safely washout medications prior to laser treatment

- Allergies to fluorescein, timolol, dorzolamide, or sulfa

- Excessive (3+) trabecular meshwork pigmentation