A study recently published in Ophthalmic and Physiological Optics assessed the potential impact of scleral lenses (SLs) on intraocular pressure (IOP) in patients with keratoconus (KC).
Give me some background.
Previous studies have suggested that the compressive effect of the SL on the globe and its support on the sclera may affect IOP—specifically by altering aqueous outflow through the episcleral veins.
KC appears to induce changes in the optic nerve and the distribution of ganglion cells that could modify the capacity of the optic nerve and neural fibers to accommodate significant variations in IOP.
Meaning…
Due to potential modifications of the optic nerve structure seen at the early stages of KC, this patient population may be more prone to damage if placed under inappropriate pressure (ex., with long-term SL wear).
Further: As KC is a form of collagen disease, the lamina cribrosa at the optic nerve head may have a structural weakness.
- As such: This makes it more susceptible to deformation and to the development of glaucomatous neuropathy.
Now talk about the study.
The research team included 18 KC patients (mean age: 35.1 years; 60% female) who were successful SL wearers for at least 3 months.
- Note: They only evaluated the results from one eye of each patient.
Plus: During the first session, corneal biomechanics were assessed using an air tonometer and Scheimpflug technology.
Then: A scan of the optic nerve was carried out using optical coherence tomography (OCT) at 2-hour intervals for 6 hours total.
- Their focus: Identifying the Bruch’s membrane opening-minimum rim width (BMO-MRW), which represents the smallest distance between the BMO and the internal limiting membrane.
These tests were repeated, respecting the time at which the initial measurements were taken.
Tell me more about BMO-MRW.
Previous research has shown that an increase in BMO-MRW is significantly correlated with a decrease in IOP.
- Meaning: Its value decreases as IOP increases and vice versa.
Due to its inverse relationship with IOP, BMO-MRW has recently been introduced as a new parameter for assessing the optic nerve in glaucoma screening and follow-up.
What about the SL parameters?
The SLs worn were, on average, designed with:
- A sagittal depth of 4,725 μm
- A spherical equivalent power of -3.75 D (20% were front toric)
- The majority (70%) of patients had a 16.0-mm diameter; the remainder had a 17.0-mm diameter
In addition, half of the lenses were designed with bi-elevation and 80% featured toric peripheral curves.
Findings?
A statistically significant decrease of -10.5±3.6 μm (p =.0.02) in BMO-MRW was reported after 6 hours of SL wear, indicating an increase in IOP.
- Conversely: Measurements without lenses were 4.8±3.4 μm (p = 0.18).
The fluctuation was greater in participants with KC than found in a previous study of regular corneas.
Expert opinion?
A healthy eye can typically withstand relatively small variations in IOP. as the optic nerve can autoregulate its blood flow.
- However: This may not be the case for patients who already have glaucomatous abnormalities.
As such: SL wear should be considered with caution in glaucoma patients—and in cases where a SL is the only possible alternative to restore the patient’s vision, it would be advisable to:
- Use lenses allowing tear exchange or designed with fenestrations to reduce compression and sub-atmospheric pressure
- Follow-up with patients closely
Limitations?
These included:
- As this was a pilot study, the sample size was limited, which may have led to a loss of statistical power
- Not all subjects wore SLs for the same length of time (some had worn SLs for several years, while others had been fitted more recently)
- As such, there may have been differences in short- and long-term modifications.
- The BMO-MRW was only measured with a single instrument (Heidelberg Spectralis)
Take home.
These findings suggest that SLs can influence IOP pressure after 6 hours of wear, as BMO-MRW became significantly thinner compared to measurements without lenses.
The study authors’ recommendation: Clinicians should closely monitor optic head changes in KC patients as they may be at risk of lamina cribrosa structural changes with long-term SL wear.
Next steps?
These results should be compared with future longitudinal studies with a larger cohort of patients, added the research team.