New Electroretinogram Parameters Identify Eye Disease

Laird Harrison

November 11, 2016

ANAHEIM, California — Newly developed parameters for electroretinography — a-wave fractional sensitivity and amplitude — could provide more information about the way photoreceptor function changes with disease.

There is a correlation between fractional sensitivity and age, said Kristen Bowles, OD, from the University of Houston College of Optometry.

"Depending on what we find in diseased eyes, it could be quite useful," she told Medscape Medical News.

Clinicians already use the a-wave of dark-adapted electroretinograms to assess rod photoreceptor function.

The original a-wave models were based on recordings of currents around rod outer segments, but the newer model incorporates resistance and capacitance of the outer nuclear layer, Dr Bowles explained here at the American Academy of Optometry 2016.

Previous research suggested that the voltage recovery toward baseline after the a-wave trough for strong stimulation is generated by capacitive currents flowing in and around the outer nuclear layer during rod hyperpolarization (Prog Retin Eye Res. 2014;39:1-22). The investigators proposed expressing sensitivity as fractional sensitivity, defined as the peak a-wave response produced by isomerization of 1 rhodopsin molecule per rod.

But for this measurement to be useful, normal variations and age-related changes must be established.

So Dr Bowles and her colleagues set about determining the normal variation, and changes with age, for both fractional sensitivity and maximum voltage.

They recruited 81 people 15 to 78 years of age, 28 of whom were 20 to 29 years of age.

 
I think there could be many more diseases where it's useful.
 

The researchers recorded dark-adapted electroretinograms using DTL fiber electrodes and an Espion E2 system with ColorDome stimulator (Diagnosys LLC).

They then calculated fractional sensitivity from the 10% to 90% rise time of the a-wave. They determined maximum voltage for an individual by scaling the model curve for the determined fractional sensitivity to the measured a-wave voltages.

The researchers found consistent fractional sensitivity measures between 10 and 100 candela-seconds per meter squared (cd-s/m²).

Linear regression showed that fractional sensitivity decreased about 0.1 unit per decade (r² ≤ 0.45; P < 0.0001), whereas maximum voltage was unchanged with age.

"The linear curve fit the logged data very well," Dr Bowles reported.

The finding that fractional sensitivity continues to decline when implicit time no longer increases provides additional information about photoreceptor function in aging subjects, and potentially in diseased subjects, she explained.

To see if the new parameters add to the clinical utility of electroretinography, the researchers compared a-wave implicit time with fractional sensitivity and a-wave amplitude with maximum amplitude.

They found a direct correlation between the increase in a-wave amplitude and maximum voltage.

The researchers are planning to study the parameter in patients with eye diseases. Dr Bowles said she has found patients with degenerative diseases who have decreased fractional sensitivity but normal clinical rod parameters, which suggests that fractional sensitivity could detect rod dysfunction earlier.

"I think there could be many more diseases where it's useful," said Dr Bowles. "Hopefully, in the next year, I will be able to figure that out."

Generating an a-wave that is specific to rods is very difficult, so information about the new parameters could prove helpful, said session moderator Jeff Rabin, OD, PhD, from the UIW Rosenberg School of Optometry in San Antonio, Texas.

"I think it's great," he added. "We get all kinds of challenging cases from retina specialists."

The study was supported by a grant from the National Eye Institute. Dr Bowles and Dr Rabin have disclosed no relevant financial relationships.

American Academy of Optometry (AAOpt) 2016: Abstract 160050. Presented November 10, 2016.

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