August 01, 1999
3 min read

MERG distinguishes optic neuropathy from retinopathy

The multifocal electroretinogram test takes about 8 minutes per eye.

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IOWA CITY, Iowa — Multifocal electroretinogram (MERG) is extremely useful in distinguishing retinal visual loss from non-retinal visual loss, eliminating the need for neuro-imaging in many cases.

Randy H. Kardon, MD, PhD, spoke about MERG at the North American Neuro-Ophthalmology Society meeting. The technique, which is about a decade old, has been made available for clinical use in the past 2 years.

“It’s been very, very helpful for answering clinical questions such as trying to sort out when a visual field defect is functional or due to optic neuropathy or retinopathy,” Dr. Kardon said.

An abnormal retinal disease results in an abnormal MERG from the photoreceptor or bipolar cell layer, not the ganglion cell layer. An optic neuropathy generates a normal MERG for a first-order response. A retinal problem will generate a matching visual field defect on the multifocal ERG in all cases. At a research level, more sophisticated stimulation and analysis techniques are being developed to also detect optic nerve function in the future.

The test takes about 8 minutes per eye, and gives replicable results. The machine costs $50,000, and was developed by Eric Sutter, PhD, of the Smith-Kettlewell Research Foundation, San Francisco, Calif. Electrodiagnostic Instruments (EDI Inc., Jackson, N.J.) offers a research and clinical version of the instrument, termed VERIS.

“It’s really worth the time and effort for the definitiveness that you can say where to direct your work-up,” Dr. Kardon said.

Sequenced hexagons

---3D density plot demonstrates a loss of amplitude in the superior hemifield. There is a region in the inferonasal field which demonstrates diminished amplitude. Both findings are consistent with the patient’s clinical and perimetry exam.

To measure the exam’s efficacy, researchers prospectively tested 65 patients with visual field loss to assess the clinical usefulness of MERG in distinguishing visual field loss due to optic neuropathy from retinopathy.

Right and left eyes were recorded monocularly under photopic conditions with the fluorescent room lights on and with gold bipolar corneal electrodes. MERG uses 103 black and white hexagons at about 200 candela/m2 per scale. Each hexagon is scaled so the smallest one is in the center. Each hexagon goes through a binary m-sequence of 214 possible configurations of on or off. Each hexagon goes through the same exact sequence, only displaced in time. Patients see random flickering elements, but they are not random.

The overall signal coming off the corneal electrode does not resemble anything, Dr. Kardon said, but a cross-correlation of each one with MERG produces a clear signal at each location that displays a response density.

Changing the intensity of those hexes creates an intensity-response function. Increasing the intensity boosts the response until it levels off at very high luminance.

A decreased retinal sensitivity results in a decreased response. While visual fields are a threshold test, MERG is a suprathreshold test.

Of the 65 patients, 14 did not show obvious findings of optic neuropathy or retinopathy before undergoing MERG. Nine of those patients showed an unequivocal focal ERG waveform reduction or loss, uncovering a retinal pathology. The other five patients showed normal first-order MERG wave forms and were subsequently diagnosed with retrobulbar optic neuritis, anterior is chemic optic neuropathy, Leber’s hereditary optic neuropathy and functional visual loss.

Clinically useful

---3D density plot demonstrates an area of diminished b-wave amplitude corresponding to the visual field seen on HVF 30-2 and extending from the blind spot. Amplitude throughout the rest of the field appears to be within normal limits.

MERG also distinguishes between abnormal signals coming from different retinal layers. Abnormal wave forms and depressions can result from the fluid between any of the retinal layers, or from retinal inflammations such as choroiditis and autoimmune retinopathy.

Pathologies that might not show up on a standard Ganzfeld ERG with diffuse bright flushes but do show up with MERG include trauma, ischemic branch retinal artery or branch vein occlusions, drug toxicities from gentamicin or vancomycin, and different types of degenerations and dystrophies or Stargardt’s.

The damage to inner retina will show a reduction in the wave form only if the damage penetrates into the bipolar cell layer.

A branch retinal artery occlusion shows definite decreasing responses if the ischemia penetrates into the bipolar layer. A complete visual field defect will knock off the entire nerve fiber layer but only part of the bipolar cell layer. This shows up as a relative defect on MERG.

“We have found it very useful if the MERG is located right in the clinical setting for differentiating some of these cases and guiding your work-up and avoiding imaging in some cases,” he said.

For Your Information:
  • Randy H. Kardon, MD, PhD, practices at the University of Iowa Hospitals, Iowa City, IA 52242; (319) 356-2260; fax: (319) 353-7996. Dr. Kardon has no direct financial interest in any of the products mentioned in this article, nor is he a paid consultant for any companies mentioned.