Changes in the electroretinogram in patients with optic nerve lesions

Electrophysiological reactions to intraoperative irritation of the optic nerve. Case report and review of possible mechanisms

BACKGROUND

Intraoperative control of optic nerve function conservation during neurosurgical operations currently relies mainly on visual evoked potential monitoring. Unfortunately, this detects peril only when the visual pathways are already compromised, sometimes irreversibly. In contrast, electrophysiological stimulation mapping of the nerves can be a fully preventive measure. However, direct sensory nerve mapping requires the patient to be awake during surgery, which is unfeasible for surgeries targeting the optic nerve area. Another possible approach to sensory nerve mapping involves unconditioned electrophysiological responses evoked by sensory nerve stimulation. The key point for this approach is the possibility of obtaining such responses for a particular sensory nerve under surgical anesthesia.

CASE REPORT

A 52-year-old woman presented with meningioma in the area of right optic nerve and chiasm. She underwent microsurgical removal of the tumor through the transciliary supraorbital approach. During surgery, electrodes at the inferior margin of the right orbit repeatedly recorded electrophysiological reactions following contacts and displacements of the right optic nerve by the surgical instruments.

CONCLUSIONS

The observed reactions suggest that either the unconditioned blink reflex or antidromic electroretinographic response to optic nerve irritation was conserved under total intravenous anesthesia. This observation might be of value for development of intraoperative optic nerve mapping. This in turn could increase patient safety by identifying the exact optic nerve location before any negative impact on it.

Efferent influences on the bioelectrical activity of the retina in primates

PURPOSE

The existence of retinopetal (sometimes referred to as "efferent" or "centrifugal") axons in the mammalian optic nerve is a topic of long-standing debate. Opposition is fading as efferent innervation of the retina has now been widely documented in rodents and other animals. The existence and function of an efferent system in humans and non-human primates has not, though, been definitively established. Such a feedback pathway could have important functional, clinical, and experimental significance to the field of vision science and ophthalmology.

METHODS

Following a comprehensive literature review (PubMed and Google Scholar, until July 2016), we present evidence regarding a system that can influence the bioelectrical activity of the retina in primates.

RESULTS

Anatomical and physiological evidences are presented separately. Improvements in histological staining and the advent of retrograde nerve fiber tracers have allowed for more confidence in the identification of efferent optic nerve fibers, including back to their point of origin.

CONCLUSION

Even with the accumulation of more modern anatomical and physiological evidence, some limitations and uncertainties about crucial details regarding the origins and role of a top-down, efferent system still exist. However, the summary of the evidence from earlier and more modern studies makes a compelling case in support of such a system in humans and non-human primates.

Electroretinogram findings in unilateral optic neuritis

To report the electrophysiological findings in patients with unilateral optic neuritis (ON), with particular reference to the electroretinogram (ERG). A retrospective analysis of full-field ERG, pattern ERG (PERG) and pattern visual evoked potential findings from 46 patients with clinical and electrophysiological findings in keeping with unilateral ON. ISCEV standard ERGs did not significantly differ between the optic neuritis and fellow eyes, nor between patients with and without MS. Differences were present in the N95 component of the PERG, which was significantly lower in the affected eye, and the pattern reversal visual evoked potential, which showed significantly longer peak time (latency) in the affected eye. In addition, there was a significant difference between patients with and without multiple sclerosis (MS). No significant inter-ocular asymmetry in ISCEV standard ERGs was present in these cases of unilateral optic neuritis, either as a clinically isolated syndrome or as part of multiple sclerosis. All ERGs recorded were normal.

Flash and pattern electroretinogram changes with optic atrophy and glaucoma

We investigated recent reports that, contrary to common belief, glaucoma can affect flash as well as pattern electroretinograms. An extensive flash and pattern electroretinogram test protocol was used in a large sample of glaucoma patients and age-matched controls who were either visually normal or had other optic nerve diseases. All electroretinogram parameters were reduced and delayed in normal people > 55 years of age. The effect did not increase in later decades. In patients aged < or = 55 years, flash electroretinograms showed mild reductions and delays from optic atrophy alone. Glaucomatous ERG changes were larger and increased with disease severity. Pattern electroretinograms and oscillatory potentials were almost equally reduced in optic atrophy and all degrees of glaucoma. Mildly affected patients > 55 years of age had similar electroretinogram change to age-matched normals in most conditions. Advanced glaucoma patients showed similar differences from normal irrespective of age. This suggests that direct diagnostic application of these results to older patients will be difficult, that the ERG changes in glaucoma cannot be attributed simply to optic atrophy and that additional widespread outer retinal damage occurs in glaucoma.

Traumatic optic neuropathy

Knowledge concerning the pathophysiologic mechanisms of traumatic optic neuropathy is limited. The optic nerve is a tract of the brain. Therefore, the cellular and biochemical pathophysiology of brain and spinal cord trauma and ischemia provide insight into mechanisms that may operate in traumatic optic neuropathy. The dosage of methylprednisolone (30 mg/kg/6 hours) which was successful in the National Acute Spinal Cord Injury Study 2 (NASCIS 2) evolved from the unique pharmacology of corticosteroids as antioxidants. The management of traumatic optic neuropathy rests on an accurate diagnosis which begins with a comprehensive clinical assessment and appropriate neuroimaging. The results of medical and surgical strategies for treating this injury have not been demonstrated to be better than those achieved without treatment. The spinal cord is a mixed grey and white matter tract of the brain in contrast to the optic nerve which is a pure white matter tract. The treatment success seen with methylprednisolone in the NASCIS 2 study may not generalize to the treatment of traumatic optic neuropathy. Conversely, if the treatment does generalize to the optic nerve, NASCIS 2 data suggests that treatment must be started within eight hours of injury, making traumatic optic neuropathy one of the true ophthalmic emergencies. Given the uncertainties in the treatment, ophthalmologists involved in the management of traumatic optic neuropathy are encouraged to participate in the collaborative study of traumatic optic neuropathy.

Changes in sensitivity of the dark-adapted eye during concurrent light adaptation of the other eye

Thresholds for detection of light by a dark-adapted test eye were measured while the other, non-test eye was either similarly dark adapted or while it was exposed to an intense red adapting field. An interocular effect that depends on the retinal location of the test was found: compared to the threshold during binocular dark adaptation, sensitivity decreased during contralateral light adaptation when the test was presented to the foveola and up to 4 deg above it; but sensitivity increased when the test was between 7 and 12 deg, showing a reversal at 5 deg.

The incidence of abnormal pattern electroretinography in optic nerve demyelination

This report describes the pattern electroretinogram (PERG) findings in 141 patients with optic nerve demyelination in one or both eyes. The overall incidence of PERG abnormality in the 199 eyes with abnormally delayed pattern visual evoked potential (PVEP) P100 component was 39.2%, with 84.6% of these PERG abnormalities being confined to the N95 component. The incidence of abnormal PERG was greater (53.3%) in those eyes with a history of retrobulbar neuritis than in those with sub-clinical demyelination (22.8%). The importance of stimulus parameters is noted. The value of the PERG in the improved interpretation of an abnormal PVEP is discussed.

A review of the clinical applications of the pattern electroretinogram

The pattern electroretinogram (PERG) has recently been introduced as a clinical procedure. It has been thought by many to represent activity of the retinal ganglion cells, although this is still a matter of contention. The exciting prospect of a selective test of ganglion cell function led to the application of the PERG in a variety of ophthalmological conditions. In the course of these investigations the PERG was found to be diminished in cases of maculopathy, optic atrophy, optic neuritis, toxic optic neuropathy, neurotransmitter disorders, glaucoma and ocular hypertension and in retinal vascular disorders such as diabetes. It was also affected in some cases of amblyopia. This paper briefly describes the techniques used to record the PERG and reviews current literature pertaining to its clinical application.

Screening for mouse retinal degenerations. I. Correlation of indirect ophthalmoscopy, electroretinograms, and histology

Mice with hereditary retinal degeneration have provided excellent models for human disease of the biochemical and physiological events occurring in retinal degeneration. Since a number of mouse models are available for other human conditions, more mouse retinal degenerations would be expected to be known; however, finding new models has proved difficult since the search has usually involved laborious histologic screening. We applied the clinical technique of indirect ophthalmoscopy to screen mice for retinal degeneration and then used electroretinography and histology to determine whether true retinal degeneration was present. A Dawson-Trick-Litzkow microfiber corneal electrode was used to record the electroretinogram since the fiber does not occlude the pupil in these small eyes. Normal control values were developed. As an example of the success of the technique, one strain, lethal spot (ls) on indirect ophthalmoscopy appeared to have a retinal degeneration, but these mice had a normal electroretinogram indicating a primary optic atrophy. Likewise, one ls heterozygote that was tested as a control animal and was not suspected of having a retinal degeneration had an abnormal electroretinogram and peripheral retinal degeneration.

The electroretinogram in multiple sclerosis and demyelinating optic neuritis

The electroretinogram (ERG) to flashes of white light presented under photopic conditions and the pattern reversal visual evoked potentials (PR-VEPs) from both eyes were recorded from 14 patients with multiple sclerosis (MS) with monocular demyelinating optic neuritis (DON) and from 11 patients soon after presenting with monocular demyelinating optic neuritis alone. Fifteen and 10 normal subjects, matched for age and sex, were used as controls for each group of patients respectively. In the DON group of patients and controls the flicker following ERG (FF-ERG) to white flashes of light at 40 Hz was also recorded. Skin electrodes and averaging procedures were used for all the recordings. The PR-VEP elicited with stimulation of the affected eye was absent or abnormally delayed, and the amplitude of the 'b' wave of ERG of the affected eye was diminished in all patients. The 'b' wave latency, however, was similar in both affected and non-affected eyes and the controls. There was no difference in 'a' wave amplitude and latency between eyes of patients and normal subjects. The FF-ERG in 8 out of 10 patients with satisfactory recordings was diminished in the affected eye. These results provide neurophysiological evidence that retinal damage is not due to loss of myelin but is an early feature of demyelinating optic neuritis. This damage preferentially affects the retinal elements associated with the generation of the 'b' wave of the ERG, probably the glial cells of Müller.

Electrophysiological testing of eyes with opaque media

Electrodiagnostic tests as applied to the eye are now widely used in the investigation of patients with inherited retinal degenerations but their value in patients with opaque media is perhaps less well recognised. The choice of stimulus is all important and a flash VEP and flash ERG probably provide the best combination of tests at present. A comparison with the fellow eye, if it is normal, greatly improves the value of the result. A pitfall of this type of test is its failure to detect amblyopia and a carefully recorded history is important in helping to interpret the traces.

The electroretinogram in advanced open-angle glaucoma

Fourteen patients with advanced open-angle glaucoma were evaluated prospectively by electroretinography and the results were compared with normal controls who were matched for age and sex. All glaucoma patients had visual acuity of 20/50 or better, cup-to-disc ratios of 0.7 or greater in at least one eye, and visual field loss consistent with advanced glaucoma. Numerous electroretinographic parameters were significantly abnormal when compared with the control group.

Pattern electroretinogram in multiple sclerosis

The pattern electroretinogram obtained in patients affected by multiple sclerosis were compared with those in a normal population. The pattern electroretinogram amplitude in multiple sclerosis optic neuritis frequently appeared normal, but pattern electroretinogram amplitude abnormalities were found in patients suffering from recurrent multiple sclerosis optic neuritis. Pattern electroretinogram examination appears a useful method in the differential diagnosis between axonal and demyelinating optic nerve impairment.

Supernormal scotopic ERG in cone dystrophy

Three patients with a bull's-eye macular lesion and other signs characteristic of cone dystrophy gave an unusual ERG finding. In response to a white flash of moderate intensity the scotopic b-wave amplitude was considerably larger than normal. One patient had elevated rod thresholds and nyctalopia, while the other 2 had normal rod sensitivity associated with the supernormal scotopic b-wave amplitude. In the latter 2 patients the abnormal ERG pattern was unchanged for 4 years and 7 years respectively. This atypical finding, of a supernormal scotopic b-wave amplitude in response to light of moderate intensity, appears to characterise a subgroup of patients with cone dystrophy, probably of autosomal recessive inheritance. The pathogenesis of the abnormal ERG remains uncertain.

Electroretinographic responses to alternating gratings before and after section of the optic nerve

Electroretinographic (ERG) responses to sinusoidal gratings reversed in contrast (pattern-reversal ERG) were recorded from both eyes of cats before and after unilateral section of the optic nerve. In the eye ipsilateral to the section, the pattern-reversal ERG remained unaltered for a few days after the section, the progressively decreased in amplitude, first at low and then at high spatial frequencies, to disappear completely about 4 months after the section, when ganglion cell degeneration was practically complete. The flash ERG remained unaltered. No alteration was observed in the contralateral eye.

The application of visually evoked cerebral potentials in ophthalmological diagnosis

A short survey is presented of what can be done with electrophysiological techniques in ophthalmological diagnosis. It is stressed that more investigation has to be done to elucidate the discrepancies between electrophysiological and psychophysical disturbances. These discrepancies can be used for the differential diagnosis. More, though still too little, is known about differences between a disturbance of latency time and that of amplitude. It seems that latencies are more disturbed in active pathological processes like demyelination, compression of nerve fibres and oedema, whereas isolated reduction of amplitudes occurs in optic atrophies. Some optic nerve diseases are found to be combined with ERG abnormalities, such as tobacco-alcohol neuropathy and some hereditary diseases.

Neurophysiological investigation in optic nerve disease: combined assessment of the visual evoked response and electroretinogram

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