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Show Journal of Neuro- Ophlhalmology 18( 1): 15- 16, 1998. © 1998 Lippincott- Riiven Publishers, Philadelphia Editorial Occult Maculopathy and the Focal ERG In this issue of JNO, ten Hove and associates report on three patients with acquired cone dysfunction, whose diagnoses were " secured" by abnormalities in the focal electroretinogram ( ERG). In each case, bilateral visual acuity loss, cecocentral scotomas, and color vision disturbance were noted. In case 1, mild temporal optic disc " pallor" was noted while the maculae were considered " normal". In cases 2 and 3, blunting of the foveal reflex was noted. In these cases, and in others reported, distinguishing optic neuropathy caused by toxic ( e. g., alcohol, digoxin, etc.), metabolic ( e. g., vitamin B12, folate deficiency) or genetic ( e. g., dominant optic atrophy, often occurring in young people) processes from maculopathy on a clinical basis is not always straight forward. When should the clinician consider maculopathy? Always. The question is whether or not the focal ERG is necessary for this purpose. The symptom of daylight vision impairment ( hemer-alopia) is certainly not typical of optic neuropathy, and suggests cone dysfunction. The finding of absolutely " normal" maculae on ophthalmoscopy should raise the possibility of an early cone dysfunction syndrome; according to some investigators this is the most frequent funduscopic finding ( 1). One should recall that " normal' ' maculae in the face of central visual loss may also be seen in early stages of Stargardt's disease ( and care should be taken to avoid making the erroneous diagnosis of cases of macular branch retinal artery occlusion). I have examined an elderly patient with bilateral visual loss, central scotomas, and color vision defects with " normal" maculae ( with contact lens examination) who showed shallow retinal pigment epithelial detachments due to occult choroidal neovascularization on fluorescein angiography. This demonstrates that a " normal" macula on ophthalmoscopy does not rule out maculopathy. According to Krill and associates ( 2), fundus findings when noted with cone degeneration include the bull's-eye lesion ( similar to that detected with chloroquine toxicity), diffuse pigment clumping, and regional choroidal vascular atrophy. Fluorescein angiography in such cases may demonstrate subtle changes when ophthalmoscopy is unremarkable. Visual field defects early in the course of the disease may show minor paracentral scotomas ( best seen on central 10 degree threshold perimetry), sometimes in a ring distribution, and may spare fixation. Thus, a patient may present with reading difficulty at a time when visual acuity is excellent. Interestingly, optic nerve pallor/ atrophy ( as in case 1) are not unusual in cone degeneration. Optic atrophy may occur secondary to transsynaptic degeneration ( previously called " consecutive optic atrophy"). Heckenlively has reported the finding of temporal optic atrophy along with telangiectatic disc vessels ( a la Leber's optic neuropathy) in such patients ( 3). Although Krill and associates ( 2) noted color vision to be normal early in the clinical course of cone dystrophies, by the time the visual acuity was impaired to a 20/ 40- 20/ 60 level, color vision was usually severely impaired, as opposed to maculopathy in general in which color vision loss more often than not parallels visual acuity loss. Thus, cone degeneration is expected to result in color vision loss out of proportion to visual acuity. One of the very bothersome findings in all three of Ten Hove's patients is the " good" color vision relative to visual acuity. In case 1, 6 of 10 Ishihara color plates were correctly identified with each eye at a time when visual acuities were 20/ 120 and 20/ 80, respectively. In case 2, " Farnsworth color testing suggested a deuteranopic axis of confusion" when visual acuities were 20/ 100 and 20/ 140. I would have expected color vision to have been so poor that determining an axis of confusion would be irrelevant. In case 3, hue discrimination, as tested by Farnsworth D- 15 test, was normal in both eyes, although visual acuities were good ( 20/ 30, 20/ 40). Maculopathy vs. cone dystrophy- how does the electroretinogram help? With diffuse cone dysfunction, the full field ERG will typically demonstrate the following: diminished response to flicker stimuli; abnormal double humped B- wave with the absence of the cone portion after scotopic red stimulus; diminished response to a white light under photopic conditions. The diminished response to flicker stimuli is most specific while the other responses are helpful in cases in which the flicker stimuli result in unreadable artifacts. In cases with normal full field cone response, an abnormal focal ERG ( performed with flicker stimuli focused on the macula region only), would imply that the central cones ( which represent a minority of the total amount of cones) are abnormal. In fact, impairment of the central cones and/ or their connections in the inner retina i. e., Mueller cells which are responsible for the B- wave, will give rise to an abnormal focal ERG. 15 16 EDITORIAL A caveat: as with any laboratory test, incorrect administration may give rise to false positive results. If the flicker stimulus is misdirected and instead stimulates parafoveal, rather than foveal, cones, a diminished amplitude will be noted ( 4). Cone degeneration with a preference to macular cones, macular degeneration, Star-gardt's disease, macular scars, and macular branch retinal artery occlusion may give rise to the picture of a normal full field ERG but an abnormal focal ERG. In Ten Hove's case 3, bilateral cecocentral scotomas associated with ring scotomas were noted. Cone and rod dysfunction were undoubtably present. With enough photoreceptor abnormalities to result in such a marked diffuse visual field disturbance, why was the full field ERG normal? Patients with amblyopia, and primary optic atrophy have been shown to have a normal ERG ( 4). Some years ago, it seemed evident to me that full field ERG would be helpful to " easily" distinguish incomplete central retinal artery occlusion from primary ischemic optic neuropathy. Central retinal artery occlusion eyes should show diminished B- waves compared to their fellow eyes, while anterior ischemic optic neuropathy should, theoretically, be normal. On reviewing the literature at that time, I was dismayed to discover optic atrophy prominently displayed in a list of causes of diminished B-waves( 5). The optic atrophy was thought to have to be long- standing, resulting, most likely, in ganglion and bipolar cell ( that is, mid- retinal) dropout ( transsynaptic degeneration). If some patients with long- standing optic atrophy through retrograde degeneration show abnormalities on the full field ERG, why wouldn't some patients with long- standing cecocentral scotomas due to optic neuropathy result in diminished focal ERG? The answer is two- fold: 1) Over 60 patients with primary optic atrophy have now been reported as having normal focal ERG ( 4, 6, 7); and 2) There is no evidence to support the original contention that primary optic atrophy actually results in transsynaptic degeneration or an abnormal ERG ( personal communication: Ronald E. Carr, M. D. August 1996). Laboratory tests alone will not replace clinical acumen, but the focal ERG may be a powerful tool when dealing with visual loss due to occult maculopathies. Michael L. Slavin, M. D. Long Island Jewish Medical Center Great Neck, New York REFERENCES 1. Weleber RG, Eisner A: Cone Degeneration ( Bull's- Eye Dystrophies") and Color vision defects in retinal dystrophies and degenerations. Ed, Newsome DA. Raven Press, New York, 1988; 241- 2. 2. Krill AE, Deutman AF, Fishman M: The cone degenerations. Doc Ophthalmol 1973; 35: 1. 3. Heckenlively JR, Martin DA, Rosales TO: Telangiectasia and optic atrophy in cone- rod degenerations. Arch Ophthalmol 1981 ; 99: 1983. 4. Jacobson SG, Sandberg MA, Effron MH, et al: Foveal cone elec-troretinograms in strabismic amblyopia. Comparison with juvenile macular degeneration, macular scars and optic atrophy. Trans Oph-thal Soc U. K. 1979; 99: 353- 6. 5. Carr RE, Siegel IM: Visual Electrodiagnostic Testing. A practical guide for the clinician. Williams & Wilkins, Baltimore, 1982; 51. 6. Sandberg MA, Baruzzi CM, Berson EL: Foveal cone electroret-inogram in optic neuropathy. Inv Oph Vis Sci 1986; 27 ( supplement) 104. 7. Biersdorf WR: The clinical utility of the foveal electroretinogram: A review. Doc Ophthalmologica 1990; 73: 313- 24. J Neuro- Ophlhalmol, Vol. 18, No. I, 1998 |
