Sir,

We agree with Mahroo’s1 helpful suggestions on the utility of electroretinography (ERG) in suspected transient retinal artery occlusion (TRAO) cases.

TRAO is a recently proposed clinical entity supported by OCT findings.2, 3 The ERG features of TRAO are yet to be described, and may be sought as part of a prospective case-finding study. Two limitations of Ganzfeld ERG in TRAO are: (1) branch pattern TRAO may not be detected; and (2) b-wave attenuation on ERG reverses fully after 30 min in experimental models of transient retinal ischaemia.4 ERG evidence of widespread ischaemia may vanish before testing takes place.

However, if there is perimetric or OCT evidence of ongoing retinal ischaemia when ERG is performed, it is likely that ERG abnormalities would be detectable. If the ischaemic changes extend beyond the obviously affected area of the retina, the Ganzfeld ERG might provide evidence of retinal ischaemia in the form of b-wave amplitude reduction and increased 30-Hz photopic flicker implicit time.

The multifocal ERG (mfERG) may provide evidence of localised ischaemic changes and might have contributed to the diagnosis in cases 2 and 3.2 Branch retinal artery occlusion attenuates the N1, P1, and N2 components in the distribution of ischaemic retina on mfERG.5 mfERG is capable of identifying wider retinal dysfunction than that suggested clinically.6 It has been used to demonstrate functional recovery following retinal artery occlusion7 and to detect subclinical retinal dysfunction in Susac’s syndrome.8 Pattern ERG may also demonstrate reduced amplitude or delayed P50 in cases of macula-involving TRAO, and may have supported a diagnosis in case 1.2

The mechanism underlying TRAO is unknown. If vasospasm or thromboembolism was responsible, it is conceivable that retinal ischaemia may persist beyond the specific occlusive episode and may be detectable on ERG or mfERG. It is also possible that reperfusion after prolonged TRAO would produce ERG changes, although we are not aware of this having been studied in human subjects.

Although a normal ERG may not exclude TRAO, it may therefore be considered in suspected TRAO when OCT findings are inconclusive or where persistent retinal ischaemia is suspected.