Statistics from Altmetric.com
Multifocal electroretinography (mfERG) is a valuable technique in assessing macular function in retinal disease objectively as it provides spatial information. Altered responses give an estimate of the extent of central retinal dysfunction.1–4 Fixation is known to be an important technical factor in mfERG recording.5–7 We present findings in a patient with asymptomatic intermittent exotropia that reinforce the importance of adequate consideration of potential fixation errors.
The patient was a 52 year old man with maternally inherited diabetes and deafness (MIDD) consequent upon a mitochondrial DNA nucleotide A3243G point mutation, and examined as part of a series of patients with MIDD.8 Visual acuity was 20/20 (ETDRS chart) bilaterally. Fundi showed symmetrical bilateral irregular patches of retinal pigment epithelium atrophy at the posterior poles. The foveas were normal.
Macular function was assessed initially by mfERG recorded binocularly with a stimulus size of 61 hexagons using the RETIscan System (Roland Consult, Wiesbaden, Germany). The patient fixated on the centre of a large diagonal cross, centred over the central hexagon, at a viewing distance of 33 cm. Pupils were dilated. Refractive errors were corrected with −6.25 dioptres (D) right eye and −6.25 spherical dioptres combined with −0.75 cylindrical dioptres at 5° left eye. Additional +3D were given for a viewing distance of 33 cm. Each recording session consisted of eight trials over about 20 minutes.
Upon binocular recording, changes reflecting the retinal dystrophy were visible in the right eye trace array outside the central hexagon. The normal foveal response was consistent both with normal foveal function and central fixation throughout testing (fig 1A). Amplitude reduction was observed in many left eye traces with an additional “off centre” peak also visible in three dimensional plot (fig 1B). These findings are not suggestive of MIDD. The mfERG was repeated monocularly. The left eye findings now showed a normal central response and alterations in parafoveal function consistent with MIDD (figs 2 A, B).
Subsequent orthoptic examination revealed a near type intermittent exotropia with poor motor fusion and additional microtropia. The latent deviation of the left eye was 2 prism dioptres base-in at 6 metres and 18 prism dioptres base-in at 33 cm. A small vertical height component was demonstrated on the Hess chart. Stereopsis was subnormal.
Patients with intermittent exotropia can be completely controlled having binocular vision or may have a manifest exotropia.9 Under binocular mfERG stimulation, the left eye presumably fixated in exotropia at times of fusional decompensation, and the stimulus pattern shifted by the extent of the squint deviation. At times of positive binocular vision the fixation was located almost centrally. The fixation was slightly shifted because of the microtropia (fig 1B) which was not detected by direct observation.
Examiners should always be aware that not only retinal disease can affect the mfERG. Asymptomatic strabismus is a reason for fixation instability and represents a potential dilemma in the interpretation of binocular mfERGs. Even with direct observation a small intermittent strabismus may be not be detected. This could result in a broadened central peak rather than the double peak seen in our patient and thus be mistaken for macular dysfunction. This would be potentially disastrous in a patient with optic nerve disease where the mfERG should be normal.2,3
Supported by the European Commission No QLK6-CT2000-51262 (Marie Curie Individual Fellowship) and by the Faun-Foundation to CB.
We thank Roland Consult for mfERG facilities.