The wide field multifocal electroretinogram reveals retinal dysfunction in early retinitis pigmentosa

The diagnosis of retinitis pigmentosa (RP) is made on the basis of characteristic retinal pigmentation changes, visual field defects, and reduction in amplitude or loss of rod scotopic standard electroretinography (ERG) responses, with a possible history of night blindness and a positive family history of RP.¹ Multifocal electrophysiology (mfERG), is a new technique that constructs a topographical map reflecting retinal function.² Reports have suggested that the spatial resolution of mfERG is sufficient to detect focal changes in retinal function as RP progresses.^3–5

We describe a case of early RP in which the amplitude and implicit times of the patient's standard ERG rod and maximal responses were normal in the right eye and equivocal in the left eye. However, the peripheral retinal mfERG amplitude and implicit times were reduced and delayed. These abnormalities were obtained using a custom built wide field mfERG,⁶ which facilitates assessment of a 90 degree retinal field.

Case report

A 29 year old woman was referred to the eye clinic by her optometrist. Abnormal retinal pigmentation was found in both fundi on routine examination. She had no visual problems and was otherwise systemically well. Her 51 year old mother is known to have RP.

On examination, she had 0.50 dioptres of hypermetropia in both eyes. Her best corrected logMAR visual acuities were −0.075 in the right eye and −0.025 in the left. Her colour vision, anterior segment examination, and intraocular pressures were normal in both eyes. Fundal examination revealed semicircular arcs of intraretinal “bone spicule” pigmentation in the inferior mid-periphery of each retina. Her optic discs appeared normal and there was no evidence of attenuation of the retinal vasculature.

A Humphrey 120 point threshold related perimetry test was performed and the patient maintained fixation throughout the test. There was an arc of absolute visual field defect in the superior field, symmetrical in both eyes, 40 degrees from fixation, which was more extensive superotemporally than superonasally.

A Ganzfeld ERG test was performed in accordance with international standards.⁷ The implicit times and amplitude of the scotopic rod, photopic cone and flicker responses of the patient were normal. The scotopic maximal b-wave amplitude was reduced by 14% in the left eye and was normal in the right eye (Fig 1).

Figure 1.

The Ganzfeld ERG responses of a normal eye compared with the patient's eyes. The rod b-wave of patient's right and left eye compared with control (normal range of rod b-wave amplitude 72–367 μV) (A); the maximal b-wave response of patient's right and left eye compared with controls (normal range of maximal b-wave amplitude 241–709 μV) (B); the cone b-wave response of patient's right and left eye compared with normal control (normal range of cone b-wave amplitude 68–222 μV) (C).

Wide field mfERG was performed, using a technique previously described.⁶ The amplitudes of the central and peripheral mfERG responses were grouped and averaged (Fig 2A) and compared with similar responses from normative data (Fig 2B). The average amplitude of the central mfERG response was 75 nV in the right and 101 nV in the left (normal range 74–122 nV) (Fig 2A). The average peripheral retinal mfERG responses were 29 nV in the right eye and 45 nV in the left eye (normal range 61–108 nV). The normal range is derived from a group of 40 controls, aged 20–40 years. In addition, the mfERG responses were reduced in areas that had normal visual field sensitivities.

Figure 2.

(A) Multifocal amplitude responses of patient's right eye with the central and peripheral responses grouped and averaged. The central responses are generated by the central 60 degrees of the retinal field. (B) Multifocal responses of a normal right eye with central and peripheral mfERG responses grouped and averaged. Note that the patient's averaged central response is normal (normal range 74–122 nV). However, the averaged peripheral mfERG response of the patient is reduced at 29 nV (normal range 61–108 nV).

Comment

Retinitis pigmentosa in its early stages of evolution is characterised by rod dominated photoreceptor dysfunction.¹ Although mfERG is a photopic response, thought to predominantly reflect cone function,⁵ the nature of mfERG stimulation (that is, stimulation frequencies from 5 Hz to 75 Hz),⁸ indicates that this composite response may contain contributions from rods, in addition to cones and post receptor cells.

The global nature of the Ganzfeld ERG requires approximately 30% of the retina to be dysfunctional before abnormalities can be detected.⁹ In this case report, the standard ERG did not help to confirm the diagnosis of RP. However, the spatial resolution of the peripheral wide field mfERG indicated peripheral retinal dysfunction, suggestive of RP.

We conclude that wide field mfERG may have advantages over Ganzfeld ERG in the electrophysiological diagnosis of some forms of early RP. Further evaluation of wide field mfERG in the diagnosis and follow up of early RP is indicated.