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Thompson DA, Lyons RJ, Liasis A, Russell-Eggitt I, Jägle H, Grünewald S. Retinal On-Pathway Deficit in Congenital Disorder of Glycosylation Due to Phosphomannomutase Deficiency. Arch Ophthalmol. 2012;130(6):712–719. doi:10.1001/archophthalmol.2012.130
Author Affiliations: Clinical and Academic Department of Ophthalmology (Drs Thompson, Lyons, Liasis, and Russell-Eggitt) and Department of Metabolic Medicine (Dr Grünewald), Great Ormond Street Hospital for Children, London, England; and University Eye Clinic, Regensburg, Germany (Dr Jägle).
Objective To describe novel electroretinographic (ERG) findings associated with congenital disorder of glycosylation due to phosphomannomutase deficiency (PMM2-CDG) (previously known as congenital disorder of glycosylation type 1a).
Methods Two male siblings with genetically confirmed PMM2-CDG underwent full-field ERG to a range of scotopic and photopic flash luminances that extended the International Society for Clinical Electrophysiology of Vision standard protocol and included scotopic 15-Hz flicker and photopic prolonged on-off stimulation.
Results Photopic prolonged ERGs were profoundly electronegative with absent b-waves but preserved oscillatory potentials. Prolonged off-responses and off-oscillatory potentials were preserved. Transient full-field photopic ERGs revealed a broad a-wave and narrow b-wave, and the photopic 30-Hz flicker ERG had a sawtooth waveform. The scotopic b-waves of both cases were attenuated to the fifth percentile, whereas scotopic a-wave amplitudes were at the 50th to 75th percentile, giving a reduced a:b ratio. The scotopic a-wave waveform was well defined to bright flash luminance. The number of scotopic oscillatory potentials was preserved, although amplitudes were smaller than average. Scotopic 15-Hz flicker ERGs were evident to a range of flash luminances and showed an expected phase cancellation between −1.5 and −1.0 log scotopic td (troland) • s, but phase increased only for the fast rod pathway.
Conclusions We find, for the first time to our knowledge, an association of PMM2-CDG with a selective on-pathway dysfunction in the retina. This ERG phenotype localizes the site of retinal dysfunction to the on-bipolar synapse with photoreceptors. Modeling the unusual combination of ERG findings helps our understanding of the role of N -glycosylation at this synapse and provides a focus for future studies of potential intervention.
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