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RC Caruso, P Lopez, LA Ayres, JA Smith, R Schiffmann; Assessment of Visual Function in Mucolipidosis IV Using ERG, VEP and TAC . Invest. Ophthalmol. Vis. Sci. 2002;43(13):1197.
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Purpose: Mucolipidosis type IV (ML-IV) is an autosomal recessive neurodegenerative lysosomal storage disease due to mutations in the gene encoding mucolipin 1. It causes a progressive degeneration of the retina and visual pathways. The objective of this study was to measure the degree of visual loss in patients with ML-IV. Electroretinography (ERG), visual evoked potentials (VEP), and visual acuity measurements with Teller Acuity Cards (TAC) were used for this purpose. Method: To date, 6 patients with ML-IV have been evaluated (4 female, 2 male, from 2 to 24 years of age). TAC measurements and VEP recordings were performed while awake, and ERGs were recorded under anesthesia with propofol and nitrous oxide. ERGs and VEPs were elicited by Ganzfeld flash stimuli and recorded following standard ISCEV procedures. The half-life of these variables as a function of age was calculated. Results: (1) ERG. In the youngest two patients (ages 2 and 4), cone responses, flicker responses, and the a-wave of mixed rod and cone ("maximal") responses were normal. However, the b-wave of rod responses and "maximal" responses already showed a reduction in amplitude. This gave the "maximal" responses a "negative" configuration. ERG amplitude declined rapidly as a function of age, and in the oldest patient all responses were non-recordable ("extinguished") with conventional averaging. Their half lives were 2.72, 3.45, 3.03, 3.66, and 3.70 years for rod response, "maximal" response a-wave, "maximal" response b-wave, cone, and flicker responses, respectively. (2) VEP. Similarly, flash VEP amplitude (N2-P2) was normal in the younger patients and declined to noise levels in the oldest subject (half-life: 4.75 years). VEP (P2) implicit time was delayed in all but one 4 year-old patient. (3) TAC. Acuity measurements could only be obtained in the 3 younger patients; acuity ranged between 20/80 and 20/100. The 3 older patients were unable to fixate and thus could not perform a preferential looking task. Conclusion: Retinal degeneration in ML-IV is present very early in life; the site of the initial abnormality is either at, or proximal to, the photoreceptor terminals (in bipolar or Muller cells), resulting in a reduced ERG b-wave amplitude. Soon thereafter, additional photoreceptor degeneration causes further reduction of all ERG components. VEP abnormalities and acuity loss are probably due to degeneration of both the retina and visual pathways. Clinical and genetic heterogeneity may modify these findings.
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