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Linda K McLoon, Matthew D Kunz, Joost Felius, David Stager, Jill S. Anderson, Erick D, Bothun; Examination of Extraocular Muscles and Their Innervation from Subjects with Infantile Nystagmus Syndrome and Optic Nerve Hypoplasia. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):3998. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Infantile nystagmus syndrome (INS) is an eye motility disorder characterized by involuntary conjugate oscillatory eye movements. INS is often associated with structural abnormalities in the afferent visual pathways. Our previous work suggested that the idiopathic form of INS and INS associated with albinism may be related to dysregulation of innervation. INS may also occur in conjunction with optic nerve hypoplasia (ONH). Our working hypothesis is that the extraocular muscles (EOM) from INS patients with ONH will have innervational adaptations that may provide insight into possible mechanisms that cause INS.<br />
We have examined EOM specimens removed as part of the normal surgical treatment of 2 patients with INS and ONH (age 6 and 10 years) compared to 3 age matched normal controls and 3 age matched subjects with idiopathic INS. They were examined for neuromuscular junction density and neuromuscular junction morphology. All studies were approved by the IRB at the University of Minnesota.
The EOM from the subjects with INS and ONH showed a number of features suggestive of innervational dysregulation. Neuromuscular junctions were almost 50% smaller than in control EOM, but resembled the smaller neuromuscular junctions of the idiopathic INS and INS/albinism EOM. Additionally, there was an increase in the density of slow myofibers, at 28% of total myofibers, compared to idiopathic INS and normal controls, at 22% and 16% respectively. Most striking was the presence of the immature gamma subunit of the acetylcholine receptor on fast myofibers in the neuromuscular junctions from the muscles obtained from INS patients with ONH, at almost 100%, compared to EOM from patients with idiopathic INS and to controls, where this occurred only rarely.
Similar to our previous results for muscles from subjects with INS and albinism, it appears that the EOM in patients with INS and ONH have changes suggestive of a dysinnervation syndrome. This includes increased density of neuromuscular junctions which are reduced in size, as well as retention of the gamma subunit on fast myosin-expressing myofibers. These results suggest potential new approaches that focus on neuromuscular junction maturation that might be used to alleviate the abnormal oscillatory movements associated with INS.
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