Extraocular muscles in patients with infantile nystagmus: adaptations at the effector level

Abstract

Objective: To test the hypothesis that the extraocular muscles (EOMs) of patients with infantile nystagmus have muscular and innervational adaptations that may have a role in the involuntary oscillations of the eyes.

Methods: Specimens of EOMs from 10 patients with infantile nystagmus and postmortem specimens from 10 control subjects were prepared for histologic examination. The following variables were quantified: mean myofiber cross-sectional area, myofiber central nucleation, myelinated nerve density, nerve fiber density, and neuromuscular junction density.

Results: In contrast to control EOMs, infantile nystagmus EOMs had significantly more centrally nucleated myofibers, consistent with cycles of degeneration and regeneration. The EOMs of patients with nystagmus also had a greater degree of heterogeneity in myofiber size than did those of controls, with no difference in mean myofiber cross-sectional area. Mean myelinated nerve density, nerve fiber density, and neuromuscular junction density were also significantly decreased in infantile nystagmus EOMs.

Conclusions: The EOMs of patients with infantile nystagmus displayed a distinct hypoinnervated phenotype. This represents the first quantification of changes in central nucleation and myofiber size heterogeneity, as well as decreased myelinated nerve, nerve fiber, and neuromuscular junction density. These results suggest that deficits in motor innervation are a potential basis for the primary loss of motor control.

Clinical relevance: Improved understanding of the etiology of nystagmus may direct future diagnostic and treatment strategies.

Figure 1

Light microscopic examination of control and patient extraocular muscles stained with hematoxylin-eosin. Comparison of hematoxylin-eosin stains of muscles from adult controls (A), the child control (paraffin embedded) (B), and patients with infantile nystagmus (C). Arrows indicate myofibers with centrally located myonuclei. Bar is 20 μm.

Figure 2

Quantification of the mean myofiber cross-sectional area in normal rectus muscles compared with all the muscles from patients with infantile nystagmus. The mean values are not statistically significantly different. Mean myofiber area for the child control muscle is displayed separately for comparative purposes. Because there were only 2 such muscles, both from 1 donor, no comparative statistics were performed. However, the mean values overlap, strongly suggesting that there is no difference between the aggregated control muscles and the control muscles from the child donor. Error bars represent SE.

Figure 3

Histograms of myofiber size demonstrating increased heterogeneity of myofiber cross-sectional areas in muscles from patients with infantile nystagmus. A, Histogram of myofiber cross-sectional areas from medial rectus muscles from 1 control (case 2) and 1 patient with infantile nystagmus (patient 5). The y-axis is in log₁₀. B, Histogram of aggregated myofiber cross-sectional areas from extraocular muscles from controls and patients with infantile nystagmus. The y-axis is in log₁₀.

Figure 4

Central nucleation in controls and in patients with infantile nystagmus. A, Hematoxylin-eosin–stained cross section obtained from the medial rectus muscle of a patient with infantile nystagmus (patient 10). Arrows indicate myofibers with central nucleation. Bar is 20 μm. B, Quantification of the percentage of myofibers with central nucleation in normal rectus muscles compared with muscles from patients with infantile nystagmus. The 2 muscles obtained from the 7-year-old control are depicted separately to demonstrate their similarity with data from the adult control muscles; however, for statistical analysis, all the control muscles were pooled. *P≤.05. Error bars indicate SE.

Figure 5

Sections from controls, patients with infantile nystagmus, and a patient with vertical strabismus that have been immunostained for the presence of Schwann cell myelin. A, Typical appearance of muscle from a patient with infantile nystagmus showing extremely sparse innervation (n=10). B, Typical appearance of myelinated nerve fiber content in control extraocular muscles (n=10). C, Typical appearance of myelinated nerves from a patient with vertical strabismus (n=1). Bars are 20 μm. D, Quantification of mean Schwann cell area per total muscle area from controls (n=10), a child control (n=1), and patients with infantile nystagmus (n=10). The 2 muscles obtained from the 7-year-old control are depicted separately to demonstrate their similarity with data from the adult control muscles; however, for statistical analysis, all the control muscles were pooled. *P≤.05. Arrows indicate Schwann cell–positive nerve bundles; error bars, SE.

Figure 6

Quantification of mean nerve fiber area per total muscle area from controls (n=10) and patients with infantile nystagmus (n=10). The 2 muscles obtained from the 7-year-old control are depicted separately to demonstrate their similarity with data from the adult control muscles; however, for statistical analysis, all the control muscles were pooled. *P≤.05. Error bars represent SE.

Figure 7

Immunostaining for visualization of neuromuscular junctions. Three-dimensional reconstructions of the neuromuscular junctions from the rectus muscles of a patient with infantile nystagmus (A) and a control monkey (B), with the monkey muscle having the same length as the human muscle selected. Each red dot represents 1 neuromuscular junction. Every 10th section was analyzed morphometrically. C, Quantification of the density of neuromuscular junctions from rectus muscles from controls (n=10) and patients with infantile nystagmus (n=10). *P≤.05. Error bars represent SE.