Abnormal nerve structures seen in congenital nystagmus muscle tendons
Researchers suspect anomalous nerve terminals are linked to the disruption of oculomotor function in patients with nystagmus.
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SEATTLE — Abnormal neural and vascular structures can be seen in the extraocular muscle tendons of patients with congenital nystagmus, according to a study presented here.
“We found that neurovascular abnormalities in the enthesial part of the extraocular muscles may be part of disease pathogenesis,” said Richard W. Hertle, MD, FACS, FAAP, here at the meeting of the American Association of Pediatric Ophthalmology and Strabismus.
Dr. Hertle and colleagues found the abnormal neural activity at the enthesial segment of extraocular tendons, where muscles attach to scleral fibers.
“This enthesial tissue is bound posteriorly by predominant, regularly arranged tendon fibers, and anteriorly by predominant, irregularly arranged scleral fibers,” he explained. He said this is the area where surgical intervention is most often initiated for the purpose of treating eye movement disorders.
“Finding incompletely developed nerve terminals and other structural faults in this area leads us to believe that visual processing and oculomotor control in nystagmus may be affected directly by these irregularities,” he said.
Surgical extraction
Figure 1: A crescent knife is used to make a scleral groove anterior to the insertion of the rectus tendon on the globe. |
Figure 2: The crescent knife is used to dissect the tendon, with an unattached flap of underlying sclera off the surface of the globe. |
Figure 3: The muscle is cut and cauterized anterior to the hemostat. The severed end of the muscle is attached to the globe at its original insertion site. |
Researchers examined 14 rectus muscle specimens for atypical features. Eight samples were gathered from four patients with congenital nystagmus and one patient with sensory exotropia. Five healthy muscles were taken from human and macaque donor eyes.
Tissue samples were obtained through a surgical procedure, which was specially designed for the study.
“Each tendon was placed on a slight stretch. Then a crescent knife was used to make a scleral groove 0.75 mm anterior to the insertion of the rectus tendon on the globe,” Dr. Hertle explained (figure 1).
The crescent knife was then used to dissect the tendon, with an attached flap of underlying sclera off the surface of the globe, he said (figure 2).
“The muscle was cut and cauterized anterior to a hemostat. The severed end of the muscle was attached to the globe at its original insertion site,” he added (figure 3).
According to Dr. Hertle, there was no scleral perforation during any steps of the procedure.
Once muscle tendon specimens were obtained, they were gently stretched using forceps during the fixation process.
“This ensured normal anatomic relationships. After 24 hours, this process was accomplished again prior to embedding and preparing specimens for histological analysis,” he said.
For analysis, all tissue samples were stained and examined with a transmission electron microscope.
Anomalous nerve terminals
“Enthesial neurovascular structures were identified in all specimens,” Dr. Hertle said.
The enthesial area showed structures consisting of myelinated and unmyelinated nerve fibers with supporting vascular capillaries. Additionally, qualitative analysis showed abnormal eurovascular structures in the enthesis of patients with congenital nystagmus.
“Electron micrographic photos showed anomalous, or incompletely developed, dysmorphic or dysgenic nerve terminals in the enthesial area of patients with infantile nystagmus,” he said.
Researchers also found nerve endings with dilated rough endoplasmic reticula in the tissue of a 19-year-old patient with acquired strabismus.
Additionally, in a patient with congenital nystagmus, researchers found aberrant capillaries with edematous or swollen endothelial cells.
“This same specimen had nerve endings encircled by capsular fibrocytes and pre-terminal axons,” he added.
In an 18-month-old infant with idiopathic congenital nystagmus, researchers found anomalous unmyelinated nerve fibers, which were degenerating and swollen.
“This case showed fragmented neurolemma, in addition to possible dystrophic nerve terminals,” Dr. Hertle added.
Finally, in a 2-year-old patient with congenital nystagmus, researchers found poorly developed nerve endings. The axons contained fragmented and unevenly distributed myelin.
More research needed
According to Dr. Hertle, the development of these abnormalities may be due to a number of factors.
“These findings may reflect immaturity of postnatal development, or may reflect postnatal development particular to the specific oculomotor disorder,” he said.
However, it is too early in the investigative process to make any definitive conclusions about the etiology of the abnormalities, he added.
“For now we await the continued, cooperative investigations of neurophysiologists and neuroanatomists to find deeper meaning in our findings,” he said.
For Your Information:
- Richard W. Hertle, MD, FACS, FAAP, can be reached at Pediatric Ophthalmology, Strabismus and Eye Movement Disorders and The Ocular Motor Neurophysiology Laboratory at Columbus Children’s Hospital and The Ohio State University, 555 S. 18th Street, Suite 4C, Columbus, OH 43205; (614) 224-6222; fax: (614) 241-5232; e-mail: hertler@chi.osu.edu.