Axon Diameter in Normal Human Optic Nerve

PURPOSE: The morphometric change of optic nerve axons is one of the parameters for evaluating the optic nerve function in pathologic conditions such as optic neuritis, anterior optic neuropathy, glaucomatous optic nerve damage, and etc. There are various reports about the diameter of optic nerve axon and its change with age. This study was performed to observe the distribution and aging change of axon diameter, and morphometric characteristic of normal human optic nerve axon. METHODS: Eight enucleated eyes(36~80-year-old), which had no specific lesion in the optic nerve, were enrolled in the study. Optic nerve specimens at central and peripheral portions in each anterior and posterior sides of lamina cribrosa were obtained. The optic nerve axons were observed using transmission electron microscopy and the diameters of optic nerve axons were measured. RESULTS: The mean diameter of optic nerve axon was 1.172 micrometer(0.299 micrometer~6.432 micrometer, SD : 0.326). The mean diameter of optic nerve axon in anterior side of lamina cribrosa was 1.018 micrometer, central nerve fiber was 1.015 micrometer and peripheral nerve fiber was 1.022 micrometer. The mean diameter of optic nerve axon in posterior side of lamina cribrosa was 1.306 micrometer, central nerve fiber was 1.257 micrometer and peripheral nerve fiber was 1.355 micrometer. As age increased, the diameter of optic nerve axon had a tendency to increase(1.08 micrometer in 36-year-old, 1.373 micrometer in 80-year-old). CONCLUSIONS: The optic nerve axons in anterior side of lamina cribrosa were larger than those in posterior side of lamina cribrosa and the peripheral nerve fibers were larger than the central nerve fibers. This study demonstrated the morphometric variability of optic nerve fibers according to their locations and ages.

Palpebral myiasis

Myiasis is most prevalent in Mexico, central and south America, tropical Africa, and the southwestern United States. Although dermal myiasis is rare in most of the United States, it is a disorder that may be seen in international travelers. In the United States, external myiasis is usually caused by the cattle botfly. We report here a case of ophthalmomyiasis involving the left upper eyelid of a child. We examined a six-year-old boy who presented to the Massachusetts Eye and Ear Infirmary (MEEI) in September 1998. He complained of persistent swelling of his left upper eyelid for the previous ten days. The edema and erythema were unresponsive to warm compresses and oral antibiotics. Ocular examination revealed a mild preseptal cellulitis of the left upper eyelid with a small draining fistula. On slit-lamp examination, we found one larva protruding intermittently from the fistula site. The larva was extracted with forceps, wrapped in a moist towel and sent in a jar to the parasitology laboratory. The specimen was identified as a Cuterebra larva by a parasitologist at the Harvard School of Public Health. One week later, the patient's eyelid edema and erythema had completely resolved.