Abstract
The trochlear and abducens nerves (TN and AN) control the movement of the superior oblique and lateral rectus muscles of the eyeball, respectively. Despite their immense clinical and radiological importance no morphometric data was available from a wide spectrum of age groups for comparison with either pathological or other conditions involving these nerves. In the present study, morphometry of the TN and AN was performed on twenty post-mortem samples ranging from 12-90 years of age. The nerve samples were processed for resin embedding and toluidine blue stained thin (1µm) sections were used for estimating the total number of myelinated axons by fractionator and the cross sectional area of the nerve and the axons by point counting methods. We observed that the TN was covered by a well-defined epineurium and had ill-defined fascicles, whereas the AN had multiple fascicles with scanty epineurium. Both nerves contained myelinated and unmyelinated fibers of various sizes intermingled with each other. Out of the four age groups (12-20y, 21-40y, 41-60y and >61y) the younger groups revealed isolated bundles of small thinly myelinated axons. The total number of myelinated fibers in the TN and AN at various ages ranged from 1100-3000 and 1600-7000, respectively. There was no significant change in the cross-sectional area of the nerves or the axonal area of the myelinated nerves across the age groups. However, myelin thickness increased significantly in the AN with aging (one way ANOVA). The present study provides baseline morphometric data on the human TN and AN at various ages.
Highlights
Aging can affect vision in many ways
Elevated intracranial pressure causing brainstem displacement can lead to compression of the AN in the Dorello canal and this is responsible for up to 30% cases of isolated VI nerve palsy observed with pseudotumor cerebri [13]
Detailed knowledge of each anatomical segment and components of the ocular motor nerves is crucial for investigations, diagnosis and management of various conditions arising in these nerves
Summary
Aging can affect vision in many ways. Cataract and presbyopia are the commonest manifestations of aging in the general population. The oculomotor (ON), trochlear (TN) and abducens (AN) nerves work in tandem to dynamically control the muscles that either fix or move the eyeball in order to attain the optimum view of the world around Diseases affecting these nerves can cause double-vision, altered visual acuity or a general disorientation in space. These nerves may be involved either individually or collectively in various physiological or pathological conditions that result in defects of gaze and/or of the accommodation reflex. Reliable baseline morphological data that describes size, numbers and distribution of nerve fibers, derived from un-biased stereological methods [18], are required from across the ages to assess changes that may define either diseases or abnormal conditions. To our knowledge and after extensive search of scientific literature we did not find any study that described the TN and AN at various ages and we have undertaken a detailed morphological study of the connective tissue organization and fiber composition of these nerves using un-biased stereological methods
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