An anatomical study of the central myelin portion and transitional zone of the oculomotor and abducens nerves
Authors:   , Quanchareonsap Wiyada 
Published: September 2023

Neurovascular syndrome is a dysfunction of an individual cranial nerve which is compressed by vessels. Several neurovascular compression syndromes are well known such as trigeminal neuralgia, hemifacial spasm, and glossopharyngeal neuralgia. Having neurovascular conflicts at oculomotor nerves and abducens nerve cause ocular neuromyotonia and abducens nerve palsy. Site of compression is still controversial because of unclear definition. The term root entry/ exit zone (REZ) is defined variously and used interchangeably with transitional zone making it hard to compare between studies. It is generally thought to be at transitional zone which is defined as junction between central and peripheral nervous system. Lately, central myelin and transitional zone of trigeminal, facial, and glossopharyngeal nerves has been studied with clear definition. However, microanatomical knowledge about oculomotor and abducens nerves is limited. The aim of study was to evaluate microanatomy of central myelin and transitional zone of both nerves.

Twenty-nine oculomotor and 53 abducens nerves were obtained from 46 cadavers’ brains but only 10 of each nerve were included. These specimens were serially sectioned with 5 µm thickness, stained, and photographed under the microscope. Five distances were measured: diameter of cranial nerve, extent of central myelin, diameter of transitional zone, depth of transitional zone, and length of central myelin on the far side of the brainstem. Length of central myelin was 2.75 ± 0.83 mm in oculomotor nerve and 1.66 ± 1.39 mm in abducens nerve. Longest central myelin length was 4.30 ± 1.26 mm in oculomotor nerve and 1.88 ± 1.40 mm in abducens nerve. Depth of transitional zone was 0.23 ± 0.07 mm in oculomotor nerve and 0.16 ± 0.08 mm in abducens nerve. Length of the central myelin portion on the far side of the brainstem was 1.47 ± 0.90 mm in oculomotor nerve and 1.42 ± 1.50 mm in abducens nerve. Positive weak correlation between depth of transitional zone and length of central myelin of each nerve bundle in oculomotor nerve (r +0.310, p<0.05) and abducens nerves (r +0.413, p<0.05) were found. Depths of transitional zone varied between nerves and nerves bundles. Transitional zone usually takes up to 20% of central myelin. For oculomotor nerve, longest central myelin was seen on first nerve bundles and then length of central myelin was gradually decreased from lateral to medial side. For abducens nerves, morphological patterns were classified into type A-D which type A and B tend to have longer segment of central myelin than type C and D. Also, longer central myelin tends to have longer transitional zone in both nerves.

Detail of microanatomy of central myelin and transitional zone is clearly stated. Clinicians could benefit from well-defined parameters which help to locate lesion precisely and understand etiology more. Moreover, it is previously known that peripheral nervous segment is more resistant to compression compared to central nervous segment so knowing microanatomy of cranial nerves could provide safer surgeries.