J. Anat. (1990), 170, pp. 199-201 199 With 4 figures Printed in Great Britain Anatomical Note

The superficial origin of the trochlear with special reference to its vascular relations

K. K. BISARIA*, I. C. PREMSAGARt, P. K. LAKHTAKIAt, R. C. SAXENA* AND S. D. BISARIA§ * Department of Anatomy, t Department of Neurosurgery, $ Department of Orthopaedic Surgery, K. G. Medical College, Lucknow, U.P. and § Department Of Ophthalmology, Vivekananda Polyclinic, Lucknow, U.P. India (Accepted 14 November 1989)

INTRODUCTION A full description of the roots of the is not commonly found in the literature. Gardner, Gray & O'Rahilly (1960), Hamilton (1976), Romanes (1964), Snell (1981), Hollinshead & Rosse (1985) and Williams, Warwick, Dyson & Bannister (1989) do not mention the possibility of two roots of origin of this nerve. Crosby, Humphrey & Lauer (1962) and Truex & Carpenter (1969) have mentioned that the nerve may emerge as a single root while Paturet (1964) and Nathan & Goldhammer (1973) have mentioned the possibility that more than one root may be present. The relations of the trochlear nerve to the adjoining blood vessels have not been described.

MATERIALS AND METHODS For the present work 162 trochlear of 90 formalin-fixed brains were studied (in 18 brains only one of the two nerves could be studied as the other was torn during the removal of the brain from the cranial cavity). The origin of the nerve and its relationships with neighbouring blood vessels was noted.

RESULTS In all the 90 brains, the trochlear nerves arose in the usual situation. In 111 nerves out of 162, the nerve had a single root; in 51, double rootlets were present, the rootlets being up to 4 mm apart (Fig. 1). In 14 nerves out of the 51 with double nerve roots, the two roots were of different thickness while in the remaining 37 nerves, the rootlets were almost equal in diameter. When two rootlets were present they were usually orientated alongside one another in the transverse plane but occasionally they lay one behind the other or were arranged obliquely. The distance from the point of origin of each rootlet to its union with the other varied from 1 to 7 mm. Fifty nerves were closely related to the superior cerebellar artery which ran anteroposteriorly over the nerve before ramifying on the superior surface of the cerebellum (Fig. 2). In 20 cases the superior cerebellar artery divided into two branches (Fig. 3) while in 60 cases the superior cerebellar artery was deep to the nerve (Fig. 4)...... : ::.... :. :....: :.: ...:;.: ..i .....

1 .: r ....

Fig. 1. The rootlets of the trochlear nerve on the right side are of different thickness (arrows) and on the left side of equal thickness (arrows). Note that the rootlets arise along an oblique line. Fig. 2. The superior cerebellar artery (arrow) crossing over the trochlear nerve on the left side. Fig. 3. The right trochlear nerve (arrow) passing in between the two branches of the superior cerebellar artery. Fig. 4. The left superior cerebellar artery (arrow) lying deep to the trochlear nerve. Origin of trochlear nerve 201

DISCUSSION Sano (1941), Huber et al. (1943) and Crosby et al. (1962) have demonstrated that the nucleus of the fourth nerve is split into two, a main cranial nucleus and a smaller accessory one more caudally located. Pearson (1943) has mentioned that the accessory nucleus is inconstant and may be bilateral or unilateral. Tozier (1912), Pearson (1943) and Crosby et al. (1962) have commented that sensory as well as proprioceptive fibres may be present and it may be that these separate nuclei account for the double roots outside the brain in the specimens studied. The relationship of the trochlear nerve with the superior cerebellar artery is very variable. The most important relationship, from a functional point of view, is that in which the nerve lies between two branches of the superior cerebellar artery. Sunderland (1948) has stressed that even a normal vessel which is congenitally displaced in position may compress the nerve. Such contact may produce a defect in the conduction of impulses in the nerve which in turn, might be expected to produce a superior oblique palsy. This paper was presented at the first Afro-Asia Oceania Congress of Anatomists, 29 August to 3 September 1988, New Delhi, India.

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