J. Anat. (1988), 159, pp. 113-116 113With 1 figurePrinted in Great Britain

Duplication of optic canals in human skulls

REWA CHOUDHRY, S. CHOUDHRY* AND C. ANAND

Department of Anatomy, Lady Hardinge Medical College and S.S.K. Hospital and*ICARE, New Delhi, India

(Accepted 13 October 1987)

INTRODUCTION

Bilateral duplication of the optic canal is a very rare anomaly (Warwick, 1951). Hestates that early workers reported duplicate canals but did not record them withexamples. They found that the larger canal carried the optic nerve with the meningeswhile the smaller transmitted the ophthalmic artery. White (1924) and Whitnall (1932)also documented this condition. Keyes (1935) saw it unilaterally present in 5 cases.A search of the literature showed that this condition is rare, and rarer still is the

bilateral duplication of the canals. Bilaterally duplicate canals have been methodicallyreported only in four instances - by Visconti (1885), Zoja (1885), Le-Double (1903) andWarwick (1951).

OBSERVATIONS

We found three adult human skulls showing duplication of the optic canals ofwhichtwo skulls had bilateral, while one had unilateral duplication. For convenience ofdescription the skulls are referred to as Skulls I, II and III.Each duplicated canal comprised a larger canal, which was in the usual position and

is referred to as the main canal. The smaller canal was inferolateral to the main canaland is termed the accessory canal. The measurements of the optic canals and theiropenings were taken from the cranial and orbital ends. Both canals were consideredas a single entity for the purpose of measurement. Each main canal was much longerthan the accessory one and its length was measured along the medial wall of the maincanal. The distance between the medial ends of the right and left orbital and cranialopenings was also measured. These measurements for the three skulls are tabulated(Table 1).

It was not possible to measure the length and thickness of the septa separatingthe duplicate canals or the lateral wall of the accessory canal because of theunapproachability of the region.

Site and direction of the canalThe main optic canal and its accessory counterpart were placed parallel to each

other. The main canal was directed posteromedially and upwards and was continuouswith the chiasmatic sulcus. The accessory canal on the cranial side was in line with theanterior end of the sulcus for the internal carotid artery.The cranial opening of each canal was horizontally oval. On the orbital side the long

axis of the opening was in an oblique direction, the axis passing downwards andmedially at an angle of about 600 with the horizontal.

REWA CHOUDHRY, S. CHOUDHRY AND C. ANAND

Table 1. Dimensions of the canal in mm

Skull I Skull II Skull III

Right Left Right Left Right Left

Measurement Orbital 7 x 4 6 5 x 4 7 x 5 6 5 x 5 6x4 6 x 4of openings Cranial 6x35 6x35 6x4 6x4 6x4 7x5Length of canal 11 11 10 9 10 10Distance between Orbital 25 26 30openings Cranial 12 16 18

The septa and the lateral wall of the accessory canalIt was found that the thicker the septa between the canals, the longer it was in

its anteroposterior measurements. In Skull I both septa were thick with greateranteroposterior measurements (Fig. 1 a, b). In Skull II the septa were very thin,represented merely by spicules of bone, the left one interrupted in the middle. Skull IIIhad a moderately thick septum on the right side whereas on the left side a projectionon the inferolateral wall resulted in the formation of a groove in the floor of thecanal.

In all three skulls the lateral wall of the accessory canal showed little variation in itsanteroposterior length and was smaller than the lateral wall of the optic canal in anormal skull. The wall seemed deficient at the cranial end.

These skulls had a tendency to excessive bone formation, shown as prominentmiddle clinoid processes, and the presence of sutural bones and pterygospinousbridges. A projecting middle clinoid process was found on the right side in Skull I. Onthe left side, in Skull III, it was united with the anterior clinoid process, forming acarotico-clinoid foramen.

DISCUSSION

Duplication of the optic canal in human skulls was a chance finding during routineteaching in this medical college. These skulls were of North Indian origin. The sex andage of the skulls are not known.

It was in the late nineteenth and early twentieth century that authenticated recordsof such cases were made. White (1924) reported 3 cases in the newborn while Whitnall(1932) made a photographic record of this condition. Bilateral duplication of opticcanals has only been recorded by four authors. Visconti (1885) saw duplication in twoskulls out of which only one was bilateral. Zoja (1885) reported five cases out of whichone was bilateral. Le-Double (1903) wrote of a single bilateral case. Warwick (1951)recorded one male child having the bilateral condition observed at postmortemexamination.The measurements of the canals of these adult skulls cannot be compared with those

recorded by Warwick (1951) as the age of the child in whom he noted the bilateralcanals was only 21 months. The length of the duplicate canals along the medial wallin this study was 9-11 mm, the range in normal skulls is 10-12 mm (Wolff, 1976). Le-Double (1903) speculated that the division of the optic canal is due to the ossificationof the dura mater covering the optic nerve. This does not appear to be the probablereason as most of the cases reported were newborn and children. Wolff (1976)attributed the condition to ossification of fibrous tissue between the dura matercovering the optic nerve and the ophthalmic artery. In some skulls the inferolateral

114

Optic canal duplication 115

Fig. 1 (a-b). (a) Duplicate optic canal seen from the right orbit. A thick septum divides the canal.Opening seen on the lateral side is the superior orbital fissure. (b) Cranial view of a left duplicatecanal.

116 REWA CHOUDHRY, S. CHOUDHRY AND C. ANAND

wall showed a small projection resulting in the formation of a groove in the floor ofthe canal. Keyes (1935) reported it in 36 out of 2187 skulls examined. It is postulatedthat the bony projection, when large, could result in the division of the optic canal intotwo parts, the upper and larger for the optic nerve and the lower for the ophthalmicartery. Why and how it occurs still remains a matter of speculation.

SUMMARY

Three human skulls showing duplication of the optic canal are reported. Two hadbilateral duplication while one had duplication on one side only. The measurementsof the whole canal were within the range of normal. Septa of variable thicknessseparated the main and the accessory canals in these skulls and the skulls also showeda tendency to excessive bone formation.

We thank Mr Vasant Thakur (of the Vasant Commercial Photographer) for thephotographs.

REFERENCES

KErES, J. E. L. (1935). Observations on four thousand optic foramina. Albrecht v. Graefes Archiv furOphthalmologie 13, 538-568.

LE-DouBLE, A. F. (1903). Traite des Variations des Os du Crane de l'Homme p. 372. Paris: Vigot Freres(quoted by Warwick, 1951).

VISCONTI, A. (1885). Cited by Le-Double, 1903 (quoted by Warwick, 1951).WARWICK, R. (1951). A juvenile skull exhibiting duplication of the optic canals and subdivision of the superior

orbital fissure. Journal ofAnatomy 85, 289-291.WHITE, L. E. (1942). An anatomic and X-ray study of the optic canal. Boston Medical and Surgical Journal

189, 741-748 (quoted by Warwick, 1951).WHITNALL, S. E. (1932). The Anatomy of the Hwnan Orbit, 2nd ed., pp. 53, 313. London: Oxford University

Press.WOLFF, E. (1976). Anatomy of the Eye and Orbit. 7th ed., vol. 15. London: H. K. Lewis.ZOJA, G. (1885). Spora il foro ottico doppio. Bollettino scientifico, Pavia, 7, 65-69 (quoted by Warwick,

1951).