Optic canal: a CT-based morphometric study in north Indian population

Lesser wing of sphenoid joins the body of sphenoid by two roots which are separated by the optic canal. Optic canal is a communicating channel between orbit and middle cranial fossa, forming a passage for the ophthalmic artery and optic nerve. Considering the delicate neurovascular structure that traverses the narrow confines of the optic canal, knowledge of the morphometry, anatomical variations of this canal and their relations becomes an important part of diagnostic medicine and surgical management. The aim of the study was to conduct morphometric analysis and document variations of optic canal in north Indian population and study its sexual dimorphism. Type of Study Cross-sectional study. Sample size 200 Computerized tomography head axial section scans from picture archiving and communication system. The computer-assisted measurements of optic canal parameters were obtained on both right and left sides. Statistical analysis Unpaired t-test, paired t-test and ANOVA were applied. Optic canal of 110 males, 90 females were examined. The overall cranial opening diameter, orbital opening diameter and length of optic canal in males were 4.34 ± 0.74, 3.17 ± 0.58 and 9.81 ± 1.62 and in females they were 3.99 ± 0.71, 2.97 ± 0.59 and 9.38 ± 2.02 respectively. Significant gender difference is seen in cranial and orbital opening diameter, and distance from midsagittal plane. No statistical significant difference is observed in right and left side parameters. A thorough knowledge of the normal dimensions of optic canal is important for evaluation and management of cases of optic nerve decompression.


Introduction
Optic canal (OC) lies between the two roots of lesser wing of sphenoid which are attached to the body of sphenoid by a thin, flat anterior root and thick triangular posterior root.It form a communicating channel between the orbit and middle cranial fossa and transmits the optic nerve, ophthalmic artery, meningeal sheaths and sympathetic nerve fibres [1].
Studies have been done in the past regarding dimensions of OC like its length, diameter of cranial and orbital openings of the canal, area of the canal, height and other parameters [2][3][4][5][6].Our study focuses on describing variables in different sex and age groups.
The comprehension of the typical variants will be useful in surgical field to correct optic nerve decompressions (trans-cranial and trans-sphenoidal routes), lesions/tumors of the orbit, schwannoma of optic nerve, neoplasms, osseous lesions and ophthalmic artery aneurysms and provide a better and safe route to access this region [7][8][9][10][11].
When one considers the delicate neurovascular structures that traverse their narrow confines, knowledge of the anatomical variations of these canal and their relations become an important part of diagnostic medicine and surgical management.The aim of the study was to conduct morphometric analysis and document variations of optic canal in north Indian population and study its sexual dimorphism.

Materials and methods
This is a cross sectional study in which 200 Computerized tomography (CT) head in axial sections were examined.Institutional Ethics Committee clearance has been taken.These scans were collected from picture archiving and communication system of a tertiary care hospital catering to a large North Indian population.We studied high-resolution images from a Philips Brilliance 64 channel multi-detector CT scanner where we visualized spiral head CT scans in axial plane between 0.9 and 2.00 mm thickness (range of detector: 0.55-7.5 mm).

Sample size calculation
Sample size was calculated estimating for two sample comparison of means [3].
Test Ho: m1 = m2, where mi is the mean in population 1, and m2 is the mean in population 2.

Assumption
Estimated required sample size: n1 = 94, n2 = 94.So minimum 94 scans of each male and female subjects was required.

Subject selection
Inclusion: We had selected every 10 th scan from last 5 year PACS system of our Institute.CT head of 110 males and 90 females, with age ranging from 1 to 80 years without any anomaly which can change anatomy of foramen were included.The foraminal parameters were obtained on both right and left sides using computerassisted measurements.

Variables measured
Width of cranial opening, orbital opening and width at the centre of OC-a line was drawn between inside surface of medial and lateral wall such that, the points selected were analogous to the data under consideration.For example: for calculation of mid-width: mid points of medial and lateral walls were marked, and a line was drawn between them to measure length (Figs. 1 and 2).
Length of the OC-centre of the orbital and cranial openings of OC were marked and a line was drawn in between (Fig. 2).Length of intracanalicular portion of optic nerve was taken as the length of optic canal measured from centre of orbital opening to the centre of  cranial opening as previously documented by Slavian et al. [6].Distance from Mid-Sagittal Plane-the mid-point of the inner surface of medial wall was marked, and a perpendicular was drawn to the MSP (Fig. 2).

Statistical analysis
Descriptive statistics were used to analyse the results.Discrete (categorical) data were summarized in proportions and percentages (%), while continuous as mean and SD.Arithmetic mean, standard deviation, Unpaired and Paired t-test, One way ANOVA were calculated in the present study.In our study, we have taken 95% confidence interval and P value < 0.05-Significant, < 0.01-Highly Significant.

Results
A total of 400 optic canals were analysed (220 males and 180 females), with age ranging from 1 to 80 years.Observing CT scan in axial plane, lateral wall of OC was seen formed laterally by the anterior clinoid process and medially by the sphenoethmoidal cell of the posterior ethmoid sinus and the sphenoid sinus.We observed sphenoethmoidal air cell also known as onodi air cell in 4% cases unilaterally and 1% cases bilaterally.The overall cranial opening diameter, orbital opening diameter and length of OC in males was 4.34 ± 0.74, 3.17 ± 0.58 and 9.81 ± 1.62 and in females was 3.99 ± 0.71, 2.97 ± 0.59 and 9.38 ± 2.02, respectively (in mm).The mean ± SD of all the variables measured are shown in Table 1.
Statistically significant difference was seen among males, between left and right side for width at mid length (p value = 0.039).In females, no significant difference was seen between left and right side parameters of OC.
Statistically significant difference was seen between males and females in width of cranial side (p value = 0.001), and of orbital side (p value = 0.013) and in the distance from MSP (p value < 0.001).
Comparative age-related morphometry in males and females is shown in Table 2.

Discussion
Optic canal is the narrowest part at the apex of orbit that allows for the passage of optic nerve from orbit to the middle cranial fossa.Accurate knowledge of its dimensions in males, females on the right and left side and in different populations is imperative for evaluation and management of different optic neuropathies.Radiographic description of OC was first described by Winckler in 1901, and its oblique position was observed by Rhese [12].Recent advances in endoscopic endonasal Optic nerve decompression and other surgical approaches to optic nerve as in transfrontal craniotomy, orbitotomy and trans-ethmoidal and spheno-ethmoidal surgeries have emphasized the need for precise information about the morphological and metrical details of the optic canal, specific to each population [11][12][13][14].This is especially in light of the fact that no age-specific studies have been found in literature to the best of our knowledge.Shape of optic canal OC shape has been quoted in previous studies as hour-glass, cone or cylindrical shaped [15,16].In the present study, optic canal was observed to be cylindrical in shape with width slightly broader towards its cranial end in all the cases.Width at the centre of the optic canal was more on the right side than the left in males and the difference was statistically significant.
Orbital and cranial openings of optic canal Statistically significant difference was seen in width on cranial side between males and females (p value = 0.001), width on orbital side (p value = 0.013) and in the distance from MSP (p value < 0.001).These dimensions were smaller in females than in males and orbital opening was seen as the narrowest point of OC.Previous studies have documented similar difference in the volume and area of OC between females and males [17][18][19].The smaller orbital opening diameter in females could predispose to partial or complete obstruction of subarachnoid space as seen in patients with papilledema, normal tension glaucoma and thus rapid progression of disease seen more commonly in females [17,[19][20][21][22].These measurements are extremely important in light of the fact that OC connects the intracranial cerebrospinal fluid space with the subarachnoid space of the intraorbital part of optic nerve with the OC subarachnoid space size being the narrowest [23].Recent observations suggest that circulatory dysfunction of cerebrospinal fluid may be a factor responsible for normal tension glaucoma and papilledema [24][25][26].Pircher et al. described narrower OC area in Caucasian normal tension glaucoma patients compared with subjects without optic nerve disease.The importance of documenting OC dimensions in these patients is more as with a narrow optic canal there may be a discontinuity of the cerebrospinal fluid flow between the intracranial and orbital subarachnoid space in these patients [17].
OC Length In patients with traumatic neuropathy the prior knowledge of length of OC is important to decide what length of OC must be opened [27][28][29].Length of OC as seen in previous studies varies from 5 to 15 mm [26,29] (Table 3), comparable to the present study.
Similar to Berlis et al., [4] in present study, we measured the distance from mid-point of medial wall of optic canal to mid-sagittal plane and it was 10.91 ± 1.55 in males and 9.70 ± 1.99 in females.
General morphometrical parameters of OC Comparing the morphometric results of our study with previous studies, values of all parameters in the present study were slightly lower than the studies done in the past.The only comparable study was a CT-based study done on Indian population by Kalthur et al. [3].The lower values of parameters in the present study may be because of racial differences and that most of the previous studies have been done on dried skull (Table 4).
Right-left asymmetry Zhang et al. commented that optic canal asymmetry is not uncommon and documented larger dimensions of right optic canal as compared to the left.Asymmetry of OC may affect risk factors of papilledema severity and explain cases of unilateral or asymmetric papilledema and possibly asymmetric glaucoma [28].Contrarily, in the present study, no statistically significant differences were seen on right and left side.Sinanoglu et al. [6] in their study also observed no asymmetry in laterality in the same sex.
Anatomical variations Studies in the past have documented a variation called as keyhole anomaly which is seen as a result of the absence of the floor at cranial opening (3.3% cases).Another variant documented is figureof-eight OC, with a bony spicule creating a separate canal for the ophthalmic artery [4].The only variant observed in our study is the Onodi air cell, an anatomical variant of the paranasal sinuses, important due to its close proximity to the optic nerve, observed in 5% of scans in our study.
Morphometric data obtained from out study shows that despite variability there are approximate anatomic standards derived from our population that can be used in neurosurgical practice.Since injury to any part of the optic pathway result in visual defects, knowledge of the gross features of the OC and related structures is very important for surgeons.Also required is the accurate knowledge of both openings of optic canal its, length and shape before performing any surgical procedure on the canal for decompression, removal of tumour or approaches to sella.

Limitations
In the present study, neuro-arterial relations of OC have not been assessed.Also distance of OC from landmarks like nasal bone tip, middle point of tuberculum sellae and root of columella nasi which are useful for endoscopic surgeries have not been documented.
Strength The present study provides a comprehensive morphometry of OC in our population, particularly in relation to different age groups, which has not been reported in any previous studies to the best of our knowledge.

Conclusions
In males, values of all measured parameters of OC were higher in comparison with female and the differences were statistically significant.No statistical significant difference is observed in right and left side parameters.A thorough knowledge of the normal dimensions of OC is important for evaluation and management of disease involving the canal.This study provides a clear understanding of OC and its morphometric variations in the North Indian population.

Fig. 1
Fig. 1 CT head axial plane showing optic canal

Table 2
Comparative age-related morphometry in males and females

Table 3
Comparison of length of optic canal with previous studies

Table 4
Morphometry of optic canal measured in different studies