Axial length evaluation of eye ball by magnetic resonance imaging in Odisha population

Background and Objective: The ocular axial length (AL) changes in refractive errors, ocular, orbital and systemic diseases. Magnetic resonance imaging provides an accurate ocular biometry. Early diagnosis of change of ocular axial length is important to prevent visual compromise. Present study is undertaken to determine the axial length of both eye balls by use of magnetic resonance imaging and to find out possible association between the ocular axial length with age and sex of healthy individuals. Methods: Retrospective study of ocular axial length was carried out in our institution. About 200 numbers of cases with normal MRI findings of brain and orbit were retrospectively analyzed by two senior radiologists independently keeping in view of inter observer variability. Study included T2 weighted axial imaging of brain and orbit from January 2017 to June 2019. Result : The normal axial length of the right and left globe of male person are 23.16±0.840 mm and 23.139± 0.830 respectively. The normal axial length of the right and left globe of female person are 22.55±0.839 mm and 22.55±0.514 respectively. Male cases have slight higher ocular axial length than female individuals and the difference is statistically insignificant. Conclusion : The normal ocular axial dimension will help the clinician and radiologist to quantitatively evaluate patients with abnormal ocular size and/or refractive errors.


Introduction
Cataract extraction and artificial intraocular lens (IOL) implantation is the most commonly done procedure nowadays. Accurate assessment and calculation of IOL power is essential for desired post-operative refraction. Axial length, corneal curvature and anterior chamber depth data are required for accurate intraocular lens power calculations. In highly myopic adult patients, the axial length continues to increase. Older individuals with posterior staphyloma are more susceptible to having a larger increase in the axial length [1,2].
The size of eye ball changes due to eyeball trauma, cancer, congenital glaucoma, retinoblastoma, and some other disorders. In case of congenital microphthalmos, the eye ball is visibly small. But in borderline cases the distinction between the normal size and the pathologically small size eyeballis difficult for which the normal biometry of eye is essential [3]. The average axial length (AL) of newborn's eyeball is about 16 millimeters in diameter which grows slightly to a length of approximately 19½ millimeters. The large scale studies on the growth of the ocular components suggest that the eye has reached its adult emmetropic axial length by the age of 13 years.In the adult, axial length remains practically unchanged.However, the AL changesin presbyopia especially after the age of 40 [4]. Previous studies in eastern Indian population suggest the axial length of eye ball in emmetropic right and left eye as 23.35 and 23.26mm respectively [5].
The axial ocular length varies due to influence of race, ethnicity, and genetics [6]. The AL is an important dimension of eye than other diameters and is larger in myopia and smaller in hypermetropia [7]. Several methods have been described for axial length measurement such as laser doppler interferometry, sonography, computed tomography and magnetic resonance imaging. The sonographic evaluation of eye is operator dependent and requires contact of probe over

Tropical Journal of Ophthalmology and Otolaryngology
Available online at: www.medresearch.in 234|P a g e eye and may introduce infection.Computed tomographic evaluation of eye involves ionizing radiation. However, MRI of orbit is safer and accurate modality for evaluation of optic nerve, intraocular and orbital pathology. Axial length of eye ball is an important dimension of all ocular biometry. AL is measured at the level in lens in magnetic resonance imaging (MRI) objectively form anterior to posterior pole. MRI of orbits provides accurate axial length as compared to A-scan ultrasonography. Axial length measurements with MRI match reliably with axial lengths measured by Ascan ultrasonography [8].
There is no standard normative reference data of AL in eastern Indian population especially people of Odisha. We carried out this retrospective study to obtain the normative data of AL and to find out any sexual dimorphism.

Subjects and Methods
Setting of the study: This study was done in Department of Radiology, Kalinga Institute of Medical Science, KIIT university, Bhubaneswar, Odisha. Kalinga Institute of industrial technology (KIIT) university is a large private university in Odisha.
Duration and type of study: Thisretrospective study was performed by analyzing MRI data storage in Department of Radiology from January 2017 to June 2019.
Sampling methods: Data were collected by consecutive sampling from image data storage of Picture Archiving and Communication system (PACS) of thehospital.

Sample size and calculation;
Out of 250 cases, 200 cases of MRI examinations were included in study with age range of 10 to 80 years old.
Inclusion Criteria: MRI studies with good image quality are included with no history of ophthalmic pathology or refractory error.
Exclusion Criteria-About 50 studies were discarded due to motion artifact and asymmetric axial images.
Date collection procedure: T2-weighted spin echo sequences in the axial plane were studied. T2-weighted axial images were used for all ocular measurements at lens level. The ALof the eyes was obtained at lens level from posterior aspect of corneal apex to posterior globe wall-fat interface (Figure 1). Image analysis of MRI brain and orbits was done by two senior radiologists and AL evaluation was done by the average of measurements determined by both radiologists.
Data analysis: Statistical analysis was done in SPSS software version 22.0 for Microsoft Window. Values were expressed in terms of the mean and standard deviation (SD).. Correlation between different parameters was tested by Pearson correlation test. The P < 0.05 is considered statistically significant. Ethical consideration & permission: Ethical committee permission was not needed for this study due to retrospective non-interventional nature of study as per institutional research board decision.

Discussion
The first edition of Henery Gray's "Anatomy: Descriptive and Surgical" of 1858 mentioned that "the axial length of the eyeball measures about an inch and exceeds the transverse diameter by about a line" [9]. In 1912, the average measurements of the eye ball diameters were accepted by various authors were 24.26mm for the anteroposterior diameter [10]. In 1970, it was known that axial length differs in cases with myopia (24.61±1 mm), emmetropia (23.40±1.38mm), and hypermetropia (22.53±1.02mm) [11]. It is a general statement by the manuals of ophthalmology thatthe eyeball diameter differs among adults by only one or

Original Research Article
Tropical Journal of Ophthalmology and Otolaryngology Available online at: www.medresearch.in 236|P a g e two millimeters [12]. The AL of the globe is the distance from the corneal apex to the retinal pigment epithelium/Bruch's membrane [13]. Among Odisha population, the mean ocular AL was found to be 22 [21]. The difference of AL measurement in all above studies could be due to racial variation and environment influence [6].
Detorakis et al. [22] did not find any statistically significant difference between data for the right and left globes. Salaam et al reported no significant difference of AL between globes in their sonographic evaluation of eyes in Nigeria population [23].
There is no significant difference in right and left eye ball AL in the present study. Some studies shows sexual dimorphism with males having higher values than females [25,26] [25]. In the present study, there was no correlation between participants' age and the ocular AL. The studies conducted by Lee et al [18] (Beaver Dam Eye Study) and Wong et al [25]. (Tanjong-Pagar study) showed that the AL decreased with increasing age while a central Indian Eye and Medical Study by Nangia et al [14] showed that the AL increased with increasing age. Similar observation was also noted by Wickremasinghe et al [17].
The contrast observed between our findings and that of these authors may be due to racial variations, environmental influences and/or technique employed in the studies.  [26].

Limitations of the study:
The sample size is smaller. A large prospective study is needed for better normative date and statistical analysis. All the case should be screened by ophthalmologist to rule out refractive errors and other ocular abnormality before inclusion of in the study. Thinner slice and volumetric MRI data image of eye ball should be obtained in MRI scanner for accurate assessment of ocular biometry.

Conclusion
AL of globe with males having slight higher values than female which is statistically insignificant. The mean AL obtained in this study is matching with the study of central India and differs from the values obtained in China and Britain which further confirms that racial variation and environmental influences affect the AL of the globe as supported by other studies also.This normative data will help the ophthalmologist, clinician and radiologists for better evaluation of ocular pathology.

What this study adds to existing knowledge?
This study will help radiologist and ophthalmologist for assessment of refractive error, congenital microphthalmia, prognostication of high grade myopia and retinal detachment and other ophthalmological disorder in Odisha population as there is no normative data of ALof orbit in population of Odisha.