STUDY OF OPTIC NERVE SHEATH DIAMETER IN NORMAL NEPALESE ADULTS USING ULTRASOUND Affiliation

Introduc on Though invasive intracranial devices are gold standard to calculate intracranial pressure (ICP); these are not without any complica ons. Non-invasive measurement of ICP by Ultrasonography could be a safe and portable technique. Objec ves The objec ve of the study was to measure and compare values of op c nerve sheath diameter of both eyes in healthy Nepalese adults. Methodology A prospec ve cross-sec onal study of healthy adult Nepalese volunteers was performed using a 7.5 MHz linear Ultrasound probe on the closed eyelids; op c nerve sheath diameter (ONSD) was measured 3 mm behind the globe in each eye. Results Op c nerve sheath diameter (ONSD) of both eyes was measured in 100 healthy volunteers of age ranged from 15 to 75 years with a mean of 30.21 ± 14.05 years. There were 18 (18%) male and 82 (82%) female. ONSD for right eye ranged from 3.20 to 4.90 mm with mean of 4.10 ± 0.50 mm and le eye from 3.20 to 4.80 mm with mean of 4.22 ± 0.49 mm. P value for right and le eye ONSD (P = 0.06) and male and female (P = 0.12 and 0.85 for right and le ONSD respec vely) were within normal limits. ONSD has no correla on with age (P = 0.27 and 0.27 for right and le ONSD respec vely). Conclusion Mean of op c nerve sheath diameter (ONSD) is 4.10mm and 4.22 mm for right and le eye respec vely. There is no sta s cal significant difference in mean of ONSD between right and le eye.


INTRODUCTION
The op c nerve, is a second cranial nerve (CNII), derived from embryonic re nal ganglion cell.Also known as the nerve of vision because it carries the electrical impulses from re na to brain.It is myelinated nerve wrapped with three meningeal layers of which subarachnoid space is 1-3 con nuous with that of the brain.Hence, change in intracranial pressure (ICP) causes reflec ve change in intraorbital subarachnoid space pressure at the same 1,4,5 me.
Gold standard for ICP measurement are Invasive 6,7 intracranial devices.However, these devices may invite complica ons like infec on, hemorrhage and dislodgement.Besides, it cannot be performed in cases of 8-10 bleeding disorders.
There lies the significance of

RESULTS
Total of 100 subjects were included.Among them, 18 (18%) were male and 82 (82%) were female.Age ranged from 17 to 75 years, with a mean of 30.21 ± 14.05 years.
ONSD value for right eye ranged from 3.20 mm to 4.90 mm with mean of 4.10 ± 0.50 mm and median of 4.1 mm.ONSD value for le eye ranged from 3.20 mm to 4.80 mm with mean of 4.22 ± 0.49 mm and median of 4.4 mm.There was no sta s cally significant difference of mean ONSD between right and le eye (P = 0.06).This study showed no sta s cally significant associa on of ONSD between male and female (P = 0.12 and 0.85 for right and le ONSD respec vely).No sta s cally significant correla on was found between ONSD and age (P = 0.27 and 0.27 for right and le ONSD respec vely).

DISCUSSION
Most of the neurosurgical pa ents usually died of raised ICP; Several studies have observed varia on of ONSD in normal subjects by using ultrasonography as shown in Table 1

RECOMMENDATION
Should be used as a bedside screening tool in all pa ents suspicious of raised ICP.These calculated values of ONSD of healthy Nepalese adults would be used as a baseline for future reference in pa ents with raised ICP.In concordance with previous studies, our study also concluded no sta s cally significant associa on between ONSD and age, gender and side.

LIMITATIONS
Study didn't measure ICP directly which would have allowed us to compare and validate how well the ONSD and ICP would have correlated with each other.Study did not include any subjects below 16 years of age.

25 , 26 head
injury is not an excep on.An increase in ICP impedesthe cerebral blood flow (CBF) leading to27,28 ischaemia.Secondary brain-injury due to raised ICP is a poor outcome predictor a er trauma c brain injury.Treatment algorithms focuses on preven on of these 28 insult.Neurological intensive care is vital for immediate treatment a er picking up subclinical causes of secondary brain 29 injury.ICP monitoring is beneficial in many studies to 30 prevent surgery in the se ngs of normal ini al CT scan.Mortality has been decreased with effec ve treatment of 26 raised ICP.For effec ve u liza on of ONSD in the situa on of raised ICP, normal reference value is required.

KC B et al 4 .
22 ± 0.49 mm for the le eye was similar to the values observed by Maude et al in Bangladesh, Shrestha in Nepal 5,23,32 and Chan et al in Hongkong.However, our values were lower than what has been reported by Ali et al in Pakistan 20,31 and Bauerle et al in Germany.

although maximum of 5.9 mm have 19 been quoted. Normal values of ONSD can be used to infer 17,23 increase in ICP in medical prac ce. ONSD can vary with ethnic origin. Asian countries have found different normal
5,23,24in adults (3.7-4.7 mm in Hong Kong, 2.9-5.3 mm in5,23,24Korea and 4.25-4.75mm in Bangladesh).Besides, there is a paucity of local data of ONSD in Nepalese adults, so this study aims to evaluate the baseline value of ONSD among healthy Nepalese adults.Shenzhen Mindray Biomedical Electronics Co., Ltd., Shenzhen-518057, P.R. China) by a single sonographer.As low as reasonably achievable (ALARA) ultrasonography technique was used for the safety of the volunteers.Volunteers were placed in supine posi on with closed eyelid.A er applying gel at the temporal side of eyelid, transducer was kept horizontally.Transducer was adjusted ll appearance of hypoechoic op c nerve behind the globe.ONSD of both eyes were measured at 3 mm behind the globe taking transverse diameter of hypoechoic shadow right angle to the op c nerve by using electronic caliper.Coupling gel was removed with sterile paper from the transducer and clean with normal saline a er each use.Data were collected and entered into the excel sheet.Sta s cal analysis was performed using IBM SPSS Sta s cs version 22 (IBM Corpora on, New York, USA).Data were analyzed with descrip ve and inferen al sta s cs.Paired T test was used to compare the mean ONSD of right and le eye.Mann-Whitney U test was used to determine correla on between the gender and ONSD and Kruskal-Wallis test to evaluate correla on between age and ONSD.P values of < 0.05 were considered significant.
th th from 5 January 2018 to 15 February 2018.Apparently healthy hospital staffs, medical students and rela ves of the pa ents above 16 years were enrolled a er obtaining informed consent.Those with ocular diseases, neurological disorder, features of raised ICP, non Nepalese and aged 16 years or below were excluded from the study.The study was approved by ethical review commi ee of KMCH.ONSD was measured using 7.5 MHz linear array transducer of the Mindray Diagnos c Ultrasound System (Model: Z6;

Table 1 .
. Maude et al found Median ONSD was 4.41 mm with 95% of subjects in the range 4.25-4.75mm in Bangladesh.Among 100 Pakistani healthy subjects,Ali et al reported ONSD of 23,31 4.3-5.7 mm.Bauerle et al in German showed normal range of 4.3-7.6 mm among 40 subjects with mean of 5.4 ONSD varia on in different countriesMean of op c nerve sheath diameter (ONSD) in healthyNepalese adults for the right and le eye is 4.10 ± 0.50 and 4.22 ± 0.49 mm respec vely.There is no significant sta s cal associa on between ONSD of right and le eyes, ONSD and age and gender and side.