Keywords
Nepalese adult - optic nerve sheath diameter - ultrasonography
INTRODUCTION
Many neurological emergencies are complicated by raised intracranial pressure (ICP).
If not recognised early, it is associated with poor outcome and secondary brain damage.[1] Multimodal neuromonitoring has contributed significantly to improved treatments
for critically ill neurologic patients.[2] Invasive method is the gold standard for ICP measurement, but it carries the inherent
risk of intracerebral haemorrhage and infection.[3]
Availability of invasive ICP monitoring may be limited in developing countries due
to constrained resources, lack of trained man power and equipped Intensive Care Units.[4]
[5] Non-invasive ICP monitoring may be helpful in the management of patients when invasive
monitoring is not immediately available and to choose patients who need invasive monitoring.[6]
Among the various non-invasive methods, ultrasonographic measurement of optic nerve
sheath diameter (ONSD) has been extensively studied. ICP has a linear relationship
with ONSD.[7] When ICP increases, changes in ONSD occur before changes in nerve are visible on
fundoscopic examination.[8] It is increasingly being used in Emergency Departments and Intensive Care Units.[9]
[10] ONSD has a good level of diagnostic accuracy to detect intracranial hypertension.[11] The sonographic measurement of ONSD is readily learned, reproducible technique with
low intra- and inter-observer variation.[12] By limiting the examination time and by keeping the ultrasound output power as low
as reasonably achievable (ALARA principle), the technique can be performed safely.[1]
[13]
Optimal ONSD cut-off value for raised ICP is controversial. Based on the first validation
studies, most studies have used 5 mm as upper limit of normal for adults.[14]
[15] However, the values are considerably different between 5 and 5.9 mm in recent studies.[1] Knowledge of normal range of ONSD in healthy population is essential to interpret
this measurement as a marker of ICP in clinical practice and research.[9]
[16] ONSD can vary with ethnic origin. Studies in Asian countries have proposed various
normal ranges in adults (3.7–4.7 mm in Hong Kong,[17] 2.9–5.3 mm in Korea[18] and 4.25–4.75 mm in Bangladesh).[16] To the best of my knowledge, this is the first study to find the normal ONSD in
Nepalese adults.
SUBJECTS AND METHODS
This cross-sectional observational study was conducted in a private setting hospital
of Nepal after obtaining approval from hospital. Healthy hospital staffs and relatives
of the patients above 18 years were enrolled in the study after obtaining written
informed consent. Individuals with known ocular pathology were excluded from the study.
A single investigator measured ONSD using 6–13 MHz linear probe (MicroMaxx®; SonoSite, USA). The investigator had an experience of performing more than 200 ONSD
measurements. Subjects were examined in the supine position with their eyes closed
and in neutral position. Ultrasound gel was applied to the outside of each upper eyelid.
Transducer was placed horizontally. Video of each reading was recorded for later analysis
by another single blinded investigator. Electronic caliper was used to mark the point
3 mm behind the globe. ONSD was measured at that depth at right angle to the optic
nerve [Figure 1]. The widest diameter visible was recorded. An average of three readings were documented
in each eye.
Figure 1: Optic nerve sheath diameter measured 3 mm behind the globe
RESULTS
Eighty-eight healthy subjects of both sexes and of Nepalese origin were enrolled in
the study. The sample size was calculated from the pre-test of initial 20 cases, using
the formula z
2Σ2/d
2. The calculated Σ (standard deviation) from the pre-test was 0.24. Values of 1.96
for z (reliability coefficient at 95% confidence interval) and 0.05 for d (maximum tolerable error) were applied.
Age of the subjects ranged from 19 to 83 years. Of them, 55 were males and 33 were
females. The median ONSD was 4.1 mm with 95% of subjects in the range of 3.1–4.6 mm
[Figure 2]. There was no significant difference in ONSD measurement between males (3.90 ± 0.54
mm) and females (4.09 ± 0.44 mm) (P = 0.09) as tested by independent t-test. ONSD of the right (3.97 ± 0.54 mm) and left eye (3.98 ± 0.50 mm) also did not
differ significantly (P = 0.72). There was no relationship between ONSD and age (R2
= 0.06, P = 0.55) [Figure 3].
Figure 2: Distribution of optic nerve sheath diameter. The numbers in boxes represent the number
of healthy volunteers with the corresponding optic nerve sheath diameter mentioned
in X-axis
Figure 3: Relationship of optic nerve sheath diameter with age (R
2 = 0.06, P = 0.55)
DISCUSSION
This study indicates that 4.6 mm can be considered as the upper limit of normal for
Nepalese healthy adults, which is similar to the cut-off of 4.75 mm in Bangladeshi
people. However, the range was narrower in their study (4.25–4.75 mm) as compared
to this study (3.1–4.6 mm), probably due to difference in the frequency of probe used
(15 MHz probe in their study as compared to 6–13 MHz probe in our study).[16] The range in this study is in similarity with the findings of Hong Kong population
(3.7–4.7 mm),[17] but varies from the findings in other populations – 2.5 to 4.1 mm in UK adults[12] and 2.2–4.9 mm in Greek adults.[19] Differences in normal ranges in various studies can be due to difference in ethnicities
and methodology.[16] Consistent with other studies, ONSD was independent of age, gender and side of the
body.[16]
[17]
There are few limitations in this study. The study was conducted in a single centre
and by a single investigator. Interobserver variability could not be studied. Paediatric
population was excluded in the study. Direct measurement of ICP was not performed.
Hence, it is not known how well the ONSD above derived normal range predicts raised
ICP. Systemic diseases such as diabetes and hypertension may affect optic disk and
ONSD measurement. Fundoscopy to rule out the ocular effects of these disorders was
not performed.
CONCLUSION
To conclude, ONSD in healthy adult Nepalese subjects ranges between 3.1 and 4.6 mm.
There is no significant relationship of ONSD with gender, age and side of the body.
Acknowledgement
I would like to thank the hospital staffs, family members of patients and Alka Hospital
authority for participating in the study. I would specially acknowledge the support
and help from the staffs of Alka hospital ICU and Operation Theater, without which,
the study would not have been possible. I am thankful to Dr. Sanjeev Bhoi for encouragement
and for sharing the concept of study.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
