Quantitative relations between the eyeball, the optic nerve, and the optic canal important for intracranial pressure monitoring
Head & Face Medicine
Quantitative relations between the eyeball, the optic nerve, and the optic canal important for intracranial pressure monitoring
Michael Vaiman 0 1
Paul Gottlieb 2
Inessa Bekerman 2
0 33 Shapiro Street, Bat Yam 59561 , Israel
1 Department of Otolaryngology, Head and Neck Surgery, Assaf Harofe Medical Center, Affiliated to Sackler Faculty of Medicine, Tel Aviv University , Zerifin , Israel
2 Department of Radiology, Assaf Harofe Medical Center, Affiliated to Sackler Faculty of Medicine, Tel Aviv University , Zerifin , Israel
Objective: To find correlations between diameters of the optic nerve sheath (ONSD), the eyeball, and the optic canal that might be important for intracranial pressure monitoring. Methods: In a prospective cohort study, the CT data of consecutive 400 adults (18+) with healthy eyes and optic nerves and absence of neurological diseases were collected and analyzed. When the CT scans were obtained, the diameters of the optic nerve sheath, the eyeball, and the optic canal were measured and statistically analyzed. The data obtained from the left and from the right eyeballs and optic nerves were compared. The correlation analysis was performed within these variables, with the gender, and the age. Results: In healthy persons, the ONSD varies from 3.65 mm to 5.17 mm in different locations within the intraorbital space with no significant difference between sexes and age groups. There is a strong correlation between the eyeball transverse diameter (ETD) and ONSD that can be presented as ONSD/ETD index. In healthy subjects, the ONSD/ETD index equals 0.19. Conclusion: The calculation of an index when ONSD is divided by the ETD of the eyeball presents precise normative database for ONSD intracranial pressure measurement technique. When the ONSD is measured for intracranial pressure monitoring, the most stable results can be obtained if the diameter is measured 10 mm from the globe. These data might serve as a normative database at emergency departments and in general neurological practice.
Optic nerve sheath diameter; Computed tomography
Intracranial pressure monitoring by means of measuring
changes in the optic nerve sheath' diameter (ONSD)
became practical in the 1990s. It was postulated that the
presence of enlarged optic nerve sheaths suggests that
raised intracranial pressure is transmitted intraorbitally
]. While this fact is already well established and its
importance is understood, some disagreement remains
in its quantitative part. The ONSD is measured by
sonography, CT, and MRI but no generally accepted
protocol was designed. Different authors indicated normal/
abnormal threshold (a cutoff value) of the ONSD from
5 mm to 5.9 mm with numerous variations between
these numbers [
]. The recent review on methods of
intracranial pressure monitoring estimated the accuracy
of the ONSD method as low .
Numerous publications on the topic [
all of them, report measurements of the ONSD only and
do not take into account variations of forms and sizes of
the eyeball and the optic canal as like the intraorbital
part of the optic nerve is located not between these two
anatomical structures but in the open space. There is a
possibility that dimensions of these two structures might
correlate with the ONSD influencing the accuracy of the
ONSD method of intracranial pressure monitoring. If
the ONSD is used as a technique for intracranial pressure
monitoring, various additional factors are to be taken into
First, the eyeball is a constantly voluntary and
involuntary moving object even when at rest and the head of
the optic nerve moves with it [
]. For example, if at the
moment of the image taking a patient will gaze 3 mm
above the horizontal line, the distal part of the optic nerve
will move 3 mm below the horizontal line; and if fixational
eye movements will turn the eye 3 mm to the left, the
optic nerve head will move 3 mm to the right. All that
movements might change the ONSD close to the globe.
The researches ignore this possibility and they usually
measure the ONSD only 3 mm behind the globe [
The authors have chosen this location because the sheath
is wide in this area (bulging dura mater region).
Second, the axial length of the eyeball
(anterior-toposterior diameter) is different in cases with myopia,
emmetropia, and hypermetropia [
]. Third, myopia,
congenital and acquired glaucoma, retinoblastoma and
some other disorders can change the size of the eyeball
]. Forth, the optic canal of the sphenoid bone can be
wide, normal, or narrow and specifically its orbital
opening can be wide or narrow that can influence the ONSD
because the sheath acts as periosteum of the sphenoid
bone inside the canal [
]. All these variations might
influence the accuracy of the ONSD method of intracranial
The purpose of the current research was to establish
normative data of the ONSD in various locations within
its intraorbital part with the help of data obtained by
computer tomography (CT) technique and to analyze
their possible correlations with the eyeball transverse
diameter (ETD) and the optic canal diameters. We planned
to measure the ONSD in several distances from the globe
together with the diameters of the optic canal and the
eyeball, analyze possible correlations, and recommend the
most convenient approach to be used in practice in cases
when ONSD is measured for the purpose of detection of
elevated intracranial pressure.
Materials and methods
Study design and setting
In a prospective cohort study, we collected and analyzed
the CT data of consecutive 400 adult patients (18+) that
were admitted to the Department of Radiology at our
Medical Center from Jan 2011 to February 2014. The
study protocol conformed to the ethical guidelines of the
1975 Declaration of Helsinki as reflected a priori after
approval by the institution's Helsinki committee. We
examined the patients who were admitted to the Emergency
Department, were referred to the CT investigation that
included the head and neck region, and appeared to be
neurologically and ophthalmologically healthy.
Exclusion procedure was organized in two steps. First,
the patients with documented ophthalmologic, cerebral,
or neurophthalmologic disorders were excluded as well
as patients with injuries around the orbits. At this stage,
we also checked the data of the blood tests to exclude
intoxications that might affect CNS. Second, the selected
patients were examined by an ophthalmologist and by a
neurologist in order to exclude overlooked eye disorders
or cerebral pathology. Special attention was paid in
order to exclude cases with ischemic, toxic, hereditary,
nutritional, or compressive neuropathies, glaucoma,
cataract, etc. Therefore, four criteria were used to include a
case into our study: 1) a neurologist did not find any
CNS-specific pathology; 2) an ophthalmologist did find
any eye/optic nerve-specific pathology; 3) CT
investigation did not detect any cranial pathology or existing
pathology of the optic nerve; 4) blood tests did not indicate
any toxic elements that might affect the CNS.
The patient flow was as follows: from the 587
consecutive patients, 122 were excluded at the first step, 65
were excluded at the second step. The data collection
was stopped when we obtained 400 healthy cases.
1. ETD (retina to retina), 2. ONSD at 3 mm behind the
globe, 3. ONSD at 10 mm behind the globe, 4. ONSD at
3 mm from the anterior lumen of the optic canal, and
5. area of the anterior lumen of the optic canal.
Data sources and measurements
All the CT scans were obtained by the 256-slice CT
scanner (Brilliance iCT, Philips Healthcare). We
implemented the standard Philips protocols for head and neck
imaging in all cases, single slice section 3 mm [
When the CT scans were obtained, the left and right
ETD and the ONSD were measured by the computer
program (Figures 1 and 2). The transverse diameter of
the eyeball was chosen because the ONSD is usually
measured in the transverse plain. The optic canal is
rarely round in its orbital orifice; usually it is oval. That
is why we measured two diameters for its orbital opening.
Window parameters were: spine window, middle third;
WW 60, WL 360, (sometimes abbreviated as C:60,0.
W:360,0 spine), accuracy 1 pixel. All measurements were
made using the same window, contrast and brightness.
The error margin was expressed by means of the technical
error of measurement (TEM) to calculate the
intraevaluator variability and inter-evaluator variability between
two evaluators. The same equipment and
methodological procedures for measurements were adopted by
Measurements of five above mentioned variables were
analyzed. A within-group repeated measures
experimental statistical analysis was used to test the variables. To
verify the normality of the data, normal probability plots
and basic descriptive statistics (mean, standard deviation
(SD), min, and max) were calculated for every variable
(the diameters). The data obtained from the left eyeball
and the optic nerve and from the right eyeball and the
nerve was compared. The correlation analysis was
performed with gender and age groups (group I: 18-30;
group II: 30-65; group III: 65+).
The correlation analysis was performed between the
following pairs of variables: ETD to ONSD at 3 and
10 mm from the globe; ONSD converted to area at
3 mm from the lumen of the optic canal to the area of
the lumen. The correlation analysis between the optic
canal measurements and the proximal ONSD were
performed not between their diameters but between their
areas because the nerve is round and the optic canal is
oval. The data were statistically evaluated by
threedimensional analysis of variance, SPSS, Standard version
17.0 (SPSS, Chicago, IL, 2007), and χ [
] criterion using
95% confidence interval. The level of significance for all
analyses was set at p < 0.05.
In our cohort, there were 214 females and 186 males,
age range was from 18 to 94 (mean 46). Distribution
among age groups was as follows: group I (18-30) - 89;
group II (30-65) - 156; group III (65+) - 155.
Altogether, 800 eyeballs, ONSD, and optic canals were
measured. For the TEM calculation, two measurements
were obtained from each location (n = 1600
measurements for each of three variables). The difference
between the first and second measurements were then
determined and the relative TEM (technical error of
measurement expressed in %) was calculated to be 3.77
acceptable. For inter-evaluator TEM, it varied from 3.18
to 3.58 for different locations (acceptable).
Tables 1 and 2 present the results of the
measurements. Analyzing these data it was detected that
standard deviation of the mean ONSD, minimal, and
maximal variations of the ONSD are the highest at
3 mm position while at 10 mm position they are the
lowest. Variations of the proximal part of the ONSD
that is close to the anterior opening of the optic canal
are also less significant if compared with the 3 mm
position. For this location, however, strong positive
correlation exists between the ONS area (calculated from
ONSD data) and the area of the orbital orifice of the
optic canal. At the orifice itself the correlation is almost
*3 mm behind the globe.
**10 mm behind the globe.
***3 mm before entering the anterior opening of the optic canal.
100% (r = 0.96), and at 3 mm from the orifice it remains
0.82. In comparison, at 10 mm from the globe location
r = 0.44 only.
Analyzing further the obtained results, we paid
attention that ONSD taken from the middle section of the
intraorbital part of the optic nerve correlates with the
ETD of the eyeball and that this correlation can be
presented as an index. This index is calculated as ONSD
divided by the transverse diameter of the eyeball (ONSD/
ETD) and is presented in the Table 3 as 0.19 with
standard deviation of 0.01-0.02.
We did not find statistically significant differences
correlated with gender of the patients (p = 0.15), and their
age (I vs. II, p = 0.25; I vs. III, p = 0.09; II vs. III, p = 0.36).
In our cases, measurements taken from the right eyeball
and optic nerve were slightly smaller than the left side
measurements but this difference is also statistically
insignificant (p = 0.44).
*3 mm behind the globe.
**10 mm behind the globe.
***3 mm before entering the anterior lumen of the optic canal.
ETD – eyeball transverse diameter.
OC – optic canal.
0.19 ± 0.02
The pathophysiology of optic nerve sheath enlargement
as a result of intracranial hypertension has been
established well already [
]. The ONSD technique itself
is not perfected yet and some improvements might
be suggested. Analyzing the obtained data, we believe
that the 3 mm distance from the globe is not the ideal
location to measure ONSD for intracranial pressure
monitoring. We cannot ignore constant physiological tremor, slow
drifts, flicking movements, tracking movements, smooth
pursuits, saccades, and other eye movements [
Whatever method is used for the ONSD measurement –
CT, MRI, or ultrasound – images are taken from a
constantly moving object even when a patient is given
instruction to look straight forward, and even when the
eyes are closed. Currently, the quantitative estimate of
how the movements of the eyeball change shape and size
of the bulging dura mater region is lacking. We cannot
recommend measuring the ONSD close to the globe
until this question is clarified. In addition to that, the
enlargement of ONSD behind the globe was also found in
papilledema, optic nerve lesions, optic atrophy, and
endocrine orbitopathy [
We did not find statistically significant differences in
ONSD correlated with age. The optic nerves experience
the age-dependent nerve fiber loss as any other nerve in
the human body. However, while total axon count in the
optic nerve decreases with age, mean axon diameter
increases with age [
]. At the same time, the thickness of
dura mater increases with age [
]. While all these
processes take place simultaneously, we might suggest that
the ONSD remains approximately the same during a
The size of the eyeball correlates with the ONSD. This
fact can be used to our advantage. The optic
nerve/eyeball diameter index is much less variable variance than
ONSD and could be used for intracranial pressure
monitoring with more precise results.
Movements of the eyeball can change ONSD close to
the globe therefore 3 mm distance from the globe is not
an ideal location to measure ONSD to monitor
intracranial pressure. If ONSD is measured close to the orbital
orifice of the optic canal, the measurements can be
influences by the correlation between dimensions of the
ONS and the optic canal. If the canal itself and
especially if its anterior opening is wide or narrow, the
ONSD measurement will correlate with it. Therefore,
the proximal location is also not ideal for measuring the
ONSD for intracranial pressure purposes. The middle
part of the intraorbital optic nerve route experience less
variations in size in normal healthy people. We do not
dictate that ONSD should be measured at 10 mm from
the globe sharp; the intraorbital part of the optic nerve
varies in length (usually from 1.5 to 2.4 cm) and the
ONSD can be measured at 8 mm or 12 mm from the
globe but definitely not at 3 mm location. In any case,
for the most precise detecting of the elevated
intracranial pressure, we recommend to use the optic nerve/
eyeball diameter index. This index is calculated as ONSD
taken from the middle part of the intraorbital path of the
optic nerve divided by the transverse diameter of the
eyeball (ONSD/ETD). While standard deviation of the
ONSD measurements varies from 0.62 to 1.51 at various
locations, the standard deviation of the ONSD/ETD
index is 0.01-0.02 that insures very precise normative
data. From three eyeball diameters, we selected the ETD
because the majority of the authors writing on ONSD
technique for intracranial pressure monitoring measure
ONSD in the transverse plain and because
anterior-toposterior eyeball diameter varies in cases of myopia,
emmetropia, and hypermetropia significantly.
Limitations of the research
All the CT scans were obtained by the 256-slice CT Philips
scanner. It might be possible that scanners of different
trademarks could provide slightly different results of
measurements as well as MRI or sonography evaluation.
External validity of the study results is based on recent
efforts in standardization of CT nomenclature and
protocols for various CT scanner manufacturers (GE,
Philips, Toshiba, Hitachi, Siemens). All these scanner
manufacturers provide features to automatically initiate a
prescribed axial, helical or dynamic scan when a
threshold level of contrast enhancement is reached at a
specified region of interest (in our case, the orbit and the optic
In healthy persons, the ONSD varies from 3.65 mm to
5.17 mm in different locations within the intraorbital
space with no significant difference between sexes and age
groups. More precise results can be obtained through the
calculation of an index when ONSD is divided by the ETD
of the eyeball. In healthy subjects, the ONSD/ETD index
equals 0.19. When the ONSD is measured for
intracranial pressure monitoring, the most stable results can be
obtained if the diameter is measured 10 mm from the
globe. These data might serve as a normative database
when ONSD technique is used for intracranial pressure
monitoring at emergency departments and in general
The authors declare that they have no financial and non-financial conflict of
MV – study concept, study design, analysis of the data, manuscript draft,
manuscript final version; PG and IB – collection of the data, data analysis.
All authors read and approved the final manuscript.
1. Hansen HC , Helmke K : The subarachnoid space surrounding the optic nerves. An ultrasound study of the optic nerve sheath . Surg Radiol Anat 1996 , 18 ( 4 ): 323 - 8 .
2. Helmke K , Hansen HC : Fundamentals of transorbital sonographic evaluation of optic nerve sheath expansion under intracranial hypertension. I. Experimental study . Pediatr Radiol 1996 , 26 ( 10 ): 701 - 5 .
3. Helmke K , Hansen HC : Fundamentals of transorbital sonographic evaluation of optic nerve sheath expansion under intracranial hypertension II. Patient study . Pediatr Radiol 1996 , 26 ( 10 ): 706 - 10 .
4. Kimberly HH , Shah S , Marill K , Noble V : Correlation of optic nerve sheath diameter with direct measurement of intracranial pressure . Acad Emerg Med 2008 , 15 ( 2 ): 201 - 4 .
5. Geeraerts T , Launey Y , Martin L , Pottecher J , Vigué B , Duranteau J , Benhamou D : Ultrasonography of the optic nerve sheath may be useful for detecting raised intracranial pressure after severe brain injury . Intensive Care Med 2007 , 33 ( 10 ): 1704 - 11 .
6. Raboel PH , Bartek J Jr, Andresen M , Bellander BM , Romner B : Intracranial pressure monitoring: invasive versus non-invasive methods-a review . Crit Care Res Pract 2012 , 2012 :950393. doi: 10 .1155/ 2012 /950393. Epub 2012 Jun 8.
7. Moretti R , Pizzi B : Optic nerve ultrasound for detection of intracranial hypertension in intracranial hemorrhage patients: confirmation of previous findings in a different patient population . J Neurosurg Anesthesiol 2009 , 21 : 16 - 20 .
8. Tayal VS , Neulander M , Norton HJ , Foster T , Saunders T , Blaivas M : Emergency department sonographic measurement of optic nerve sheath diameter to detect findings of increased intracranial pressure in adult head injury patients . Ann Emerg Med 2007 , 49 ( 4 ): 508 - 14 .
9. Martinez-Conde S , Macknik SL : Fixation eye movements across vertebrates: comparative dynamics, physiology, and perception . J Vis 2008 , 8 ( 14 ): 28 . 1 - 16 .
10. Tomlinson A , Phillips CI : Applanation tension and axial length of the eyeball . Brit J Ophthal 1970 , 54 : 548 - 553 .
11. Charman WN : Optics of the Human Eye . In Visual Optics and Instrumentation . Edited by Cronly Dillon J. Boca Raton : CRC Press; 1991 : 1 - 26 .
12. Prado PA , Ribeiro EC , De Angelis MA , Smith RL : Biometric study of the optic canal during cranial development . Orbit 2007 , 26 ( 2 ): 107 - 11 .
13. Wintermark M , Maeder P , Verdun FR , Thiran JP , Valley JF , Schnyder P , Meuli R : Using 80 kVp versus 120 kVp In Perfusion CT Measurement Of Regional Cerebral Blood Flow . AJNR Am J Neuroradiol 2000 , 21 ( 10 ): 1881 - 1884 .
14. Smith WS , Roberts HC , Chuang NA , Ong KC , Lee TJ , Johnston SC , Dillon WP : Safety and feasibility of a CT protocol for acute stroke: combined CT, CT angiography, and CT perfusion imaging in 53 consecutive patients . AJNR Am J Neuroradiol 2003 , 24 ( 4 ): 688 - 90 .
15. Wintermark M , Fischbein NJ , Smith WS , Ko NU , Quist M , Dillon WP : Accuracy of dynamic perfusion CT with deconvolution in detecting acute hemispheric stroke . AJNR Am J Neuroradiol 2005 , 26 ( 1 ): 104 - 12 .
16. Hayreh SS : The sheath of the optic nerve . Ophthalmologica 1984 , 189 ( 1-2 ): 54 - 63 .
17. Schütz AC , Trommershäuser J , Gegenfurtner KR : Dynamic integration of information about salience and value for saccadic eye movements . Proc Natl Acad Sci U S A 2012 , 109 ( 19 ): 7547 - 52 .
18. Richard A , Churan J , Guitton DE , Pack CC : Perceptual compression of visual space during eye-head gaze shifts . J Vis 2011 , 11 ( 12 ). doi:10.1167/11.12 .1.
19. Mashima Y , Oshitari K , Imamura Y , Momoshima S , Shiga H , Oguchi Y : Highresolution magnetic resonance imaging of the intraorbital optic nerve and subarachnoid space in patients with papilledema and optic atrophy . Arch Ophthalmol 1996 , 114 ( 10 ): 1197 - 203 .
20. Skalka HW : Neural and dural optic nerve measurements with A-scan ultrasonography . South Med J 1978 , 71 ( 4 ): 399 - 400 .
21. Mikelberg FS , Yidegiligne HM , White VA , Schulzer M : Relation between optic nerve axon number and axon diameter to scleral canal area . Ophthalmology 1991 , 98 ( 1 ): 60 - 3 .
22. Jimenez-Hamman MC , Sacks MS , Malinin TI : Quantification of the collagen fiber architecture of human cranial dura mater . J Anat 1998 , 192 : 99 - 106 .
23. Kalra MK , Saini S : Standardized nomenclature and description of CT scanning techniques . Radiology 2006 , 241 : 657 - 660 .