Pilot Study of Normative Range of Field of Binocular Single Vision in Adults in Singapore
Conditions: Binocular Visual Function
Sponsor: Tan Tock Seng Hospital

DSRB Ref: 2025-1293               
NCT Number: NCT07257341 
 
 
 
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NORVIBAS Study Protocol V2.0, Dated 15 December 2025 
Pilot Study of Normative Range of Field of Binocular Single Vision in Adults in Singapore 
(NORVIBAS) 
 
Study Site: Tan Tock Seng Hospital, Singapore 
 
Principal Investigator: 
Dr. Daniel Chin Yuxiang 
Associate Consultant 
NHG Eye Institute, 
Tan Tock Seng Hospital 
NHG Health, Singapore 
 
Co-Investigators(s): 
Mr Sebastian Yue Chii Xiang 
 
 
Ms Azizah Binte Hajamohideen 
Senior Orthoptist 
 
 
 
 
Senior Orthoptist 
 
 
NHG Eye Institute, 
 
 
 
 
NHG Eye Institute, 
Tan Tock Seng Hospital 
 
 
 
Tan Tock Seng Hospital 
NHG Health, Singapore 
 
 
 
NHG Health, Singapore 
 
Mr Jackson Chng Jie Sheng 
 
 
Dr Koh Yan Tong 
Orthoptist 
 
 
 
 
 
Consultant 
NHG Eye Institute, 
 
 
 
 
NHG Eye Institute, 
Tan Tock Seng Hospital 
 
 
 
Tan Tock Seng Hospital 
NHG Health, Singapore 
 
 
 
NHG Health, Singapore 
 
Mr Soh Zheng Kuang, Noel 
Clinical Research Coordinator 
NHG Eye Institute, 
Tan Tock Seng Hospital 
NHG Health, Singapore 
 
 
 
 

DSRB Ref: 2025-1293               
NCT Number: NCT07257341 
 
 
 
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NORVIBAS Study Protocol V2.0, Dated 15 December 2025 
Background:  
The assessment and monitoring of the field of binocular single vision is a crucial aspect of 
ophthalmological care, particularly in the management of oculoplastic and strabismus. Currently, 
clinicians worldwide, including those in Singapore, rely on the normal boundary of the field of 
binocular single vision established by Doctor Robert M. Feibel & Gill Roper-Hall in 1974 as the 
gold-standard parameter for clinical assessment. However, this long-standing standard raises 
several significant concerns regarding its applicability to our current population. 
 
The original study by Feibel & Roper-Hall was conducted in St Louis, Missouri, United States of 
America, with participants who were likely of Caucasian descent. This demographic composition 
presents a potential limitation when applying these standards to Asian populations, particularly 
in Singapore, where ethnic and genetic factors may influence ocular characteristics. 
Furthermore, the study's methodology involved only "several" normal individuals, suggesting a 
sample size of more than two but fewer than ten participants. A sample size of this limited scope 
significantly increases the likelihood of Type II errors, potentially compromising the statistical 
power and reliability of the established normative values. 
 
The temporal gap of over five decades since the original study adds another layer of concern. 
Population characteristics, environmental factors, and lifestyle patterns have undergone 
substantial changes during this period, potentially affecting ocular health parameters. 
Additionally, advances in measurement techniques and an understanding of ocular physiology 
suggest the need for updated normative data that reflect current population characteristics and 
incorporate modern assessment methodologies. 
 
Given these limitations, there is a pressing need to establish population-specific normative data 
for the field of binocular single vision in Singapore's adult population. This would not only 
provide more relevant reference values for clinical assessment but also potentially improve the 
accuracy of diagnosis and monitoring of various ocular conditions in our local context. 
 
Study Objectives 
The primary purpose of this pilot study is to address these gaps in current clinical practice 
through two main objectives. First, we aim to establish the normative range of the field of 
binocular single vision specifically for adults in Singapore, creating a locally relevant reference 

DSRB Ref: 2025-1293               
NCT Number: NCT07257341 
 
 
 
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NORVIBAS Study Protocol V2.0, Dated 15 December 2025 
standard. This will provide clinicians with more appropriate benchmarks for assessing their 
patients. 
Secondly, we seek to determine whether a significant difference exists between these local 
normative values and the currently used standards established by Feibel & Roper-Hall in 1974. 
We hypothesise that there will be a significant difference in the normative range of the field of 
binocular single vision in our current adult population in Singapore compared to the historical 
standards. This hypothesis is based on the understanding that genetic, environmental, and 
temporal factors may influence ocular characteristics. 
 
The findings from this study could have substantial clinical implications. If significant differences 
are identified, it would justify the adoption of new, population-specific normative values for 
clinical practice in Singapore. This could lead to more accurate assessment and monitoring of 
subjects, potentially improving the quality of eye care delivered to our population. Furthermore, 
this study could serve as a model for other Asian countries to establish their population-specific 
normative values, contributing to the broader field of ophthalmology in the region. 
 
2. STUDY DESIGN 
This research will be conducted as a single-centre, pilot study at Tan Tock Seng Hospital, 
designed as an observational investigation regulated under the Human Biomedical Research 
Act (HBRA) of Singapore. This study will employ a cross-sectional design, with each participant 
completing all assessments in a single visit lasting approximately 60 minutes. This approach 
has been chosen to minimise participant burden while ensuring comprehensive data collection. 
 
The study will span a total duration of one year, with active data collection scheduled from 1 
November 2025 to 31 October 2026. This timeline allows for adequate recruitment, data 
collection, and subsequent analysis while accounting for potential scheduling challenges and 
the need for careful quality control of the collected data. The single-visit design also helps 
minimise dropout rates and ensures consistency in data collection procedures. 
 
Study Design: 
Study population: This study aims to recruit a balanced sample of adult participants from 
Singapore's population, with a target enrolment of 32 participants, equally distributed between 
males and females (16 of each gender) and of Asian descent. This sample size has been 

DSRB Ref: 2025-1293               
NCT Number: NCT07257341 
 
 
 
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NORVIBAS Study Protocol V2.0, Dated 15 December 2025 
carefully determined to provide sufficient statistical power while remaining feasible within the 
study's resources and timeline. 
 
Participants will be recruited primarily by word of mouth at the TTSH Ophthalmology Specialist 
Outpatient Clinic’s, focusing on accompanying next-of-kin of existing patients, TTSH staff 
members, or any other availably healthy volunteers after meeting the inclusion and exclusion 
criteria. The recruitment process will be conducted with careful attention to avoiding any 
coercion or undue influence, particularly in cases involving dependent relationships. All potential 
participants will be given adequate time to consider their participation and will be required to 
provide informed consent in the presence of a witness in compliance to the Declaration of 
Helsinki, as well as other prevailing laws and guidelines regulating clinical research. 
 
Inclusion Criteria: 
1. Age: 21 to 59 years old 
2. Gender: Males or females 
3. Ethnicity: Asian descent. 
4. Language comprehension: Able to understand verbal spoken instructions in: 
• 
British/American English 
• 
Mandarin Chinese 
• 
Bahasa Melayu 
5. Protocol compliance: Ability to comply with the study protocol, as determined by the 
investigator's judgment 
6. Consent capacity: Must be able to understand and provide informed consent, with 
signed informed consent form required before any study assessments 
 
Exclusion Criteria: 
1. Communication barrier: Unable to understand verbal spoken instructions 
2. Medical history: 
o 
Any facial trauma 
o 
Any ocular trauma 
o 
Any head trauma 
o 
Any history of ocular disease 
o 
Any history of ocular surgery 
o 
Systemic condition(s) 

DSRB Ref: 2025-1293               
NCT Number: NCT07257341 
 
 
 
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NORVIBAS Study Protocol V2.0, Dated 15 December 2025 
3. Near visual acuity: 
o 
Right eye & Left eye monocularly, binocularly unaided near vision: worse than N6 
at 40cm from participant’s nose 
4. Ocular alignment: 
o 
Prism cover test at near (30cm) of equal or greater than 10 prism diopters at 
these positions of gaze: 
§ 
Primary gaze 
§ 
Right gaze 
§ 
Left gaze 
§ 
Up gaze 
§ 
Down gaze 
5. Extraocular movement: 
o 
Extraocular movement of over-action or under-action of equal or greater than 0.5 
in any of the nine cardinal positions of gaze 
6. Confrontational Visual field: 
o 
Abnormal monocular visual field on confrontational visual field test 
7. Hess chart test 
o 
Abnormal or asymmetry in pattern of extraocular muscle action in Hess chart test 
8. Pregnant woman will be excluded from this study. 
 
Study Procedure: 
This cross-sectional study will involve 1 study visit. 
 
Pre-specified clinical outcome:  The primary outcome is the establishment of a normative 
range of field of binocular single vision measurements in Singapore adults. These 
measurements will be taken using the Takagi MT-325UD Projection Perimeter. The secondary 
outcome is the comparison with the normal boundary of the established field of binocular single 
vision by Feibel & Roper-Hall (1974) and the analysis of any significant differences between the 
two sets of measurements. 
 
Clinical Validation will be a cross-sectional study design to determine the normative range of the 
field of binocular single vision measurements in participants who are Singapore adults focusing 
on accompanying next-of-kin of existing participants and TTSH staff members after meeting the 
inclusion and exclusion criteria.  

DSRB Ref: 2025-1293               
NCT Number: NCT07257341 
 
 
 
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NORVIBAS Study Protocol V2.0, Dated 15 December 2025 
Research Procedures:  
1) Medical history assessment 
A comprehensive medical history will be obtained from each participant through a structured 
interview. The investigator will specifically inquire about any history of facial trauma, ocular 
trauma, head trauma, ocular disease, ocular surgery or any systemic condition(s), documenting 
the nature, timing, and severity of any reported incidents. Particular attention will be paid to any 
previous ocular diseases and surgical interventions. All responses will be recorded in 
standardized case report forms, with detailed documentation of the type, date, and outcome of 
any reported conditions or procedures. Participants should not have any history of facial trauma, 
ocular trauma, head trauma, ocular disease, ocular surgery or any systemic condition(s). This 
thorough screening is essential to ensure participants meet the study criteria for establishing 
normative values. 
 
2) Near Visual Acuity Testing 
Near visual acuity will be assessed right eye and left eye monocularly & binocularly using a 
standard near vision chart held at 40cm from the participant’s nose. Testing will be conducted 
under standardized lighting conditions with monocularly and binocularly and unaided. A 
measuring stick will be used to maintain the precise testing distance. The examiner will record 
the smallest line that can be read accurately, with participants required to achieve N6 or better 
to be included in the study. Any participant with near vision worse than N6 will be excluded from 
the study. 
 
3) Ocular Alignment Assessment 
A prism cover test will be performed at near of approximately 30cm from participant’s nose to 
evaluate ocular alignment. The examiner will systematically assess alignment in five positions of 
gaze: primary gaze (straight ahead), right gaze, left gaze, up gaze, and down gaze. 
Measurements will be taken using loose prisms or a prism bar, with deviations recorded in prism 
diopters. Any deviation equal to or greater than 10 prism diopters in any of the five positions of 
gaze will result in exclusion from the study. The examiner will ensure consistent testing distance 
using a measuring stick and maintain standardized testing conditions throughout the 
assessment. 
 
 
 

DSRB Ref: 2025-1293               
NCT Number: NCT07257341 
 
 
 
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NORVIBAS Study Protocol V2.0, Dated 15 December 2025 
4) Extraocular Movement Testing 
Extraocular muscle function will be examined by having the participant follow a fixation target 
through the nine cardinal positions of gaze. The examiner will assess and grade any overaction 
or underaction of the extraocular muscles using a standardized scale, where 0 represents 
normal movement. Participants demonstrating overaction or underaction equal to or greater 
than 0.5 units in any direction will be excluded from the study. The examination will be 
conducted methodically, ensuring smooth pursuit movements are carefully observed and 
documented. 
 
5) Confrontational Visual Field Testing 
Monocular confrontational visual field testing will be performed separately for each eye. The 
examiner will sit at one meter distance, directly facing the participant at eye level. Each 
quadrant of the visual field will be tested systematically using standardized targets, while the 
participant maintains fixation on the examiner's nose. Any abnormality in the visual field will be 
documented and will result in exclusion from the study. The testing sequence will remain 
consistent for all participants to ensure standardization of the assessment. 
 
6) Hess Chart Testing 
The Hess chart test will be conducted using a standard Hess screen at the recommended 
testing distance. This test will evaluate the action of each extraocular muscle, with results 
plotted on the Hess chart. The examiner will assess for any asymmetry or abnormality in the 
pattern of extraocular muscle action. Any abnormal findings on the Hess chart will be 
documented in detail and will result in exclusion from the study. The test will be performed 
following standardized protocols to ensure consistency and accuracy of results. 
 
7) Field of binocular single vision 
Participants perform the field of binocular single vision test binocularly and pursue a single 
round light stimulus presented in the bowl of the Takagi MT-325UD Projection Perimeter, and 
participants will need to look and follow through the single light stimulus as it moves throughout 
the bowl, and participants will press the buzzer if two distinct light stimuli are seen. Scores will 
be recorded and plotted on the recording sheet based on participants’ responses during the 
test. Eye dilation will not be required for this research study. 
 

DSRB Ref: 2025-1293               
NCT Number: NCT07257341 
 
 
 
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NORVIBAS Study Protocol V2.0, Dated 15 December 2025 
All examinations will be performed by trained orthoptists, with results recorded immediately in 
standardised documentation forms. The testing sequence will remain consistent across all 
participants, and any exclusion criteria met will be documented with specific measurements and 
observations. 
 
As part of the clinical evaluation of participants during the analysis phase, access and extraction 
of data will be limited to TTSH's NGEMR and Zeiss Forum Viewer. Clinical information collected 
for this study would include medical & ocular histories along with prior visual assessments 
documented by orthoptists.  
 
Additionally, NEHR cannot and will not be used for research. Access of medical records for the 
purpose of data collection will also be limited to the period dated from 1 November 2025 to 31 
October 2026. Clinical information/data collected will be done once off during, or retrospectively 
after the study visit. 
 
Sample size calculation 
This is a pilot study. Due to resource constraints, a total of 32 healthy participants will be 
recruited for this study. 
Based on the literature, the standard deviation (SD) of normal values for the selected binocular 
single vision measure ranges from 5.3 to 6.9. Assuming an SD of 6 in our study, a sample size 
of 29 will allow us to estimate the mean of normal values with a precision of 2.2 at the 95% 
confidence level. We expect drop out rate to be no more than 10%, 32 participants will be 
sufficient. 
 
References:  
1 
Feibel RM, Roper-Hall G. Evaluation of the field of binocular single vision in incomitant 
 
strabismus. Am J Ophthalmol. 1974; 78(5):800-805. 
2 
Kakizaki H, Umezawa N, Takahashi Y, Selva D. Binocular Single Vision Field. 
Ophthalmology. 2009; 116(2):364-364.e2.