1. Anterior Segment OCT in Strabismus
Ocular Motility Journal Club
16th May 2017
Mali Okada

2. BACKGROUND

3. Anterior Segment optical coherence tomography (AS-OCT) in strabismus
What is it?
What is it used for?
Previous studies

4. Anterior Segment optical coherence tomography (AS-OCT) in strabismus
Non-invasive imaging
Generated from reflected light beam
820 – 1310nm wavelength
Reflected light vs. reference value of known length
Series of A-scans to produce 2D B-scans

5. Anterior Segment optical coherence tomography (AS-OCT) in strabismus
Surgical planning
Localisation of muscles in difficult cases
? Potential for diagnosis of EOM diseases

6. Anterior Segment optical coherence tomography (AS-OCT) in strabismus
Small studies comparing AS-OCT measurements with intra-operative calipers
But only used time-domain OCT (TD-OCT)
No study of how factors (eg. age, AL) affect measurements

7. PAPER #1

8. De-Pablo-Gomez-de-Liano et al. JAAPOS June 2016
From Madrid, Spain

9. “Spectral domain optical coherence tomography to assess the insertion of extraocular rectus muscle”
To measure the distance of horizontal EOM insertion to limbus using AS-OCT
To examine effect of age, gender, AL on measurements
Prospective crosssectional study
Right eyes of 187 healthy adults
Single institution
Subset of 30 eyes assessed for inter and intra-observer reproducibility
AS-OCT
Distance from EOM insertion to limbus
Spectralis (Heidelberg)
Scanning in long-axis of muscle
Fixation in opposite gaze
Limbus determined manually by viewing external image of eye – corresponding point on OCT
AIM
DESIGN
TECHNIQUE

10. Figure from De-Pablo-Gomez-de-Liano et al. JAAPOS June 2016
21 Raster line scans
Centre most scan (line 11) used as reference scan (sometimes could not visualize muscle so closest line scan used +/- 2 scans)
Figure from De-Pablo-Gomez-de-Liano et al. JAAPOS June 2016

11. RESULTS Muscle Distance (m±SD) Range (mm) Classic Values (mm) MR
PATIENTS
Total = 187 participants
Identification of LR (87%)
Identification of MR (94%)
EXCLUSIONS
Excluded = 34 (19%)
Due to:
Poor image quality
Lack of patient cooperation
Technical issues
MEASUREMENTS
Excluded more LR than MR
Muscle
Distance (m±SD)
Range (mm)
Classic Values (mm) MR
5.22 ± 0.51
4.1 – 6.1
5.5 LR
6.47 ± 0.52
5.2 – 7.6
6.9

12. RESULTS AGREEMENT Inter-observer = 0.87 Intra-observer = 0.88
Excluded more LR than MR

13. Table from De-Pablo-Gomez-de-Liano et al. JAAPOS June 2016
Longer in males for LR and MR insertion distances
Table from De-Pablo-Gomez-de-Liano et al. JAAPOS June 2016

14. First study to measure external limbus directly from OCT
PROS
CONS
First study to measure external limbus directly from OCT
First to use SD-OCT
Good reproducibility (but only performed in subset)
Large numbers
Only looked at healthy eyes (? applicability)
Only right eyes
Only subset had reproducibility measured
Unclear how repeatable the limbus identification is from external image
Repeated measures not masked
Previous studies measured distance to AC angle + 1mm

15. PAPER #2

16. From Bascom Palmer Eye Institute
Rosetto et al. AJO April 2017
From Bascom Palmer Eye Institute

17. “Accuracy of Optical Coherence Tomography Measurements of Rectus Muscle Insertions in Adult Patients Undergoing Strabismus Surgery”
To determine accuracy of AS-OCT in measuring distance from EOM insertion to limbus
To examine effect of distance limbus-anterior chamber angle and AL on measurements
Prospective reliability study
74 adult patients with all types of strabismus
Single institution
Masked
AS-OCT
Distance from EOM insertion to limbus
Visante (Carl Zeiss) line scan (average of 3)
Fixation in opposite gaze to muscle measured
Calipers
Distance from centre of EOM insertion directly anterior to hook
Repeated after muscle dis-inserted
AIM
DESIGN
TECHNIQUE

18. Figure from Rossetto et al. AJO 2017
AS-OCT: Insertion = end of muscle cleft and sclera; Limbus = transition between corneal epithelium and conjunctival epithelium
Figure from Rossetto et al. AJO 2017

19. RESULTS PATIENTS Total = 74 patients Majority horizontal (n = 55)
Majority concomitant (n = 54)
Re-operations = 31 muscles imaged
EXCLUSIONS
Image failure = 9 muscles
Due to severely impaired ductions
Only a few had restrictive or paralytic strabismus
Image failures (7 inferior 2 medial)
Bland-altman analysis – no difference for MR at different insertion lengths but significant for LR

20. Strong correlation between AS-OCT and intraoperative calipers
HORIZONTAL MUSCLES
Strong correlation between AS-OCT and intraoperative calipers
Overall = 75% within 1mm
Less accurate in eyes with prior surgery (but not significant)
AS-OCT underestimates distance when LR insertion at larger distance
Only a few had restrictive or paralytic strabismus
Image failures (7 inferior 2 medial)
Bland-altman analysis – no difference for MR at different insertion lengths but significant for LR

21. RESULTS VERTICAL MUSCLES
Reported ‘no substantial discrepancy’ in measurement techniques
All within 1mm
But small numbers and correlation
OTHER MEASUREMENTS
No correlation between limbus and AL
Only a few had restrictive or paralytic strabismus
Image failures (7 inferior 2 medial)
Bland-altman analysis – no difference for MR at different insertion lengths but significant for LR

23. RESULTS VERTICAL MUSCLES
Reported ‘no substantial discrepancy’ in measurement techniques
All within 1mm
But small numbers and correlation
OTHER MEASUREMENTS
No correlation between limbus and AL
Only a few had restrictive or paralytic strabismus
Image failures (7 inferior 2 medial)
Bland-altman analysis – no difference for MR at different insertion lengths but significant for LR

24. Looked at effect of other parameters (eg. AL)
PROS
CONS
Uses SD-OCT
Looked at effect of other parameters (eg. AL)
Largest cohort of patients
Masked
Included all types of patients
Small numbers of vertical muscles
Some imaging failure
Low correlation values for vertical
Difficult to know how authors determined transition point for limbus
Inherent errors in intraoperative caliper accuracy

25. SUMMARY
SD-OCT can be used to measure insertion of horizontal rectus muscles
Can measure limbus directly instead of surrogate marker of AC angle
Longer limbus-to-insertion distance in men
Good reproducibility
SD-OCT can be used to measure insertion in strabismic eyes
Good correlation with intraoperative measurements for horizontal muscles
Less accurate for eyes with prior surgery (especially buckle implant)
May be useful for surgical planning especially in complex cases