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

BACKGROUND

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

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

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

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

PAPER #1

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

“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

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

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

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

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

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

PAPER #2

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

“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

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

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

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

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

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

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

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