Abstract:
Aims:
To introduce identify novel features of lenticular accommodation using high speed swept source optical coherence tomography (OCT)
Methods and Material:30 patients were imaged in OCT under an accommodative stimulus of ranging from 0 to -4D. Anterior eye scans were segmented with custom algorithms to quantify corneal and lens curvature along with anterior chamber depth (ACD).
Results:
Mean (range) age of patients was 29.5 (18,42). Len curvature was 5.75D, 5.83D, 6.16D, 6.52D and 6.87D at 0, -1, -2, -3 and -4D, respectively. Corneal curvature and thickness were unchanged. Interestingly, ACD increased from 3.39 mm to 3.51 mm from 0 to -1D but normalized to 3.4 mm by -4D stimulus. Lens thickness also increased from 3.78 mm to 3.93 mm by -4D stimulus.
Conclusions:
OCT imaging was extremely useful in understanding lens biophysics. ACD trend was novel. It remains to be seen if this holds in older patients with lower accommodation. OCT imaging can help in developing novel ray tracing tools for presbyopic patients with much greater accuracy.
Key-words:ASOCT , CASIA 2 , ACCOMMODATION
Introduction:
Accommodation is the ability of the eye to automatically change its focus from one distance to that of another. The accommodative system is controlled by the crystalline lens which adjusts its curvature and shape so as to create a proper optical power of the eye to provide a clear retinal image of objects at various distances.
Accommodation is the active and dynamic change in dioptric power of the eye when changing focus from far to near. (1)
Although accommodation is a dynamic process, objective clinical accommodation measurement has typically relied on static measurements. Static measurement is a single instantaneous snapshot taken as the eye focuses on a near target, as opposed to dynamic measurement that is a continuous higher temporal frequency measurement as the eye changes focus from a far target to a near target or as the eye maintains focus on a near target. (2)
There are very few devices which help in measuring the dynamic accommodation of the lens.
CASIA 2 ASOCT is one novel swept source anterior segment OCT whose highlight is the ability to image the anterior segment while inducing programmable grades of accommodation. (3)
It has active fixation targets to enable patient fixation.
It gives us advanced OCT based corneal topography based on 16 raster scans with anterior and posterior corneal keratometry and elevation data similar to schiempflug imaging.
Inbuilt 2D analysis gives real time change in ACD and lens thickness with accommodation.
By using this new technology, it is possible to measure from anterior cornea to posterior lens with one shot. Scanning depth is approximately 13 mm.
While capturing anterior cornea to posterior lens, it is possible to measure corneal curvature, thickness and tilt of the anterior / posterior lens.
The Casia 2 gives an Axial resolution(Depth) of 10µm or less (in tissue) , Transverse resolution of 30µm or less (in tissue) , Scan rate of 50,000 A scans / second . A Scan range Depth of 13mm and a scan range Transverse Radial Scan of φ16mm with Raster Scans: 12mm×12mm. (3)
The advantage of CASIA2 is that the light source of coherency functions is improved, and higher sensibility toward depth is realized compared to our former model.
Thus CASIA2 explores new possibilities in anterior segment analysis.
We utilized this high speed swept source OCT to introduce and identify novel features of lenticular accommodation and in turn planning of intra ocular lenses and presbyopic surgeries.
Subjects and Methods:
30 patients with clear lens underwent accommodative scans using the Casia 2 ASOCT lens protocol.
Accommodation was induced from 0 to -4D and swept source scans were taken at each step.
Images underwent edge detection of the anterior and posterior corneal surface, lens surfaces and angle points to measure the effective lens position (ELP), lens curvature and AC depth changes with accommodation. In short the anterior eye scans were segmented with custom algorithms to quantify corneal and lens curvature along with anterior chamber depth (ACD).
These images were then exported for advanced analysis of lens curvature and aberrometric changes during accommodation which is a novel area of research in understanding accommodation and presbyopia.
Figure 1: The Lens analysis Protocol on the Casia 2 AS OCT
Figure 2a: Anterior chamber depth of 2.95 mm measured using calipers before accomodation
Figure 2b: Anterior chamber depth of 2.79 mm measured using calipers after accomodation
In Figure 2a we can see that the anterior chamber depth is 2.95mm before accommodation, however we can that post accommodation i.e after myopization on the lens protocol in Casia2 ASOCT there is a decrease in the anterior chamber to 2.79mm.
Similarly using the other protocols we can also measure the lens curvature.
Lens curvature can be measured based on image analysis using Matlab software which is freely available by coding and creating an algorithm.
This can be useful in planning multifocal intraocular lenses and presbyopic surgeries.
Also we can measure the corneal curvature which can later be useful in planning corneal laser surgery.
Results:
TABLE 1 :Mean±SEM for accommodation at different stimulus (0 to -4D).
| Stimulus | |||||
| 0 | -1D | -2D | -3D | -4D | |
| Mean Lens Curv (anterior) (D) | 5.75 ± 0.27 | 5.83 ± 0.3 | 6.16 ± 0.34 | 6.52 ± 0.37 | 6.87 ± 0.3 |
| Lensthickness (mm) | 3.78 ± 0.11 | 3.8 ± 0.11 | 3.84 ± 0.1 | 3.87 ± 0.1 | 3.93 ± 0.12 |
| Mean Cornea Curv (anterior) (D) | 40.7 ± 0.4 | 40.88 ± 0.47 | 40.87 ± 0.46 | 40.69 ± 0.41 | 40.76 ± 0.29 |
| CCT (µm) | 530.8 ± 6.67 | 524.83 ± 7.25 | 528 ± 6.95 | 535.2 ± 6.64 | 525.67 ± 7.67 |
| ACD (epi) (mm) | 3.39 ± 0.1 | 3.51 ± 0.05 | 3.49 ± 0.05 | 3.42 ± 0.05 | 3.4 ± 0.05 |
| ACD (endo) (mm) | 2.89 ± 0.08 | 2.98 ± 0.06 | 2.95 ± 0.06 | 2.89 ± 0.05 | 2.87 ± 0.05 |
Mean (range) age of patients was 29.5 (18,42).
Lens curvature was 5.75D, 5.83D, 6.16D, 6.52D and 6.87D at 0, -1, -2, -3 and -4D, respectively.
Corneal curvature and thickness were unchanged.
Interestingly, ACD increased from 3.39 mm to 3.51 mm from 0 to -1D, but normalized to 3.4 mm by -4D stimulus.
Lens thickness also increased from 3.78 mm to 3.93 mm by -4D stimulus.
Discussion:
OCT imaging was extremely useful in understanding lens biophysics. ACD trend was novel. It remains to be seen if this holds in older patients with lower accommodation. OCT imaging can help in developing novel ray tracing tools for presbyopic patients with much greater accuracy.
Measurement of exact lens curvature with lens thickness changes determines the state of accommodation and helps plan presbyopic lasers, multifocal IOLs and the exact ELP from 3-dimensional lens analysis can be used in the latest IOL calculation formulae for perfect outcomes.
References:
1) Lockhart TE, Shi W. Effects of Age on Dynamic Accommodation. Ergonomics. 2010;53(7):892-903. doi:10.1080/00140139.2010.489968.
2) Win-Hall, D. M., Houser, J., & Glasser, A. (2010). Static and Dynamic Accommodation Measured Using the WAM-5500 Autorefractor. Optometry and Vision Science, 87(11), 873–882. doi:10.1097/opx.0b013e3181f6f98f
3) https://www.tomey.com/tomeycorp/product.html
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