Optician India

INTRODUCTION

AS-OCT uses the principle of low-coherence interferometry instead of ultrasound to produce high-resolution, cross-sectional images of the anterior segment of the eye. Early detection and monitoring are critical to the diagnosis and management of glaucoma, a progressive optic neuropathy that causes irreversible blindness. Optical coherence tomography (OCT) has become a commonly utilized imaging modality that aids in the detection and monitoring of structural glaucomatous damage.

ROLE OF AS OCT IN IDENTOFYING RISK FACTORS

Nongpiur et al  noted a significant correlation between angle closure and lens vault In a study examining angle closure disease in Chinese subjects, Larger values of lens vault are suggestive of increased crowding of the anterior chamber by a
thick lens.(1)

A study by Wu et al further examined the usefulness of AS-OCT biometrics as predictors for PACD,. Smaller anterior chamber area and smaller anterior chamber volume are measured by AS-OCT is used as a screening parameter (2) Aptel F et al. in his study said that Primary dynamic risk factors for PACD revolve around structural changes of the iris with dilation. Studies have shown that iris behavior in the dark versus in the light differs significantly between PACD and normal eyes.

Early studies using AS-OCT showed less reduction in iris area and iris volume with dilation in PACD patients when compared with normal subjects.(3) More recently, Zhang et al found that post-dilation, larger, more peripherally distributed irises increase the risk of angle closure(4).

ROLE OF AS OCT FOR CLOSER ANGLE GLAUCOMA

Anatomically narrow angles can be diagnosed with AS-OCT both qualitatively and quantitatively. Radhakrishnan et al. imaged 31 eyes, including both normal subjects and subjects with narrow angles, and compared results of UBM and the prototype version of AS-OCT by using the same customized software to conventional gonioscopy under similar room illumination. Values for AOD, ARA, TISA, and TICL were similar between UBM and AS-OCT. The same investigators also showed high specificity and sensitivity in detecting narrow angles with these two devices when compared with gonioscopy (5)

ROLE OF AS OCT FOR OPEN ANGLE GLAUCOMA

Dada et al analyzed 63 eyes using the 2 methods and concluded that they correlate well. In fact, AS-OCT offers better resolution and is thus able to provide sharper images of the scleral spur.However, the use of infrared light in AS-OCT  limits penetration of iris pigment epithelium, resulting in poor visualization of the ciliary body and more posterior structures. Overall, there was a tendency towards smaller measurements of angle and slightly higher measurements of the anterior chamber depth in AS-OCT compared with UBM (6)

Hong et al have shown that AS-OCT can quantify changes in angle structures post procedure, specifically reporting that Schlemm canal expands with trabeculectomy (7)

ROLE OF AS OCT FOR SURGICAL MANAGEMENT

Leung et al. used AS-OCT to describe intrableb morphology and structures, including bleb wall thickness, subconjunctival fluid collections, suprascleral fluid space, scleral flap thickness, and intrableb intensity. They demonstrated low to medium intrableb reflectivity and intrableb fluid-filled spaces in functioning blebs (diffuse and cystic blebs). Encapsulated blebs had a thick wall, high reflectivity because of dense collagenous connective tissue present in the bleb wall, and an enclosed fluid-filled space. Flat blebs demonstrated high scleral reflectivity with no bleb elevation. Although qualitative assessment can be performed, the authors did caution that specific software does not exist for quantitative analysis of blebs and that the measurements in the study may not have reflected true values. However, the measurements could be used to compare different types of blebs and monitor bleb changes over time. (8)

The noncontact nature of OCT imaging provides a much safer approach to examine the intrableb morphology. In the early postoperative period, thereby offering a unique opportunity to study the healing and remodeling process inside the blebs longitudinally. Using ASOCT, four-different patterns of intrableb morphology, including diffuse filtering blebs, cystic blebs, encapsulated blebs and flattened blebs were identified (9)

AS-OCT could be used to determine which blebs are suitable for needling, to evaluate bleb changes after laser suture lysis, and to aid in the planning of bleb revision surgeries(10)

CONCLUSION

While no technology can be a substitute for a thorough clinical examination performed by an experienced ophthalmologist, AS-OCT is a valuable adjunctive tool for anterior segment imaging, especially the angle anatomy in glaucoma suspects and patients. Its noncontact nature, high-resolution images, rapid scanning speed, storage capacity, imaging in the presence of corneal opacities, and the ability to provide both qualitative and quantitative analyses of the angle recess make it an important diagnostic tool for disease documentation, progression, and therapeutic outcomes. Its limitations should be kept in mind, including cost and its inability to image the ocular structures posterior to the iris due to blockage of wavelength by pigment.

REFERENCES

1.Nongpiur ME, He M, Amerasinghe N, et al.Lens vault, thickness, and position in Chinese subjects with angle closure.Ophthalmology, 118 (2011), pp. 474-479

2.Wu RY, Nongpiur ME, He MG, et al. Association of narrow angles with anterior chamber area and volume measured with anterior-segment optical coherence tomography. Arch Ophthalmol, 129 (2011), pp. 569-574

3.Aptel F, Chiquet C, Beccat S, et al.Biometric evaluation of anterior chamber changes after physiologic pupil dilation using Pentacam and anterior segment optical coherence tomography.Invest Ophthalmol Vis Sci, 53 (2012), pp. 4005-4010

4.Zhang Y, Li SZ, Li L, et al. Dynamic iris changes as a risk factor in primary angle closure disease Invest Ophthalmol Vis Sci, 57 (2016), pp. 218-226

5.Radhakrishnan S., Goldsmith J., Huang D., Westphal V., Dueker D. K., Rollins A. M., Izatt J. A., and Smith S. D., Comparison of optical coherence tomography and ultrasound biomicroscopy for detection of narrow anterior chamber angles,Archives of Ophthalmology. (2005) 123, no. 8, 1053–1059, 2-s2.0- 23844543530

6.Dada T, Sihota R, Gadia R, Aggarwal A, Mandal S. Gupta V Comparison of anterior segment optical coherence tomography and ultrasound biomicroscopy for assessment of the anterior segment. J Cataract Refract Surg. 2007 May;33(5):837–840.

7.Hong J, Yang Y, Wei A, et al.Schlemms canal expands after trabeculectomy in patients with primary angle-closure glaucoma.Invest Ophthalmol Vis Sci, 55 (2014), pp. 5637-5642

8.Leung C. K. S., Yick D. W. F., Kwong Y. Y. Y., Li F. C. H., Leung D. Y. L., Mohamed S., Tham C. C.Y., Chung- Chai C., and Lam D. S. C., Analysis of bleb morphology after trabeculectomy with Visante anterior segment optical coherence tomography, British Journal of Ophthalmology. (2007) 91, no. 3, 340–344, 2- s2.0-33947578799,

9.Theelen T, Wesseling P, Keunen JE, Klevering BJ. A pilot study on slit lamp-adapted optical coherence tomography imaging of trabeculectomy filtering blebs. Graefes Arch Clin Exp Ophthalmol. 2007 Jun;245(6):877–882.

10.Tominaga A, Miki A, Yamazaki Y, Matsushita K, Otori Y. The assessment of the filtering bleb function with anterior segment optical coherence tomography. J Glaucoma. 2010 Oct-Nov;19(8):551–555.