[The effect of keratoconus-associated refractive errors on the results of tomographic methods of studying the posterior structures of the eye]

PURPOSE

The study assesses the potential influence of refractive errors in keratoconus (KC) on the results of tomographic methods of studying the structures of the posterior eye segment.

MATERIAL AND METHODS

The study included 30 patients with bilateral stabilized KC of stages I-IV in classification by M. Amsler. Spherical and cylindrical components of refractions were determined using automatic refractometry, keratometry measurements - based on scanning keratotopography with Scheimpflug analyzer. Aberrometry was performed to evaluate corneal wave front according to the following parameters: root mean square for lower order aberrations (RMS LOA), root mean square for higher order aberrations (RMS HOA), vertical trefoil, vertical coma, horizontal coma and spherical aberrations. Optical coherence tomography (OCT) and laser confocal scanning ophthalmoscopy (HRT 3) data was used in morphometric analysis of the optic nerve head and peripapillary retina. The following morphometric parameters were analyzed: optic nerve head (ONH) area, optic disc cup area, optic disc cup volume, ratio of optic disc cup area to ONH area, neuroretinal rim area, neuroretinal rim volume, peripapillary retinal nerve fiber layer (RNFL) thickness. All studies were performed first without correction, and 30 minutes after installing customized scleral hard contact lenses (SHCL).

RESULTS

Compensation of the refractive errors characteristic for KC was achieved as expected with contact correction. OCT revealed a general trend for reduction in the area and volume of the optic disc cupping, ratio of area to volume of the optic disc cupping, as well as an increase in other parameters. As such, with correction the values for area and volume of the neuroretinal rim according to OCT were 2.2 and 13%, HRT 3 - 18 and 51.6%; comparable increase in mean RNFL thickness - 2.8 and 28.5%, respectively (p<0.001). According to HRT 3 data, the area and volume of optic disc cupping statistically significantly decreased (by 21 and 28%, respectively), while OCT showed statistically significant decrease only in cupping area (by 5.7%). The ratio of cupping to ONH area decreased by 6.6 and 23% relative to the initial data obtained with OCT and HRT 3, respectively. Significant decrease in ONH area amid SHCL correction was observed only with HRT 3. The revealed changes in morphometric parameters were analyzed using the fundamental principles of physiological optics. Changes in interference pattern and, consequently, morphometric parameters of structures of the eye fundus in KC are of multifactorial nature, and are mostly associated with refractive and wave artefacts occurring when the rays pass through the irregular corneal surface and cannot be optically compensated by the device. The use of SHCL as means for making the optic system relatively regular can significantly decrease the artefacts in morphometric measurements.

CONCLUSION

The results obtained in this study demonstrate the practicality of tomographic examination in KC with contact correction. The optimal choice is custom-fit SHCL, which along with proper correction of refractive errors also ensures stable position of the lens on the cornea. In standard examination specialists should take into account the «false» decrease in parameters of the peripapillary retinal nerve fiber layer and increase in ONH cupping.

[Methods of studying deformation properties of ocular structures]

Studying ocular biomechanics presents apparent interest because of certain clinical situations when in vivo evaluation of mechanical properties can help with both diagnosis and treatment. This literature review considers the approaches to studying deformation properties of various ocular structures: dynamometric methods, elastotonometry, ophthalmic mechanography, photoelasticity method, ultrasound methods, analysis of pneumatic applanation of the cornea, atomic force microscopy, holographic interferometry, optical coherence elastography. Knowledge of the particularities of tissue deformation during examination with various methods can expand our understanding of the mechanisms of pathological changes in different structures of the organ of vision, which can help develop new methods of diagnosis and treatment.

[Comparison of visometry by Sivtsev and ETDRS charts]

Most foreign authors currently recommend the protocol and charts developed for the Early Treatment Diabetic Retinopathy Study (ETDRS) as the gold standard for evaluation of visual acuity (VA) in clinical trials and studies. Not every medical facility has equipment required for performing visometry by that method, while Sivtsev tables are found universally.

OBJECTIVE

To compare ETDRS and the more common Sivtsev visometry methods.

MATERIAL AND METHODS

The study included 100 volunteers - students of the Sechenov University. All participants were examined for VA in both eyes without correction using ETDRS "R" chart and Sivtsev table. Visometry with "R" ETDRS chart involved calculation of the number of named letters, then the number was translated into logarithmic notation by the conventional method. Sivtsev visometry findings were translated into logarithmic notation by two methods: first - standard, line-by-line, second - experimental, letter-by-letter. Subjects were divided into two groups: high VA (higher than 0.4 in decimal scale) and low VA - everyone else.

RESULTS

The best agreement with ETDRS was found in letter-by-letter method of Sivtsev table interpretation, especially in the group with high VA with the lowest 95% limits of agreement (LoA) of ±0.157 logMAR, or 8 letters on ETDRS. Clinically significant were also LoA in comparison of ETDRS with line-by-line calculation method in the group with high VA and in letter-by-letter method without dividing groups by VA.

CONCLUSION

The use of Sivtsev table in standardized multicenter studies is only reasonable when the ETDRS charts are unavailable.

[Results of confocal microscopy of the cornea after bandage therapeutic-optical keratoplasty in keratoconus]

Bandage therapeutic-optical keratoplasty (BTOK) is one of the modern methods of surgical treatment of keratoconus (KC) that allows to improve visual functions by changing keratotopographic and pachymetric parameters of the cornea in the zone of ectasia. Long-term results show that changes also affect the central part of the cornea, which tends to flatten.

PURPOSE

To compare the light confocal microscopy appearance of the cornea before and after BTOK surgery.

MATERIAL AND METHODS

We examined 15 patients (15 eyes) with progressive keratoconus (7 eyes with Amsler stage I KC and 8 eyes with Amsler stage III KC), who subsequently underwent BTOCK surgery. Using the Confoscan-4 confocal light microscope (Nidek, Japan), we studied the structure of central and paracentral cornea. The exact zone of interest was determined individually depending on the location of the graft.

RESULTS

In all cases, morphological changes in the cornea characteristic of the stage II and III KC were detected. Twelve months after the operation, there was an increase in the transparency of intercellular substance of the stroma with a clearer visualization of keratocyte nuclei in the optical zone as well as at the site of the graft. In the area of interface, tissue compaction in the form of diffuse fibrosis with local scar formation could be seen. Neither significant changes in the cell structure, nor a decrease in cell density were found in the central part of ectasia and the areas of clear visualization of the endothelium under the graft. Additional postoperative assessment of the level of corneal light scattering indicated a tendency towards restoration of stromal transparency and optical uniformity.

CONCLUSION

As a result of interlayer graft implantation in the zone of corneal ectasia, a tendency to structural improvement in its central and paracentral parts was revealed. Local fibrosis in the interface area can indirectly indicate biomechanical «strengthening» of this zone.

[Retinal changes after laser interventions on anterior segment of the eyeball]

Laser surgery of the cornea, iris, angle of the anterior chamber and other parts of anterior segment of the eye sees wide application in clinical practice. The adverse effects it can cause in the anterior segment are well known and understood. At the same time, changes the treatment method can cause in the posterior segment of the eye - primarily, in macular area of the cornea, which is of great importance, have not been studied sufficiently. Purpose of the review - to consolidate previously published data on changes in the retina after laser intervention on the anterior segment of the eye.

Revealing structural modifications in thermomechanical reshaping of collagenous tissues using optical coherence elastography

Moderate heating of such collagenous tissues as cornea and cartilages by infra-red laser (IR laser) irradiation is an emerging technology for nondestructive modification of the tissue shape and microstructure for a variety of applications in ophthalmology, otolaryngology and so on. Postirradiation high-resolution microscopic examination indicates the appearance of microscopic either spheroidal or crack-like narrow pores depending on the tissue type and irradiation regime. Such examinations usually require special tissue preparation (eg, staining, drying that affect microstructure themselves) and are mostly suitable for studying individual pores, whereas evaluation of their averaged parameters, especially in situ, is challenging. Here, we demonstrate the ability of optical coherence tomography (OCT) to visualize areas of pore initiation and evaluate their averaged properties by combining visualization of residual irradiation-induced tissue dilatation and evaluation of the accompanying Young-modulus reduction by OCT-based compressional elastography. We show that the averaged OCT-based data obtained in situ fairly well agree with the microscopic examination results. The results obtained develop the basis for effective and safe applications of novel nondestructive laser technologies of tissue modification in clinical practice. PICTURE: Elastographic OCT-based images of an excised rabbit eye cornea subjected to thermomechanical laser-assisted reshaping. Central panel shows resultant cumulative dilatation in cornea after moderate (~45-50°C) pulse-periodic heating by an IR laser together with distribution of the inverse Young modulus 1/E before (left) and after (right) IR irradiation. Significant modulus decrease in the center of irradiated region is caused by initiated micropores. Their parameters can be extracted by analyzing the elastographic images.

[Public healthcare responsibilities of ophthalmologists according to professional regulations]

Modern public healthcare practices are constantly being improved by the medical community, which involves approval and implementation of professional standards such as including public health activities in the responsibilities of medical specialists. To understand how well such skills can be mastered by the specialists, we have conducted a comparative analysis of ophthalmologist qualifications found in the most often used guidelines and textbooks. With logical, analytical and hypothetical analysis, books and guides published before 2018 were found to have insufficient coverage of the public health responsibilities. Improvement of the public healthcare in terms of professional standards is an important course of development for the medical field. The continued work on Russian Federal State Educational Standard (Higher Education) for Ophthalmology - index number 31.08.59 - should include actualization of educational materials with modern public health notions, development of a universal learning evaluation database for specialist accreditation, as well as evaluation tools for the obtainment and confirmation of their qualification. The professional standards for ophthalmologists should fully account for the job responsibilities they acquire. Training of ophthalmologists in the public healthcare should utilize the following textbooks published in 2018: 'Organization of medical aid in the Russian Federation' under the editorship of V.A. Reshetnikova, 'Healthcare and public health' under the editorship of G.N. Tsarik, and 'Public health and healthcare' by V.A. Medic.

[New biophotonics methods for improving efficiency and safety of laser modification of the fibrous tunic of the eye]

The article describes a newly developed and tested diffractive optical element (DOE) that converts non-uniform radiation of the laser output into a homogeneous ring. The Gerchberg-Saxton algorithm is shown to be well suited for achieving annular intensity distribution. Testing this ring transducer on threshold-plasticity cornea demonstrated the reversibility of axisymmetric changes in the cornea. Atomic-Force microscopy of the area of maximum stresses in the corneal periphery showed no significant changes in the structure of the cornea when irradiated in the selected mode. Measurement of Young's modulus of the corneal surface areas after their irradiation also revealed no changes in the elastic properties, while examination of the corneal structure demonstrated the absence of significant structural changes in irradiated samples compared with intact ones.

Optical coherence elastography for strain dynamics measurements in laser correction of cornea shape

We describe the use of elastographic processing in phase-sensitive optical coherence tomography (OCT) for visualizing dynamics of strain and tissue-shape changes during laser-induced photothermal corneal reshaping, for applications in the emerging field of non-destructive and non-ablative (non-LASIK) laser vision correction. The proposed phase-processing approach based on fairly sparse data acquisition enabled rapid data processing and near-real-time visualization of dynamic strains. The approach avoids conventional phase unwrapping, yet allows for mapping strains even for significantly supra-wavelength inter-frame displacements of scatterers accompanied by multiple phase-wrapping. These developments bode well for real-time feedback systems for controlling the dynamics of corneal deformation with 10-100 ms temporal resolution, and for suitably long-term monitoring of resultant reshaping of the cornea. In ex-vivo experiments with excised rabbit eyes, we demonstrate temporal plastification of cornea that allows shape changes relevant for vision-correction applications without affecting its transparency. We demonstrate OCT's ability to detect achieving of threshold temperatures required for tissue plastification and simultaneously characterize transient and cumulative strain distributions, surface displacements, and scattering tissue properties. Comparison with previously used methods for studying laser-induced reshaping of cartilaginous tissues and numerical simulations is performed.