What Is Retinal Vein Occlusion?
Retinal vein occlusion (RVO) is the second most prevalent retinal vascular condition next to diabetic retinopathy and accounts for 0.77 % of the global population in the age group of 30 and over. The most prevalent cause is venous wall compression by atherosclerotic retinal arteries. Other potential reasons include disorders of the venous wall, such as vasculitis, or abnormalities in the blood components.
The symptoms of retinal vein occlusion include sudden or progressive loss of vision without any pain or abnormality in the visual field. The occlusion's location, degree, and effect on retinal perfusion determine the extent of visual impairment. RVO develops due to a full or partial blockage in the central retinal vein or branches.
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Central Retinal Vein Occlusion (CRVO) - It is a condition when central retinal vein thrombosis occurs at the retrolaminar or lamina cribrosa.
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Branch Retinal Vein Occlusion (BRVO) - It is a type of venous thrombosis that usually occurs at an arteriovenous crossing (a point where the vein and artery share a vascular sheath). It is six to seven times more prevalent than CRVO and occurs in the superotemporal quadrant, where up to two-thirds of BRVOs occur.
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Hemi-Retinal Vein Occlusion (HRVO) - It is a minor variant of chronic retinal vein occlusion (CRVO) that affects either the superior or inferior hemifield at the optic disc and has a similar natural history.
What Are the Ophthalmic Imaging Techniques for Retinal Vein Occlusion?
The ophthalmic imaging techniques for retinal vein occlusion include,
Fundus Photography:
Fundus photography permits the recording and assessment of the clinical image and could be essential in comparison with the findings of other modalities. Widefield fundus photography increases the field of vision from 30 degree to 50 degree to 200 degree, capturing roughly eighty percent of the retina in a single image. Still, it generates a static morphologic rendering.
FA (Fluorescein Angiography):
This offers data such as the degree of vascular leakage, retinal ischemia, and neovascularization. Additional characteristics of FA include the ability to differentiate between new and collateral vessels, extend the arteriovenous transit time, and delay the arm-to-retina time.
Extensive retinal hemorrhages can make it difficult to determine capillary nonperfusion on FA. Like fundus imaging, widefield FA can offer significant details on vascular activity. One major disadvantage of FA is the requirement for intravenous dye instillation, which may take significant clinic time and resources and can result in some morbidity with low risk.
OCT (Optical Coherence Tomography):
OCT has become the gold standard for qualitative and quantitative macular thickness evaluation. It has also been essential in clinical studies to determine the efficacy and outcome of intravitreal anti-VEGF therapy, laser photocoagulation, and corticosteroid administration. OCT relied on a time-domain approach. The spectral domain, or Fourier domain, has generated significantly better imaging (about three times higher axial resolution and 100 times quicker scan speed).
The most advanced technique for clinical research uses spectral-domain OCT (SD-OCT). An additional modification known as swept-source OCT uses a short-cavity swept laser rather than the superluminescent diode laser commonly used in SD-OCT. This results in a high axial resolution of 5 µm, the highest imaging speeds to date, (100,000 A-scans can be obtained per second), visualization of deeper tissues, and a better signal-to-noise ratio.
Optical Coherence Tomography Angiography (OCTA):
OCTA visualizes the perfused retinal vasculature by capturing high-speed, consecutive OCT A-scans at the same retinal locus and then using complicated digital subtraction algorithms to assess variations caused by moving blood columns. The main disadvantage of OCTA is that it can be inaccurate when imaging newly formed arteries and cannot detect leakage from the vascular system or nonperfused veins. Image quality may also be compromised due to host retina distortion, macular edema, or atrophy.
Other Imaging Modalities:
Numerous noninvasive imaging methods provide improved vascular resolution, blood flow calculation, or functional assessment like oximetry.
Blood Flow Evaluation:
Laser Doppler flowmetry uses Doppler shifts in laser light dispersed from vascularized retinal tissue to quantify the volumetric flow of capillary blood. There have been reports of reduced blood volume, flow, and velocity in BRVO areas compared to age-matched normal areas.
Retinal Function Imaging:
It uses high-resolution imaging to quantify blood flow velocity, vascular volumetrics, and oximetry. It has been shown that patients with CRVO and BRVO have reduced blood flow velocities in the arteries and veins of the macular regions. Retinal hemorrhages may restrict retinal vascular imaging, similar to FA.
Laser Speckle Contrast Imaging or Flowgraphy:
It monitors blood flow distribution in real time using speckle patterns. It is significantly associated with flow modalities and the CRVO therapy response in the retina.
Functional Assessment:
Two-wavelength oximetry measures oxygen saturation levels by analyzing the spectral fingerprints of oxyhemoglobin and deoxyhemoglobin in retinal vessels. Studies suggest that blocked veins and arteries have lower oxygen saturation levels. The diagnostic value of these approaches is limited by large intravessel variability and the absence of normative data.
Hyperspectral imaging improves two-wavelength oximetry and allows for comprehensive fundus oximetry by monitoring variations in oxygen saturation in the macro- and microcirculation of the retina.
Better Vessel Resolution:
Adaptive Optics Fundus Imaging - It uses the same optical principles as adaptive optic telescopes to decrease aberrations. It has a transverse resolution of 2.5 µm, enabling observation of capillaries and outer segment cones, 3-D cellular imaging, and detection of fluorescent signals. The diabetic retina has been shown to exhibit a number of microscopic subclinical vascular changes before they manifest clinically, including capillary blockage, recanalization, and reperfusion. The retinal layers can be seen through multispectral imaging at various wavelengths between 550 and 950 nm.
Conclusion
The most effective imaging method for treating RVO is OCT. It adds value by supporting clinical evaluation in RVO diagnosis and treatment. Both fundus photography and FA have been extensively utilized in clinical trials to assist clinicians in recording the course of the disease. The above-discussed novel diagnostic methods provide noninvasive observations of vascular function, but they need to be thoroughly validated in bigger investigations. Therefore, important clinical trials should be the foundation for RVO management guidelines, although they may eventually require revision. Newer imaging techniques should be approached with caution.
