Diagnostic usefulness of indocyanine green angiography ...

Diagnostic Utility of Indocyanine Green Angiography in Diagnosing AMD

Reviewed/Updated by the authors, July.

For additional details, please visit weiqing.

Age-related macular degeneration (AMD) is recognized as a significant contributor to serious loss of central vision and legal blindness for individuals over 55 years of age in developed nations. This condition poses a significant global public health challenge.

The exudative (neovascular) form of AMD, although less prevalent, accounted for approximately 80-90% of severe vision impairment cases. Central to this issue is choroidal neovascularization (CNV), which emerges in various forms, causing considerable complications and further loss of vision among affected individuals.

Historically, the ability to categorize CNV by its angiographic characteristics held great significance, particularly when treatment modalities were primarily confined to thermal laser ablation or photodynamic therapy (PDT). However, advancements such as spectral-domain optical coherence tomography (SD-OCT) and intravitreal anti-angiogenic injections have transformed diagnosis and monitoring of patients with AMD.

Current guidelines endorse anti-VEGF therapy as the treatment of choice for exudative AMD, with potential for alternate treatments to enhance efficacy in specific cases.

The rise of SD-OCT has significantly supplanted other traditional diagnostic techniques, lending added anatomical insights beyond the capabilities of fluorescein angiography (FA). SD-OCT delineates the retinal and choroidal layers involved with neovascular membranes, aided by the emergence of technologies such as swept-source OCT (SS-OCT), which improves acquisition speed and reduces noise levels in imaging. Over the last decade, the utilization of OCT diagnostics has proliferated, while that of FA has seen a marked decline.

Despite these innovations, we maintain that FA remains an essential diagnostic method. Its prognostic insights are invaluable, particularly in reassessing patients who exhibit suboptimal responses to treatment. The American Academy of Ophthalmology emphasizes the use of intravenous fundus FA when unexplained blurred vision is reported, as well as its role in diagnosing, assessing the extent, type, size, and location of CNV, along with monitoring for recurrence following treatment.

The retina and imaging communities are eager to evaluate whether advancements in OCT technology could inevitably phase out FA.

Indocyanine green angiography (ICGA) has emerged as a critical tool in the analysis of occult CNV (OCNV), particularly in recognizing distinct patterns associated with idiopathic polypoidal choroidal vasculopathy (IPCV) and retinal angiomatous proliferation (RAP). Precise identification of these conditions is crucial as they respond to varying treatment modalities. A key advantage of ICGA lies in its ability to scrutinize choroidal circulation and its interactions with the retina. Nonetheless, interpreting ICGA images can be challenging and may necessitate corroboration with data from other diagnostic techniques such as FA or OCT, combined with detailed biomicroscopic evaluations of the macula.

This chapter provides a thorough examination of the clinical insights gained from ICGA relative to diagnosing and monitoring AMD. It delineates the most effective applications of this technique and its ongoing relevance in clinical practice.

In conclusion, the distinctive ICGA imaging patterns observed in AMD patients form the crux of this analysis, underscoring the necessity of this technique in attaining accurate diagnoses for effective treatment outcomes in this multifaceted disease.

Overview of Indocyanine Green Angiography (ICGA)

Indocyanine green (ICG) is a dye characterized by its tricarbocyanine structure, exhibiting both hydrophilic and lipophilic characteristics. The empirical formula for ICG is C43H47N2NaO6S2, with a molecular weight of 775 Daltons. When diluted in saline, ICG tends to polymerize at high concentrations, while in lower concentrations, it remains as monomers suitable for intravenous injection.

ICG demonstrates peak absorption for monomers and polymers at 785 nm and 690 nm, respectively. Its absorption spectrum shifts to 810 nm upon binding to plasma proteins, leading to an increase in emitted fluorescence intensity. Within blood, ICG operates effectively between wavelengths of 790-830 nm.

Upon intravenous injection, ICG binds to blood lipoproteins at levels reaching up to 98%, resulting in enhanced fluorescence. The liver metabolizes and excretes ICG, primarily through bile.

While adverse reactions associated with ICG use can mirror those of fluorescein, their occurrence is comparatively rare. Reports indicated incidences of mild (0.15%), moderate (0.2%), and severe reactions (0.05%). Caution is warranted for individuals with allergies to shellfish or iodine, liver dysfunction, or end-stage renal disease. Though not tested in pregnant animal models, prevailing practices suggest a more flexible approach to administering ICG in pregnant patients.

ICGA is conducted similarly to FA, incorporating specific light excitation and barrier filters. Post dye injection, images documenting ICG's passage through the retinal fundus are recorded at defined time intervals.

Through its unique biophysical attributes, ICG is advantageous for visualizing choroidal circulation. Its infrared excitation aids penetration through retinal pigments, melanin, and vascular layers, facilitating a clear view of choroidal vasculature. Notably, ICGA can delineate CNV obscured by fluids, exudates, or retinal pigment epithelium detachments.

ICGA in AMDClinical Utility and Applications

The application of ICGA in AMD has been instrumental in the identification of choroidal neovascularization missed by traditional FA. Early studies documenting the use of ICGA provided opportunities for targeted therapies in previously challenging cases, enhancing the prognosis for individuals inflicted by exudative AMD.

ICGA continues to play a crucial role in accurately diagnosing OCNV cases, especially when simultaneously addressing serous pigment epithelium detachments, which poses significant challenges in standard imaging practices. The precise distinction of OCNV reliant on classic imaging modalities keeps ICGA at the forefront of AMD diagnostic strategies.

Throughout its history, therapeutic advancements like photodynamic therapy (PDT) prompted re-evaluations of the role of ICGA for detecting, assessing, and outlining the neovascular membranes critical for subsequent interventions.

In the realm of exudative AMD management, ICGA remains relevant as it enables the timely classification of neovascularization types essential for determining individualized treatment protocols. The technique's advantages in instances of occult and complex lesions uphold its value against newer non-invasive technologies.

As improvements in imaging technologies continue to unfold, ICGA's adaptability combined with conventional methods fosters ongoing relevance in the diagnostic landscape of AMD. Vigilance in adopting emerging modalities in conjunction with ICGA will foster even broader capabilities in monitoring and managing AMD as further studies elucidate this correlation.

ICGA, complemented by its imaging counterparts, guarantees continued enhancements in the diagnosis and treatment of age-related macular degeneration, emphasizing the importance of employing this powerful tool effectively in clinical routines.

In summary, the multifaceted functions of ICGA provide essential insights into the dynamics of ocular conditions, especially where traditional modalities may fall short of solving diagnostic dilemmas. Efforts to refine and validate its applications against a growing backdrop of innovations will undoubtedly consolidate indocyanine green angiography's role in the evolving field of vitreoretinal therapy.

Indocyanine Green Angiography - Medical Clinical Policy ...