Histology of Retina: Rods vs Cones

Retinal Anatomy and Histology

• A potential space exists between pigmented epithelium and neural layer of retina, associated with retinal detachment.
• Lack of tight junctions between these layers facilitates retinal pathologies.

Photoreceptor Differences: Rods vs. Cones

• Rods are sensitive to low light, allowing night vision (black and white perception).
• Cones enable sharp vision and color detection (RGB) but have poor sensitivity in low light.
• Rods are primarily located in peripheral retina; cones are concentrated in the macula.

Common Retinal Disorders

• Vascular diseases (e.g., diabetic retinopathy)
• Degenerative diseases
• Congenital diseases
• Neoplastic diseases

Diabetic Retinopathy (DR)

• Only 15%-20% of diabetics develop DR; primarily linked to diabetes mellitus (DM).
• Chronic hyperglycemia leads to retinal blood vessel dysfunction, causing DR.
• Initially asymptomatic; untreated, it may result in low vision or blindness.

Risk Factors for Diabetic Retinopathy

• Duration of diabetes, particularly first 5-10 years may be asymptomatic.
• Poor blood sugar control and presence of hypertension (HTN).
• Hyperlipidemia, especially hypertriglyceridemia.
• Barriers to care can hinder screening and follow-up.

Pathophysiology of Diabetic Retinopathy

• Microvasculopathy leads to retinal capillary occlusion and leakage.

Stages of Diabetic Retinopathy

• Non-proliferative stages vary in severity (mild to very severe).
• Proliferative stages involve new vessel growth (neovascularization).

Neovascularization in DR

• Occurs on retinal surface (NVD/NVE) or iris (NVI/rubeosis iridis), potentially leading to neovascular glaucoma.

Key Fundus Appearance Features

• Normal fovea is located temporal and inferior to optic nerve.
• Dot blot hemorrhages and hard exudates are early signs of DR.
• Macular edema is the primary complaint in non-proliferative DR detectable by optical coherence tomography (OCT).

Treatment Options for Diabetic Retinopathy

• Control diabetes; for macular edema, consider intravitreal anti-VEGF injections.
• For proliferative stages: Pan-retinal photocoagulation in early stages; surgery for advanced stages.
• Pars plana vitrectomy may be done for tractional retinal detachment.

Recommended Eye Examination Schedule

• Type 1 diabetes: First exam 3-5 years after diagnosis, yearly thereafter.
• Type 2 diabetes: Exam at diagnosis, yearly follow-up recommended.
• Patients with severe NPDR: follow-up every 1-3 months.

Prevention and Management of DR

• Focusing on blood glucose control.
• Laser photocoagulation for PDR.
• Intravitreal injections of anti-vascular endothelial growth factors (e.g., Avastin, Lucentis).

Hypertensive Retinopathy Overview

• Refers to retinal changes seen in patients with hypertension; includes phases: vasoconstrictive, exudative, and sclerotic.
• Can occur without arteriosclerotic changes in secondary hypertension (e.g., pregnancy-induced).

Ocular Findings in Hypertensive Retinopathy

• Arteriolar narrowing and A-V nicking.
• Cotton wool spots, retinal hemorrhages, and optic nerve swelling.
• Associated complications: Branch retinal artery/vein occlusions, neovascular glaucoma.

Clinical Presentation

• Patients may experience sudden painless vision loss.
• Clear red reflex in fundus; critical for diagnosing conditions like CRVO and BRVO.

Pathognomonic Signs

• Bright reflex associated with central retinal vein occlusion (CRVO), branch retinal vein occlusion (BRVO), and retinal artery macroaneurysm (RAMA).