Glaucoma

USMLE Guide: Glaucoma

Introduction

Glaucoma is a group of eye disorders characterized by damage to the optic nerve, which can lead to vision loss or blindness if left untreated. It is the second leading cause of blindness worldwide and affects millions of people. This USMLE guide aims to provide a comprehensive overview of glaucoma, including its types, pathophysiology, clinical features, diagnosis, and management.

Types of Glaucoma

1. Primary Open-Angle Glaucoma (POAG): This is the most common form of glaucoma and occurs when the drainage angle of the eye becomes less efficient over time, leading to increased intraocular pressure (IOP).

2. Angle-Closure Glaucoma: This type occurs when the drainage angle of the eye becomes completely blocked, causing a sudden and severe increase in IOP.

3. Normal-Tension Glaucoma (NTG): In NTG, optic nerve damage occurs despite normal IOP levels. The exact cause is unknown, but factors like vascular abnormalities or increased susceptibility to damage are believed to play a role.

Pathophysiology of Glaucoma

Glaucoma is primarily caused by increased IOP, which can damage the optic nerve. The exact mechanisms leading to increased IOP differ based on the type of glaucoma:

• POAG: Impaired aqueous humor outflow through the trabecular meshwork leads to increased resistance, resulting in elevated IOP. This increased pressure can damage the optic nerve.

• Angle-Closure Glaucoma: The iris blocks the drainage angle, preventing the outflow of aqueous humor. This causes a sudden increase in IOP, leading to optic nerve damage.

• NTG: Although IOP levels are within the normal range, other factors like blood flow abnormalities or an increased susceptibility to damage compromise the optic nerve's vascular supply, resulting in damage.

Clinical Features

• POAG: Patients are usually asymptomatic until late stages. Gradual peripheral vision loss (visual field defects) is the hallmark sign. Patients may also complain of headache or eye discomfort.

• Angle-Closure Glaucoma: Sudden onset of severe eye pain, blurred vision, halos around lights, redness of the eye, and nausea/vomiting are typical symptoms.

• NTG: Similar to POAG, NTG patients experience gradual peripheral vision loss. However, IOP levels remain normal.

Diagnosis

• Tonometry: Measures IOP using instruments like Goldmann applanation tonometry or non-contact tonometry.

• Gonioscopy: Evaluates the drainage angle of the eye to determine if it is open or closed.

• Ophthalmoscopy: Assesses the optic nerve head for characteristic cupping or damage.

• Visual Field Testing: Detects visual field defects caused by glaucoma using techniques like automated perimetry.

Management

1. Medications: Topical eye drops that decrease IOP, such as prostaglandin analogs, beta-blockers, alpha-agonists, or carbonic anhydrase inhibitors, are commonly prescribed.

2. Laser Treatment: Laser trabeculoplasty or laser peripheral iridotomy may be performed to improve aqueous humor outflow or relieve angle closure.

3. Surgery: In cases where medications or laser treatment fail to control IOP, surgical interventions like trabeculectomy or implantation of drainage devices may be necessary.

4. Regular Follow-up: Glaucoma requires long-term management, including regular eye exams, monitoring IOP, and adjusting treatment as needed.

Conclusion

Glaucoma is a significant eye disorder that can lead to irreversible vision loss if not diagnosed and managed promptly. Understanding the types, pathophysiology, clinical features, diagnosis, and management of glaucoma is crucial for healthcare professionals. By utilizing this USMLE guide, medical students can enhance their knowledge and readiness for glaucoma-related questions in exams or clinical practice.