**Question:** A patient presents with h/o evening halos and occasional headache for some months. His examination shows normal IOP but shallow AC. He is in which stage of glaucoma:

A. Early Open Angle Glaucoma
B. Intermediate Angle Closure Glaucoma
C. Advanced Angle Closure Glaucoma
D. Late Open Angle Glaucoma

**Core Concept:** Glaucoma is a group of eye diseases characterized by optic neuropathy and progressive vision loss. It is classified into two types:

1. Open Angle Glaucoma (OAG): The intraocular pressure (IOP) is elevated, and the angle between the cornea and iris remains open.
2. Angle Closure Glaucoma (ACG): The angle between the cornea and iris becomes narrowed or closed, leading to elevated IOP.

**Why the Correct Answer is Right:** The patient presents with evening halos and occasional headache, which are characteristic symptoms of ACG. His examination shows normal IOP but shallow anterior chamber (AC). This indicates that the patient is experiencing an acute phase of ACG, where the AC is not yet shallow enough to raise IOP significantly.

**Why Each Wrong Option is Incorrect:**

A. Early Open Angle Glaucoma (OAG) is characterized by normal AC or slightly decreased AC, which is not the case in this scenario.
B. Intermediate Angle Closure Glaucoma (IACG) usually presents with more advanced AC narrowing and IOP elevation, which is not the case in this scenario.
C. Advanced Angle Closure Glaucoma (AACG) typically presents with significant IOP elevation and severe AC narrowing; this is not the case in this scenario.
D. Late Open Angle Glaucoma (LOAG) is characterized by advanced OAG with secondary angle closure, which is not the case in this scenario.

**Clinical Pearl:**

The presence of evening halos and occasional headache in the context of normal IOP and shallow AC is highly suggestive of an acute phase of ACG. This clinical scenario warrants urgent management, as prompt treatment can prevent irreversible vision loss due to ACG.

**Correct Answer Explanation:**

The correct answer is B (Intermediate Angle Closure Glaucoma) because the patient is experiencing an acute phase of ACG. The symptoms and examination findings indicate that the angle between the cornea and iris is narrowed, which might progress to IACG or AACG if not promptly managed. This type of presentation is less common but more reversible compared to other stages.