A 52-year-old woman with newly diagnosed primary open-angle glaucoma (POAG) is started on topical timolol 0.5% twice daily. After 4 weeks, IOP remains 24 mmHg in both eyes (target IOP: 16 mmHg). She tolerates the medication well with no systemic side effects. Visual fields and optic disc remain stable. What is the most appropriate next step?

Explanation

## Management of Inadequate IOP Control on Monotherapy ### Clinical Context: Inadequate Response to Beta-Blocker **Key Point:** The patient has failed to achieve target IOP on timolol monotherapy (IOP 24 mmHg vs. target 16 mmHg). The standard approach is to add a second agent from a different drug class rather than escalate the dose of the same agent. ### Rationale for Adding a Prostaglandin Analogue **High-Yield:** When monotherapy fails to achieve target IOP, combination therapy with agents of different mechanisms of action provides additive IOP reduction. Prostaglandin analogues (latanoprost, travoprost, bimatoprost) are the most effective second-line agents and can reduce IOP by an additional 25–30% when added to beta-blockers. ### Mechanism of Additive Effect | Agent Class | Mechanism | Typical IOP Reduction | |---|---|---| | Beta-blockers (timolol) | Decrease aqueous humor production | 20–25% | | Prostaglandin analogues | Increase uveoscleral outflow | 25–30% | | Combination | Dual mechanism | 35–45% | **Clinical Pearl:** The combination of a beta-blocker + prostaglandin analogue is one of the most commonly prescribed and evidence-based dual therapies in POAG management. ### Why Other Options Are Suboptimal #### Increasing Timolol Concentration (0.5% → 1%) **Warning:** Increasing the concentration of the same drug class does not provide additive IOP reduction beyond what is already achieved at 0.5%. The dose-response curve for timolol plateaus at 0.5%; higher concentrations increase systemic absorption and adverse effects (bradycardia, bronchospasm, fatigue) without meaningful additional IOP benefit. #### Switching to Carbonic Anhydrase Inhibitor (CAI) Monotherapy **Reason to avoid:** CAIs (dorzolamide, brinzolamide topical; acetazolamide systemic) are less potent than prostaglandin analogues and are typically reserved for: - Adjunctive therapy in inadequately controlled POAG - Patients intolerant to prostaglandins - Acute angle-closure glaucoma (systemic acetazolamide) Switching to CAI monotherapy would represent a step backward in efficacy. #### Argon Laser Trabeculoplasty (ALT) Immediately **Clinical Pearl:** ALT is indicated when: 1. Medical therapy has been optimized (usually 3–4 agents) 2. Patient is intolerant to or non-compliant with medications 3. Target IOP cannot be achieved with maximal medical therapy ALT is **not** a first-line intervention for inadequate monotherapy response; medical optimization takes precedence. ### Treatment Algorithm for POAG ```mermaid flowchart TD A[POAG diagnosed, target IOP set]:::outcome --> B[Start monotherapy<br/>Prostaglandin or Beta-blocker]:::action B --> C{IOP at target?}:::decision C -->|Yes| D[Continue monotherapy<br/>Monitor 3-6 monthly]:::action C -->|No| E[Add second agent<br/>Different class]:::action E --> F{IOP at target?}:::decision F -->|Yes| G[Continue dual therapy<br/>Monitor]:::action F -->|No| H[Add third agent or<br/>Consider laser/surgery]:::action H --> I{IOP controlled?}:::decision I -->|No| J[Laser trabeculoplasty<br/>or Filtration surgery]:::urgent ``` **Key Point:** The stepwise approach to POAG is: monotherapy → dual therapy → triple therapy → laser/surgery. This patient is at the dual therapy stage. [cite:American Academy of Ophthalmology Preferred Practice Pattern: Primary Open-Angle Glaucoma 2023]