Definition
Ophthalmic anesthetics are topical agents used to numb the surface of the eye (cornea and conjunctiva) for short-term procedures. These agents block sodium channels in neuronal membranes, preventing depolarization and the transmission of nerve impulses, resulting in temporary loss of sensation. They are administered as drops and have rapid onset but short duration, typically lasting between 10 to 30 minutes.
They are essential in ophthalmic diagnostics and surgical procedures, but their use is strictly professional and short-term due to potential corneal toxicity and abuse risk with prolonged use.
1. Mechanism of Action
Ophthalmic anesthetics act by reversibly inhibiting voltage-gated sodium channels in sensory neurons of the cornea and conjunctiva. This blocks signal conduction, thereby eliminating pain or discomfort. The blockade affects free nerve endings responsible for pain and temperature sensation.
2. Primary Indications
-
Tonometry (intraocular pressure measurement)
-
Removal of foreign bodies or corneal sutures
-
Gonioscopy
-
Schirmer test (for tear production)
-
Contact lens fitting (in specific diagnostic procedures)
-
Superficial corneal or conjunctival surgeries
-
Before administering ophthalmic dyes (e.g., fluorescein)
Note: They are not prescribed for home use or for chronic pain relief, as repeated exposure leads to epithelial damage and delayed healing.
3. Common Ophthalmic Anesthetics
| Generic Name | Brand Name | Duration | Onset |
|---|---|---|---|
| Proparacaine hydrochloride | Alcaine, Ophthetic | 10–20 minutes | 20–30 seconds |
| Tetracaine hydrochloride | TetraVisc, Pontocaine | 15–30 minutes | 30–45 seconds |
| Benoxinate hydrochloride + fluorescein | Fluress | 10–15 minutes | 20–30 seconds |
| Lidocaine (gel) | Akten (3.5% gel) | 20–30 minutes (longer contact) | ~1 minute |
| Oxybuprocaine (Europe) | Benoxil | 15–20 minutes | 20–40 seconds |
4. Formulations and Routes
-
Eye Drops: Most common, quick onset, short duration (Proparacaine, Tetracaine)
-
Gels: Longer duration, useful for laser or cataract surgery (Lidocaine 3.5%)
-
Combination Drops: Benoxinate + fluorescein, for procedures like tonometry
5. Comparison of Key Agents
| Agent | Potency | Duration | Notable Uses |
|---|---|---|---|
| Proparacaine | Moderate | 10–20 min | Tonometry, foreign body removal |
| Tetracaine | High | 15–30 min | Gonioscopy, minor procedures |
| Benoxinate | Mild | 10–15 min | Tonometry (with fluorescein) |
| Lidocaine gel | High | 20–30 min | Cataract surgery, LASIK |
6. Pharmacokinetics
-
Onset: 20–60 seconds (rapid)
-
Duration: 10–30 minutes, depending on the agent and concentration
-
Metabolism: Ester-type agents (e.g., proparacaine, tetracaine) are hydrolyzed by plasma esterases
-
Absorption: Minimal systemic absorption, but can be increased with epithelial disruption
7. Contraindications
-
Known hypersensitivity to local anesthetics
-
Damaged corneal epithelium (unless under specialist supervision)
-
Use in infants or children is limited and must be clinically justified
8. Adverse Effects
| Short-Term Use | Prolonged Use |
|---|---|
| Burning or stinging | Epithelial toxicity and erosions |
| Blurred vision | Delayed corneal healing |
| Photophobia | Corneal ulceration and stromal melting |
| Allergic conjunctivitis | Secondary infections |
| Lacrimation | Irreversible visual loss (rare cases) |
9. Special Precautions
-
Not for home or chronic use: Prescribing is limited to trained ophthalmic professionals
-
Avoid contact lenses during and after administration
-
Do not reuse for pain management
-
Storage: Most products require refrigeration (2–8°C)
10. Drug Interactions
While systemic interactions are rare due to limited absorption, caution is advised with:
-
Other topical ophthalmic agents: May alter pH or stability of anesthetic drops
-
Alcohol-based formulations or preservatives: May increase corneal epithelial permeability
-
Topical NSAIDs or corticosteroids: Co-administration can delay epithelial healing
11. Abuse Potential and Misuse
There are documented cases of topical anesthetic abuse, particularly among individuals with chronic eye pain, photophobia, or ocular surface disease. This can lead to:
-
Corneal melting
-
Severe infections
-
Permanent vision loss
Commonly abused agents include tetracaine and proparacaine. Abuse is more prevalent in unsupervised settings or where access to over-the-counter anesthetics is possible.
12. Use in Surgical and Diagnostic Procedures
| Procedure | Anesthetic Preferred |
|---|---|
| Applanation tonometry | Proparacaine or Benoxinate + fluorescein |
| Foreign body removal | Tetracaine or Proparacaine |
| Cataract surgery | Lidocaine gel |
| Gonioscopy | Tetracaine |
| Schirmer’s test with anesthesia | Proparacaine |
| Contact lens fitting (diagnostic) | Proparacaine |
13. Availability and Regulatory Notes
-
United States: Prescription-only, used in-office under professional supervision
-
UK and Europe: Prescription-only or administered by trained personnel (ophthalmologists, optometrists)
-
OTC Availability: Generally banned or tightly restricted due to abuse potential
14. Patient Education Points
-
Expect mild burning or stinging upon administration
-
Do not rub eyes after receiving drops
-
Vision may be temporarily blurry—do not drive immediately
-
Do not use at home or without professional direction
-
Inform clinician of any drug allergies, particularly to local anesthetics
15. Summary of Selected Products
| Product Name | Formulation | Main Ingredient |
|---|---|---|
| Alcaine | Ophthalmic solution (0.5%) | Proparacaine hydrochloride |
| TetraVisc | Ophthalmic solution (0.5%) | Tetracaine hydrochloride |
| Fluress | Drops (Benoxinate + Fluorescein) | Benoxinate hydrochloride + fluorescein |
| Akten | Gel (3.5%) | Lidocaine hydrochloride |
| Benoxil (EU) | Drops | Oxybuprocaine hydrochloride |
No comments:
Post a Comment