Adverse Effects of Topical Anaesthesia – Risks, Complications & Clinical Insights

Topical anaesthesia is an essential tool in modern medicine, particularly in ophthalmology, dermatology, dentistry, and minor surgical procedures. By numbing the surface tissues without the need for injections, it provides comfort, improves compliance, and allows clinicians to perform diagnostic and therapeutic interventions with minimal pain.

However, while topical anaesthetics are widely considered safe, they are not free from side effects. Prolonged use, overdose, or inappropriate application can result in a range of adverse effects — from mild irritation to significant ocular or systemic complications. This article explores the adverse effects of topical anaesthesia, their pathophysiology, clinical manifestations, and strategies to minimize risks.

What is Topical Anaesthesia?

Topical anaesthesia refers to the application of anaesthetic drugs directly to the skin, mucous membranes, or ocular surface to block sensory nerve endings. Common drugs include:

• Lidocaine (Lignocaine)
• Proparacaine
• Tetracaine
• Benzocaine
• Oxybuprocaine

These agents act by blocking sodium channels in nerve membranes, thereby preventing depolarization and transmission of pain signals.

Clinical Uses of Topical Anaesthesia

Ophthalmology: Tonometry, foreign body removal, cataract surgery initiation

Dermatology: Minor excisions, laser treatments, wound cleaning

Dentistry: Oral mucosal numbing before injections or procedures

ENT and Gastroenterology: Endoscopies, laryngoscopy

Emergency Medicine: Superficial wound care, catheterization

Adverse Effects of Topical Anaesthesia

The following are the major complications associated with the use of topical anaesthesia:

1. Epithelial and Endothelial Toxicity

Topical anaesthetics, especially when used excessively in ophthalmology, can damage corneal epithelial cells. Prolonged exposure leads to epithelial defects, delayed wound healing, and endothelial dysfunction, which compromises corneal transparency.

Mechanism: Inhibition of epithelial mitosis and interference with cell membrane stability.

Clinical Signs: Persistent epithelial defects, corneal thinning, stromal oedema.

Risk Factors: Self-medication, repeated application, higher concentration formulations.

2. Allergy to Drug

Allergic reactions to local anaesthetics can range from mild hypersensitivity (itching, redness, conjunctival congestion) to severe anaphylaxis. Ester-type anaesthetics (e.g., procaine, tetracaine) have a higher risk of causing allergic responses compared to amide-type drugs (e.g., lidocaine).

Symptoms: Redness, itching, swelling, tearing, difficulty breathing (rare systemic reaction).

Management: Immediate discontinuation, antihistamines, corticosteroids in severe cases.

3. Alteration of Lacrimation

Topical anaesthetics may interfere with tear film stability and tear production. This leads to dry eye symptoms, ocular surface irritation, and blurred vision.

Mechanism: Disruption of corneal sensory nerves reduces reflex tearing.

Clinical Impact: Patients may experience foreign body sensation and chronic discomfort.

Prevention: Limited use, artificial tears supplementation, avoidance in pre-existing dry eye disease.

4. Surface Keratopathy

Repeated use of topical anaesthesia can cause toxic keratopathy. This condition is characterized by punctate epithelial erosions, corneal opacities, thinning, and even perforation in severe cases.

Clinical Signs: Pain, photophobia, reduced visual acuity.

Long-term Risks: Scarring, permanent vision loss if untreated.

Important Note: Abuse of topical anaesthetic eye drops is a recognized cause of severe keratopathy, often seen in patients who self-medicate for chronic eye pain.

Systemic Adverse Effects

Though rare with topical administration, systemic absorption of anaesthetics (especially through mucous membranes or damaged epithelium) can cause neurological and cardiovascular side effects:

Central Nervous System (CNS) Toxicity: Dizziness, tremors, seizures, confusion.

Cardiac Toxicity: Bradycardia, hypotension, arrhythmias.

Methemoglobinemia: Seen with benzocaine use, leading to cyanosis and hypoxia.

Risk Factors for Adverse Effects

1. Overuse and Abuse – particularly in ophthalmic settings.

2. Pre-existing Ocular Disease – dry eye, keratoconjunctivitis, or corneal ulcers.

3. Drug Sensitivity – history of drug allergies.

4. Inappropriate Concentration – high-dose formulations.

5. Application on Large Surfaces – increased absorption, systemic risk.

Prevention and Safety Measures

• Use the lowest effective concentration for the shortest possible duration.
• Avoid prescribing topical anaesthetics for self-medication, especially for chronic pain.
• Screen patients for drug allergy history.
• Monitor for ocular surface toxicity in long-term users.
• Educate patients on proper use and potential complications.

Comparison of Common Topical Anaesthetics

Drug	Onset of Action	Duration	Risk of Toxicity	Special Notes
Proparacaine	20–30 seconds	10–15 min	Moderate	Common in ophthalmology
Tetracaine	30 seconds	10–20 min	High	Higher allergy risk
Lidocaine	1–2 minutes	20–30 min	Moderate	Widely used, safe
Benzocaine	30 seconds	10–15 min	High (MetHb risk)	ENT, dental use
Oxybuprocaine	20 seconds	10–15 min	Moderate	Ophthalmic tonometry

Frequently Asked Questions (FAQ)

Q1. Can topical anaesthesia damage the eyes?

Yes. Prolonged or abusive use can cause corneal epithelial damage, delayed healing, and surface keratopathy, which may lead to permanent vision loss.

Q2. Are allergic reactions to topical anaesthesia common?

Allergic reactions are uncommon but can occur, especially with ester-type anaesthetics such as tetracaine and benzocaine.

Q3. Can topical anaesthetics cause systemic side effects?

Yes, though rare. Excess absorption may lead to dizziness, seizures, cardiac arrhythmias, or methemoglobinemia.

Q4. Why should patients not self-medicate with topical anaesthetics?

Because unsupervised use can lead to severe keratopathy, corneal perforation, and irreversible vision impairment.

Q5. How can doctors minimize the risks of topical anaesthesia?

By using the lowest effective dose, limiting duration, monitoring for toxicity, and avoiding self-prescription.