Topical anesthetic

A topical anesthetic is a local anesthetic that is used to numb the surface of a body part. They can be used to numb the front of the eye, the inside of the nose, the throat, the skin, the ear, the anus, and the genital area [1]. Topical anesthetics are available in creams, ointments, aerosols, sprays, lotions, and jellies [2]. Examples include benzocaine, butamben, dibucaine, lidocaine, oxybuprocaine, pramoxine, proparacaine (Alcaine), proxymetacaine, and tetracaine (AKA amethocaine).

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Usage

Topical anesthetics are used to relieve pain and itching caused by conditions such as sunburn or other minor burns, insect bites or stings, poison ivy, poison oak, poison sumac, and minor cuts and scratches [3].

Topical anesthetics are used in ophthalmology in order to numb the surface of the eye (the outermost layers of the cornea and conjunctiva) for the following purposes:

• In order to perform a contact/applanation tonometry.
• In order to perform a Schirmer's test (The Schirmer's test is sometimes used with a topical eye anesthetic, sometimes without. The use of a topical anesthetic might impede the reliability of the Schirmer's test and should be avoided if possible.).
• In order to remove small foreign objects from the uppermost layer of the cornea or conjunctiva. The deeper and the larger a foreign object which should be removed lies within the cornea and the more complicated it is to remove it, the more drops of the topical anesthetic are necessary to be dropped onto the surface of the eye prior to the removal of the foreign object in order to numb the surface of the eye with enough intensity and duration.

Some topical anesthetics are also used in otolaryngology, like for example oxybuprocaine.

Duration of topical anesthesia in the eye

The duration of topical anesthesia might depend on the type and amount applied, but is usually about half an hour.^{[citation needed]}

Abuse when used for ocular pain relief

When used excessively, topical anesthetics can cause severe and irreversible damage to corneal tissues^{[1][2][3][4][5]} and even loss of the eye.^[6] The abuse of topical anesthetics often creates challenges for correct diagnosis in that it is a relatively uncommon entity that may initially present as a chronic keratitis masquerading as acanthamoeba keratitis or other infectious keratitis.^{[1][2][6][7][4]} When a keratitis is unresponsive to treatment and associated with strong ocular pain, topical anesthetic abuse should be considered,^[4] and a history of psychiatric disorders and other substance abuse have been implicated as important factors in the diagnosis.^[1][6][7] Because of the potential for abuse, clinicians have been warned about the possibility of theft and advised against prescribing topical anesthetics for therapeutic purposes.^[2][6]

Some patients who suffer from eye pain, which is often considerably strong neuropathic pain caused by the irritation of the nerves within the cornea and/or conjunctiva, unfortunately try to illegally obtain oxybuprocaine or other eye anesthetics (for example by stealing them at their ophthalmologist, by forging medical prescriptions or by trying to order it via an online pharmacy) and secretly use the substance to numb their eye pain, often ending up with irreversible corneal damage or even destruction (which is a vicious cycle and causes even much more pain). Often, such patients finally require corneal transplantation.

In case of prolonged or chronic eye pain, especially neuropathic eye pain, it is highly advisable to use centrally acting substances like anticonvulsants (pregabalin, gabapentin and in more serious cases carbamazepine) or antidepressants (for example SSRIs or the tricyclic antidepressant amitriptyline). Even very small amounts of an anticonvulsant and/or an antidepressant can almost completely stop eye pain and does not damage the eye at all.

Notes

See also

• Local anesthesia