How does local anesthesia work?

In the form of sprays, gels, patches, injected... Local anesthesia is a type of intervention that can be do in multiple ways but that in all of them implies numbing a small region to avoid feeling pain.

Local anesthetics are used both at home, to treat a sore throat or discomfort in the gums, such as in medical professional consultations, such as at the dentist or in the dermatologist.

The mechanism of action of the drugs used in these procedures has the particularity that they affect the nerves, something that we will see in greater depth by seeing how local anesthesia works in a summarized way.

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How does local anesthesia work?

Local anesthesia consists of the use of drugs to temporarily numb a small part of the body. This type of anesthesia is applied before performing a minor medical procedure, such as a skin biopsy and, also, in the dentist's office for the extraction of a tooth or application of a filling.

Unlike general anesthesia, local anesthesia does not put the patient to sleep, intervening in the nerves of the area where it has been applied. To understand how it works we must first review how the nervous system works in general.

As we already know, in the nervous system we find cells called neurons, which have a membrane with many channels that allow ions, electrically charged molecules, to pass through them. The ions that are involved in the transmission of the nerve impulse are mainly three: sodium (Na +), potassium (K +) and chlorine (Cl-).

When a stimulus affects a sensory cell, a nerve signal is produced. This signal can be experienced in different ways such as, for example, in the form of temperature, pressure or, related to today's topic, pain.

For a nerve impulse to be generated it is necessary that the threshold of electrical activation of the neuron is reached and, if it is reached, it will lead to a process called depolarization.

When the neuron is at rest, its exterior is positive and its interior is negative. This changes when a sufficiently strong stimulus is received, something that causes the channels of the membrane to open, introducing Na + to the nerve cell and causing K + to come out, in a ratio of 3 to 2. In this way, the interior of the neuron becomes more positive, allowing the action potential to occur and thus transmitting the nerve impulse.

Having understood this, what does it have to do with how local anesthesia works? The truth is that a lot, since the mechanism of action of local anesthetic drugs directly affect the ability of neurons in the intervened area to emit an impulse. Local anesthesia blocks the Na + channels of the neuronal membrane, preventing this ion from entering the neuron and making its interior positive. Since the interior remains negative, depolarization cannot occur.

Local anesthesia prevents depolarization of the neuron, which makes the area under the effects of this intervention cannot transmit sufficient signals and, therefore, pain is not felt in the region.

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Properties of these drugs

There are a variety of local anesthetics and therefore their properties vary, although they all have in common being slightly alkaline. Because of this, these drugs do not work very well in acidic environments such as inflammation or infection, which is why they are not used in these medical conditions.

Chemically, local anesthetics are molecules made up of an aromatic lipophilic ring linked to a hydrophilic group through an intermediate bond that can be of the ester or amide type. It is just the type of link that can be found in the anesthetic that makes it receive one name or another, in addition to influencing the way in which the body metabolizes the substance.

Amide-type anesthetics are metabolized in the liver, while ester-type anesthetics are metabolized by pseudocholinesterases in the blood. The ester anesthetics, once they are metabolized, give as a para-aminobenzoic acid metabolite which can cause allergies in some people.

As a general rule, you can tell if a local anesthetic belongs to the ester group or the amide group by looking at its name.. In the case of amides, in their name there is another "i" apart from the one that forms the suffix -caine, such as lidocaine, mepivacaine, prilocaine or ropivacaine, while in esters there is only the "i" of -caine, as we can see in chloroprocaine, procaine, cocaine and benzocaine.

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Types of local anesthesia

There are two main types of local anesthetics depending on how they are applied.

Topical anesthetics

Topical anesthetics are applied directly to the skin or mucous membranes, such as the inside of the mouth, nose, and throat. They can also be applied to the surface of the eye. Topical anesthetics are marketed and applied in multiple ways:

• Liquids
• Creams
• Gels
• Sprays
• Patches

In some cases, the doctor may use a combination of local anesthetics to have a longer lasting effect long-term.

Examples of procedures that use topical local anesthesia include:

• Apply or remove points
• Prick with a needle
• IV insertion
• Catheter insertion
• Laser treatments
• Cataract Surgery
• Endoscopy

Most of the local anesthetics that we find in a pharmacy are topical, benzocaine compounds in many cases, and are used to manage pain from:

• Teeth, gums, or mouth ulcers
• Open wounds
• Throat pain
• Minor burns
• Poison ivy rash
• Bug bites
• Hemorrhoids

Injected anesthetics

Local anesthetics can be administered through injections. This type of anesthetics They are usually used for minor procedures in which it is necessary to numb the intervention area rather than for pain management. Among the procedures where local anesthesia is injected we find:

• Dental intervention, as in root canal treatment.
• Skin biopsy
• Elimination of a growth under the skin
• Removal of moles or deep warts
• Pacemaker insertion
• Diagnostic tests such as lumbar puncture or bone marrow biopsy

The type of anesthesia required for the specific case will vary. depending on the particularities of the procedure and the patient's own characteristics. For example, in the case of cataract surgery, this type of intervention can be done with both topical and injected anesthesia. The doctor will determine the best type of anesthesia to apply based on the following factors.

• The duration of the procedure
• The size and location of the area to be numbed
• If there is an underlying health condition that may affect the procedure
• Medication the patient takes

How is it administered?

The patient does not have to do much while preparing to administer the local anesthetic; however, you need to inform your doctor or anesthetist of any inconvenience that could affect both the effectiveness of the local anesthetic and the possibility of presenting side effects. The information that the doctor should know about the patient before applying local anesthesia is:

• If there are any open wounds near the affected area
• If you take any type of medication, especially blood thinners
• If you have a bleeding disorder, such as hemophilia or Von Willebrand disease

The patient will receive local anesthesia shortly before the procedure begins, allowing time enough to the anesthetic to begin to take effect and perform the intervention for the duration of the numbness.

Most likely, the operation will last a few minutes in which the patient should not notice any pain, although he may notice certain sensations of pressure in the intervened area. In case you notice pain, it will be necessary to apply a higher dose of the local anesthetic.

Local anesthesia is usually applied for an hour, but the patient will notice slight numbness for a few more hours. As its effects wear off, the patient may notice tingling and spasms.. It is advisable for the patient to be careful with the numb area, since as it is not so noticeable, there is a greater probability of suffering an injury without realizing it.

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Side effects of local anesthesia

Local anesthetics are generally safe and do not cause side effects, except for the tingling and small spasms that may be noticed in the numb area after the intervention. However, if a higher dose than normal has been administered, the injection was made into a vein instead of a tissue or simply that the patient is more sensitive than the average to the anesthetic, the following effects are likely to appear secondary:

• Ringing in the ears
• Dizziness
• Numbness
• Spasms
• Metallic taste

In extremely rare cases where there has been an administration of too much dose of local anesthesia the following effects can occur:

• Seizures
• Low blood pressure
• Slow heart rate
• Trouble breathing

An allergic reaction to the anesthetic is also possible, although this is a rare situation and research suggests that only 1% of the general population would be allergic to local anesthetics.