Clinical Uses of Local Anesthetics — MCQs
What is the current medical recommendation regarding topical lignocaine use for teething pain relief in infants?
Which of the following local anesthetics is the most common cause of methemoglobinemia?
Which local anesthetic is considered the most cardiotoxic?
In ophthalmology, if a patient is allergic to aminoesters, which local anesthetic can be safely used?
Which of the following statements about EMLA is true?
Which approach of brachial plexus block targets cords of the brachial plexus:-
A patient undergoing a minor surgical procedure is given lignocaine injection. Assertion: Local anaesthetics act by blocking nerve conduction. Reason: Small fibers and non-myelinated fibers are blocked more easily than large myelinated fibers.
Which nerve is targeted in the nasociliary nerve block?
The following nerves are blocked for repairing inguinal hernia by local anaesthetic except:
Which local anaesthetic is known to cause methemoglobinemia?
Clinical Uses of Local Anesthetics Indian Medical PG Practice Questions and MCQs
Question 1: What is the current medical recommendation regarding topical lignocaine use for teething pain relief in infants?
- A. Once a day
- B. Twice a day
- C. Three or four times a day.
- D. Topical lignocaine is contraindicated in teething (Correct Answer)
Explanation: ***Topical lignocaine is contraindicated in teething*** - Current medical recommendations strongly advise against the use of **topical lignocaine** for **teething pain relief** in infants. - This is due to the risk of **systemic absorption**, which can lead to serious **adverse effects** such as seizures, cardiac arrest, and brain injury, even with small doses. *Once a day* - This recommendation is incorrect because **topical lignocaine** is not considered safe for daily use in infants for teething due to potential toxicity. - Even infrequent use carries risks of **systemic toxicity** in infants whose smaller body mass and developing metabolic systems make them more vulnerable. *Twice a day* - Administering **topical lignocaine twice a day** for teething is not medically recommended and increases the risk of **adverse systemic effects**. - There is a lack of evidence supporting the safety and efficacy of repeated daily application of lignocaine for this indication in infants. *Three or four times a day.* - Frequent application of **topical lignocaine** (three or four times a day) significantly elevates the risk of **lignocaine toxicity** in infants, which can be life-threatening. - Such usage exceeds any conceivable safe therapeutic window and is strictly advised against by health authorities.
Question 2: Which of the following local anesthetics is the most common cause of methemoglobinemia?
- A. Lignocaine
- B. Benzocaine (Correct Answer)
- C. Chloroprocaine
- D. EMLA Cream (Lignocaine + Prilocaine)
- E. Prilocaine
- F. Dibucaine
Explanation: ***Benzocaine***- **Benzocaine** is an ester-type local anesthetic that is the **most common cause of methemoglobinemia** among local anesthetics, especially when used in high doses or on mucous membranes due to its rapid absorption.- Its metabolic byproducts, particularly **aniline derivatives**, are potent oxidizers of hemoglobin, converting the ferrous iron (Fe2+) to ferric iron (Fe3+), thus forming methemoglobin which cannot bind oxygen.- **FDA warnings** have been issued regarding benzocaine-induced methemoglobinemia, particularly with topical spray preparations.*Lignocaine*- **Lignocaine** (lidocaine) is an amide-type local anesthetic and is **rarely associated** with methemoglobinemia.- While it can theoretically cause it in very high doses, it is significantly **less potent** in this regard compared to benzocaine.*Chloroprocaine*- **Chloroprocaine** is an ester-type local anesthetic with a very **short duration of action** due to rapid hydrolysis by plasma cholinesterases.- This rapid metabolism typically limits systemic exposure and makes it an **uncommon cause** of methemoglobinemia despite being an ester.*Prilocaine*- **Prilocaine** is an amide-type local anesthetic that can also cause methemoglobinemia, particularly at **higher doses (>600mg)** [1, 2].- It works through its metabolite, **o-toluidine**, which is an oxidizing agent [1].- However, **benzocaine** is more consistently linked to this adverse effect in clinical practice and has more documented case reports.
Question 3: Which local anesthetic is considered the most cardiotoxic?
- A. Procaine
- B. Prilocaine
- C. Ropivacaine
- D. Bupivacaine (Correct Answer)
Explanation: ***Bupivacaine*** - **Bupivacaine** is an amide-type local anesthetic associated with significant **cardiotoxicity** due to its high lipid solubility and slow dissociation from cardiac sodium channels. - This can lead to severe **arrhythmias** and myocardial depression, making it particularly dangerous in systemic overdose. *Procaine* - **Procaine** is an ester-type local anesthetic with a relatively low potential for cardiotoxicity. - Its rapid metabolism by **plasma pseudocholinesterase** limits systemic exposure and reduces the risk of cardiac effects. *Prilocaine* - **Prilocaine** is an amide-type local anesthetic that is generally less cardiotoxic than bupivacaine. - Its primary concern is the potential to cause **methemoglobinemia** at higher doses, a side effect not directly related to cardiotoxicity. *Ropivacaine* - **Ropivacaine** is an amide-type local anesthetic developed as an alternative to bupivacaine with a reduced cardiotoxicity profile. - It exhibits a more favorable **therapeutic index** for cardiac effects due to its chemical structure and faster dissociation from cardiac sodium channels.
Question 4: In ophthalmology, if a patient is allergic to aminoesters, which local anesthetic can be safely used?
- A. Procaine
- B. Cocaine
- C. Prilocaine (Correct Answer)
- D. Tetracaine
Explanation: **Local anesthetics are classified into two chemical groups: esters (aminoesters) and amides. Allergies to esters typically do not cross-react with amides.** ***Prilocaine*** - **Prilocaine** is an **amide-type local anesthetic**, and allergies to **aminoesters** typically do not cross-react with **amides**. - It is a safe alternative in patients with a known allergy to **ester-type local anesthetics**. *Cocaine* - **Cocaine** is an **ester-type local anesthetic**, sharing a similar chemical structure with **aminoesters**. - Patients allergic to **aminoesters** are likely to experience a **cross-reaction** with **cocaine**. *Procaine* - **Procaine** is a classic **ester-type local anesthetic** (an aminoester). - An allergy to aminoesters directly implies an allergy to **procaine** due to its chemical classification. *Tetracaine* - **Tetracaine** is also an **ester-type local anesthetic** (an aminoester). - It is contraindicated in patients with an allergy to **aminoesters** due to the high risk of **allergic reaction**.
Question 5: Which of the following statements about EMLA is true?
- A. Can be used for intubation
- B. Faster acting
- C. Used only in children
- D. Mixture of local anesthetics (Correct Answer)
Explanation: ***Mixture of local anesthetics*** - **EMLA** (Eutectic Mixture of Local Anesthetics) is a cream composed of a 1:1 mixture of **lidocaine** and **prilocaine**. - This eutectic mixture has a lower melting point than its individual components, allowing it to penetrate the skin more effectively for **topical anesthesia**. *Can be used for intubation* - EMLA is a **topical anesthetic cream** designed for skin surface application rather than mucosal surfaces or deep tissue. - Its slow onset and limited depth of penetration make it **unsuitable for intubation**, which requires rapid and profound anesthesia of the airway. *Faster acting* - EMLA requires an application time of **at least 60 minutes** (and sometimes longer for deeper anesthesia) to achieve maximal effect. - This **slow onset** is a significant limitation and is much slower than injected local anesthetics or other rapid-acting topical agents. *Used only in children* - EMLA is indeed **commonly used in children** to reduce pain from venipuncture, vaccinations, and minor procedures. - However, it is also widely used in **adults** for similar indications, such as before IV insertions, laser treatments, or skin biopsies.
Question 6: Which approach of brachial plexus block targets cords of the brachial plexus:-
- A. Infraclavicular (Correct Answer)
- B. Supraclavicular
- C. Axillary
- D. All of the options
Explanation: ***Infraclavicular*** - The **infraclavicular approach** targets the brachial plexus at the level of the **cords**, which lie deep to the pectoralis muscles and medial to the coracoid process. - This block is particularly useful for surgeries involving the **distal upper limb**, as it blocks all divisions of the cords. *Supraclavicular* - The **supraclavicular approach** targets the brachial plexus at the level of the **trunks**, specifically where they emerge between the anterior and middle scalene muscles. - While it provides good anesthesia for the entire upper limb, it is proximal to the cord level. *Axillary* - The **axillary approach** targets the terminal nerves (e.g., median, ulnar, radial, musculocutaneous) after the brachial plexus has divided into individual nerves in the **axilla**. - This block is distal to the cords and is often used for surgeries of the forearm and hand. *All of the options* - This option is incorrect because only the **infraclavicular approach** specifically targets the cords of the brachial plexus. - The other approaches target either the trunks (supraclavicular) or the terminal nerves (axillary).
Question 7: A patient undergoing a minor surgical procedure is given lignocaine injection. Assertion: Local anaesthetics act by blocking nerve conduction. Reason: Small fibers and non-myelinated fibers are blocked more easily than large myelinated fibers.
- A. Assertion is false, but Reason is true
- B. Both Assertion and Reason are true, and Reason is not the correct explanation for Assertion (Correct Answer)
- C. Both Assertion and Reason are true, and Reason is the correct explanation for Assertion
- D. Assertion is true, but Reason is false
Explanation: ***Both Assertion and Reason are true, and Reason is NOT the correct explanation for Assertion*** - The **Assertion** is true: Local anesthetics (like lignocaine) block nerve conduction by inhibiting **voltage-gated sodium channels**, preventing the depolarization necessary for action potential propagation - The **Reason** is also true: Small diameter and non-myelinated fibers (like C and Aδ pain fibers) are blocked more easily than large myelinated fibers (like Aα motor fibers), which explains the **differential blockade** pattern seen clinically - However, the **Reason does NOT explain WHY** local anesthetics block nerve conduction—it describes **WHICH** nerve fibers are blocked preferentially. The mechanism of blocking conduction is sodium channel inhibition, not fiber size selectivity - The differential sensitivity is a consequence of fiber characteristics (surface area-to-volume ratio, number of nodes of Ranvier), not the explanation for the blocking mechanism itself *Both Assertion and Reason are true, and Reason is the correct explanation for Assertion* - While both statements are individually true, the Reason does not explain the **mechanism** by which local anesthetics block nerve conduction - The Reason addresses fiber **selectivity**, which is a separate pharmacological property from the **mechanism of action** (sodium channel blockade) *Assertion is true, but Reason is false* - The Assertion is demonstrably true—local anesthetics block nerve conduction - The Reason is also true—this is well-established pharmacology: autonomic (small) > sensory (medium) > motor (large) fiber blockade sequence *Assertion is false, but Reason is true* - The Assertion is fundamentally correct and represents the primary pharmacological action of local anesthetics - Blocking nerve conduction is the therapeutic goal of local anesthetic administration
Question 8: Which nerve is targeted in the nasociliary nerve block?
- A. Greater palatine nerve
- B. Sphenopalatine nerve
- C. Anterior ethmoidal nerve
- D. Nasociliary nerve (Correct Answer)
Explanation: ***Nasociliary nerve*** - A nasociliary nerve block specifically targets the **nasociliary nerve** itself. - This block is used to anesthetize the sensory innervation of structures supplied by the nasociliary nerve, such as parts of the **nasal cavity**, **eyeball**, and **skin of the nose**. *Greater palatine nerve* - The **greater palatine nerve** supplies sensation to the posterior hard palate and is targeted in a **greater palatine nerve block**. - This nerve is a branch of the **maxillary nerve** and is primarily involved in dental and palatal anesthesia. *Sphenopalatine nerve* - The **sphenopalatine nerve**, or pterygopalatine ganglion, contains sensory fibers for the nasal cavity, palate, and pharynx, and its block is distinct from a nasociliary block. - A **sphenopalatine ganglion block** is mainly used for conditions like cluster headaches and facial pain, not for direct eyeball sensation. *Anterior ethmoidal nerve* - The **anterior ethmoidal nerve** is a branch of the nasociliary nerve, but a nasociliary nerve block targets the main trunk, which includes all its branches. - While the anterior ethmoidal nerve supplies the anterior part of the nasal septum and lateral wall, it is a **component** of the nasociliary innervation rather than the sole target.
Question 9: The following nerves are blocked for repairing inguinal hernia by local anaesthetic except:
- A. Ilio-inguinal
- B. Femoral (Correct Answer)
- C. Genito-femoral
- D. Ilio-hypogastric
Explanation: ***Femoral*** - The **femoral nerve** primarily innervates the anterior thigh muscles and provides sensory supply to the anterior thigh and medial leg; its block is not typically required for **inguinal hernia repair**. - Blocking the femoral nerve would primarily affect **motor function** of the quadriceps and sensation in the distribution of the saphenous nerve, which is not the surgical field for an inguinal hernia. *Ilio-inguinal* - The **ilio-inguinal nerve** provides sensation to the inguinal region, scrotum/labia majora, and the medial aspect of the thigh, making its block essential for anesthesia during **inguinal hernia repair**. - It lies in the **inguinal canal** and is typically targeted with local anesthetic to cover the incision site and surgical area. *Genito femoral* - The **genitofemoral nerve** has both genital and femoral branches, providing sensation to the scrotum/labia majora and a small area of the femoral triangle, respectively, and is therefore often included in an **inguinal block**. - Its blockade helps to cover the sensory innervation of the **spermatic cord** and a portion of the inguinal region, contributing to effective pain control. *Ilio-hypogastric* - The **ilio-hypogastric nerve** provides sensory innervation to the suprapubic and gluteal regions, and its blockade is important for covering the **upper part of the surgical incision** for an inguinal hernia repair. - It runs parallel to the ilio-inguinal nerve and is often blocked concurrently to ensure **comprehensive analgesia** of the abdominal wall.
Question 10: Which local anaesthetic is known to cause methemoglobinemia?
- A. Procaine
- B. Prilocaine (Correct Answer)
- C. Ropivacaine
- D. Etidocaine
Explanation: ***Prilocaine*** - **Prilocaine** is metabolized into **ortho-toluidine**, which can oxidize hemoglobin to **methemoglobin**, especially at higher doses or in susceptible individuals. - **Methemoglobinemia** symptoms include **cyanosis**, **dyspnea**, and in severe cases, central nervous system depression, due to reduced oxygen-carrying capacity of blood. *Procaine* - **Procaine** is an ester-type local anesthetic. It is metabolized to **para-aminobenzoic acid (PABA)**, which can cause allergic reactions, but it is not associated with methemoglobinemia. - It has a relatively **short duration of action** and is less commonly used now compared to amide-type local anesthetics. *Etidocaine* - **Etidocaine** is an amide-type local anesthetic that is known for its **long duration of action** and high potency. - While it can cause systemic toxicity with high doses due to its cardiac and neurological effects, **methemoglobinemia** is not a characteristic side effect. *Ropivacaine* - **Ropivacaine** is an amide-type local anesthetic similar to bupivacaine, known for its **motor-sparing effect** and use in regional anesthesia. - It is associated with a lower risk of **cardiotoxicity** compared to bupivacaine but does not cause methemoglobinemia.
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