Before elective surgery, a patient gives a history that his brother remained paralysed for six hours after succinylcholine during anaesthesia. Plasma cholinesterase activity is measured and found to be normal. Which additional test would best identify the genetically atypical enzyme responsible for prolonged apnea?

• A Nerve conduction study of the recurrent laryngeal nerve
• B Repeat enzyme assay after incubation with fluoride only
• C Serum butyrylcholinesterase mass spectrometry
• D Measurement of the dibucaine number ✓

Explanation

The dibucaine number measures the percentage inhibition of plasma cholinesterase by dibucaine, a local anaesthetic. Normal enzyme is inhibited about 80 percent (dibucaine number approximately 80), while the atypical enzyme produced by the K variant related mutations is resistant, giving values around 20 in homozygotes and 45 to 60 in heterozygotes. Total enzyme activity can be normal in heterozygotes yet still cause clinically significant prolongation of succinylcholine apnea, so qualitative testing is required.

Reference: Harper's Illustrated Biochemistry, 31st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.