Saturday, February 7, 2009

Myasthenia Gravis and Local Anesthetics

I have come across a second patient now with MG that exhibited poor analgesia from local anesthetic. A middle-aged female with MG and Hep C underwent external levator repair yesterday. Pain control was poor despite generous amount of subcutaenous infiltration. Paralytic effect was also poor, as orbicular muscle contracted vigorously during unipolar incision. Extra amount of local anesthetic was required, resulting in eventual paralysis of the levator muscle and inablity to assess function of repair on the table. Patient was also exquisitely sensitive to stretching of the levator muscle, even though already immune from pain when muscle pinched by forceps. She is complicated by (1) her hepatic disease which may alter metabolism of lidocaine, and (2) a hyperacute personality which may be equating tactile sensation to pain.

A year ago I performed a cataract phacoemulsification of a 80 year of female with MG. A retrobulbar anesthesia was given due to unsteady eye position pre-op and squeezing of lids. Despite a good RBB, patient regained movement of eye shortly after beginning of case, making rest of procedure more challenging. The procedure was otherwise uncomplicated.

I wonder if MG somehow hinders the efficacy of local anesthetic. A brief PubMed search under "MG and local anesthetic", "MG and lidocaine", and "MG and analgesia" has not turned out report of similiar cases or mechanisms proposed. MG affects the neuromusclar junction via antibody against the nitoctinic ACH receptor. ACh receptor activation normally leads to dipolarization of the muscle cell, involving Voltage-gated Na channels. Lidocaine works by blocking this channels. Therefore one mechanisms of poor akinesia by lidocaine in MG may be a subsequent up-regulation of these Na channels due to chronic ACh receptor suppression. Literature search found only one paper on the subject, which reported DECREASE of Ach and Na channels at the endplate. Marx A, Siara J, RĂ¼del R Sodium and potassium channels in epithelial cells from thymus glands and thymomas of myasthenia gravis patients.

Association may be complicated as MG comes with other autoimmune conditions. Also, a second type of the MG has been identified that affects MuSK (muscle specific kinnase).

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3 comments:

wolverin said...

hlo sir..... procaine n xylocaine s contraindicated in MG so which s the safe local n general n epidural tat cn b used.....it would b grt help if u would ki9ndly help me in tis....thank u....saran_poison@yahoo.com

wolverin said...

hlo sir..... procaine n xylocaine s contraindicated in MG so which s the safe local n general n epidural tat cn b used.....it would b grt help if u would ki9ndly help me in tis....thank u....saran_poison@yahoo.com

Unknown said...

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