a 60-year - old woman ( height , 162 cm ; weight , 61 kg ) visited pain clinic with tactile allodynia and electric shock - like pain in the left dorsal scapular area around the t3 dermatome , which had been diagnosed as phn about 1 month previously and attack of the herpes zoster was 1 year ago .
the 100-mm visual analogue scale ( vas ) of allodynia and electric shock - like pain was rated between 70 and 80 mm on a scale from 0 ( no pain ) to 100 ( worst pain imaginable ) .
the interlaminar epidural block was performed at the t3 - 4 space by the paramedian approach with 5 ml of 0.2% ropivacaine and 20 mg of triamcinolone acetate .
pregabalin and morphine at doses of 150 mg and 10 mg , respectively , twice a day , amitriptyline at a dose of 10 mg before sleep and topical lidocaine patches were prescribed .
dosages of all drugs were adjusted depending on the side effects during the follow - up period .
epidural blocks were repeated twice with a 1-week interval and the continuous intravenous infusion of ketamine ( 60 mg ) was performed over a period of 1 hour twice a week under careful monitoring .
after 1 month elapsed , electric shock - like pain was reduced to a vas score of 30/100 , but allodynia was not diminished ( vas score of 70/100 ) .
after 4 months elapsed , we decided to administer magnesium sulfate via the intravenous route . and
it was done with continuous intravenous infusion of 1,000 mg of magnesium sulfate in 50 ml of normal saline for 1 hour . before and after infusion , the serum magnesium levels were checked . after magnesium therapy
, she felt very good about her pain and the vas of allodynia was reduced to 40 - 50 . at 1-week follow - up , she was very satisfied with the treatment and reported the reduction of allodynia on the dorsal scapular area of up to 50% ( vas 25 - 30/100 ) . however , the serum magnesium level had increased above normal range ( 2.3 meq / l to 2.9
although it was below the serum level reveals of the adverse effect , we decided to stop intravenous infusion of magnesium sulfate . for more accurate and safe delivery of magnesium to the target site , we applied magnesium using the transforaminal epidural injection technique . with the patient 's informed consent , left t3 tfemi
identification of the t3 nerve root sheath and epidural space was performed using contrast media ( fig .
1 ) . then , 100 mg of magnesium sulphate and 1 ml of 0.2% ropivacaine ( total volume , 2 ml ) was carefully injected .
tfemi was repeated twice with a 1-week interval ( total of three times ) and the degree of pain decreased gradually during the follow - up period .
one week after the last procedure , the vas score of allodynia decreased to 15/100 and all medications except pregabalin were discontinued .
the vas was 10/100 throughout 1-month follow - up , and pregabalin had also been tapered .
to our knowledge , no previous report has described about the magnesium administration by the transforaminal epidural route in patients with neuropathic pain . here
, in our report of phn patient , this treatment resulted in effective pain relief .
previous studies have demonstrated the anti - allodynic effects of nmda receptor antagonists in neuropathic pain disorders . among the currently available nmda receptor antagonists ,
ketamine is the most widely used one for the treatment of neuropathic pain . however , ketamine is not always effective and psychomimetic side effects are frequent .
magnesium can antagonize nmda receptor channels by blocking calcium influx in a voltage - gated manner .
intravenous administration of magnesium is efficacious in the management of various conditions associated with neuropathic pain , including phn .
demirkaya and colleagues revealed 1 g i.v . mg sulfate is effective in the treatment of migraine attacks   and collins and colleagues reported that 70 mg / kg magnesium sulphate infusions in 4 hours for 5 days reduced pain in patients with complex regional pain syndrome .
whether intravenous administration of magnesium can achieve a sufficient concentration in the cerebrospinal fluid to block nmda receptors is unclear   and studies have reported on the limited efficacy of magnesium when administered via the intravenous route .
furthermore , even if the dose of intravenously administered magnesium is not sufficient to present toxicity , patients are still at risk of magnesium overdose .
neuraxial administration of magnesium is an " off - label " use , and the safety of this technique in human subjects is still undetermined .
however , animal studies   showed that intrathecally administered magnesium was free of neurotoxicity , and recent studies have demonstrated the safety of magnesium administration via the epidural   or intrathecal   route in humans .
in fact , the exact site of action of epidurally administered magnesium ( i.e. , spinal or supraspinal ) remains unclear .
however , comparison with previous reports regarding intravenous magnesium administration suggested that the low dose epidural magnesium used in our patient was unlikely to result in systemic effects . in conclusion , tfemi showed a favourable result in the treatment of intractable allodynia associated with phn .
this study was performed in only a single case , and further investigations are required to determine the efficacy of tfemi in the management of allodynia in patients with phn .
 although postherpetic neuralgia ( phn ) is a common chronic pain syndrome , the pathophysiology of this disorder is not well known and management is often very difficult . 
 n - methyl - d - aspartate ( nmda ) receptor antagonists are known to be effective in phn , and magnesium , a physiological blocker of nmda receptors , is widely used to treat various chronic pain disorders . here , 
 we present a case of the phn refractory to conventional treatment , which was treated successfully with transforaminal epidural injection of magnesium sulphate at the affected dermatome . 
CASE REPORT
DISCUSSION