histological evaluation of a peripheral nerve is often the final diagnostic work - up for a neuropathy of unknown origin , and is most informative when the nerve is clinically affected . a distal sensory nerve , including the sural , superficial peroneal , or superficial radial nerve , is usually biopsied , and a motor nerve branch is rarely considered [ 2 , 3 , 4 ] . here , we report a case with painful unilateral neuropathy in the upper arm . in this case , a biopsy of a pronator teres motor branch provided useful information in consideration of the pathogenesis . a previously healthy 19-year - old japanese woman noticed a skin rash on her lower limbs . after 4 months , the rash spread to her hands and forearms . in february 2013 ( 9 months from the onset of the exanthema ) , she was admitted to our hospital because of sudden paresis and pain in her left upper limb . at the first presentation , she showed livedo reticularis in all four limbs , with a temperature of 36.5c , a pulse of 65 beats per minute , and a blood pressure of 112/62 mm hg . examination showed decreased sensation in the radial side of her left forearm and dorsum of the hand . manual muscle testing was uncompleted in her left forearm because of severe neuralgia , although it was normal in the left proximal arm and lower limbs . laboratory findings were normal for sedimentation rate ( 8.0 mm / h ) , leukocyte counts ( 5,880/mm ) , serum c - reactive protein ( 0.01 mg / dl ) , serum creatinine ( 0.49 mg / dl ) , and urinalysis . tests for serum antibodies , including antinuclear , anti - dna , anti - rnp , anti - ssa and anti - ssb , igg anticardiolipin , and anti - beta-2-glycoprotein i , were negative . serum antineutrophil cytoplasmic antibody was negative in both perinuclear and cytoplasmic staining patterns , and antimyeloperoxidase and proteinase 3 specificity were not detected . the results of the serological tests for acute infection with epstein - barr virus , herpes simplex virus , varicella - zoster virus , cytomegalovirus , human parvovirus b19 , and mycoplasma pneumoniae were all negative . antibodies to human immunodeficiency virus ( hiv ) and hepatitis b and c viruses were also negative . cerebrospinal fluid analysis showed a normal cell count of 1/mm ( mononuclear cells ) with normal total protein concentration ( 26 mg / dl , normal < 40 mg / dl ) . mri scans of the brain , whole spine , left upper limb , cervical nerve roots , and brachial plexus and whole - body computed tomography scans showed no abnormal findings . skin biopsy specimens from the livedo reticularis in her left lower limb showed no specific findings of vasculitis or thrombophlebitis . in neurophysiological studies , the patient 's motor nerve conduction in the median , ulnar , and radial nerves was within the normal range ; however , the compound muscle action potential and conduction velocity in the left radial nerve ( 8.4 mv , normal > 7.0 mv ; 64.4 m / s , normal > 60 m / s , respectively ) were decreased in comparison with the right radial nerve ( 10.6 mv and 91 m / s , respectively ) . the result of the sensory nerve conduction study was normal . whilst we were unable to record the motor unit potentials during voluntary contraction of the left forearm muscles because of the patient 's severe pain , ultrasound examination of the ulnar , median , and radial nerve in the left forearm showed no abnormalities . livedo reticularis and severe neuralgia pointed to vasculitic neuropathy as the most likely pathological condition in this patient . due to the acute progression of the motor impairment , a single course of intravenous immunoglobulin ( 400 mg / kg body weight daily for 5 consecutive days ) with methylprednisolone pulse therapy ( intravenous methylprednisolone 1 g daily for 3 consecutive days ) was commenced . improvement of the patient 's neuralgic pain and motor dysfunctions was observed within a few days of commencement of the therapy . however , severe tenderness along the median nerve in her left forearm and muscle weakness mainly in the left median nerve territory remained . manual muscle testing showed muscle weakness in the left median nerve territory : flexor digitorum profundus ( second finger : 3/5 ; third finger : 4/5 ; intact in fourth and fifth finger ) , flexor digitorum superficialis muscle ( second finger : 2/5 ; third finger : 2/5 ; fourth finger : 4/5 ; intact in fifth finger ) , and pronator teres muscle ( 2/5 ) . a second mri of the left upper limb showed new abnormal signal intensities in the median nerve at the forearm and muscles innervated by the radial and median nerve , including flexor pollicis longus , flexor digitorum profundus , flexor digitorum superficialis , extensor carpi ulnaris , and extensor digitorum ( fig . 1 ) . these findings were suggestive of the presence of mononeuropathy multiplex involving the left median and radial nerves ( predominant in the former ) . despite vasculitic neuropathy being the most likely causative pathological condition in this patient , histological confirmation was required in order to justify the length of her treatment with potentially cytotoxic medications . a superficial radial nerve biopsy was not encouraged because there were no apparent findings of its involvement . we performed a combined biopsy of the left pronator teres muscle and a motor branch of the median nerve to the pronator teres . we found that her left pronator teres motor nerve had two branches , one proximal branch going to the superficial ( humeral ) head and the other distal branch going to the deep ( ulnar ) head of the pronator teres muscle . we biopsied the latter , which was a very thin nerve branch , to reduce potential motor complications of the procedure . the specimens from a motor branch , that contained only one nerve bundle , showed severe loss of myelinated fibers with a sectional distribution , myelin ovoids with many foamy macrophages , and marked edema in the perineurium and subperineurium ( fig . the specimens from the pronator teres muscle showed typical neurogenic features including small group atrophy with small - angulated muscle fibers ( fig . as expected , vasculitis seemed to be the most probable cause of the condition , although direct findings ( e.g. , fibrinoid necrosis of wall vessels ) were not seen in these specimens . as a result of these histological findings , two cycles of a 3-day regimen of high - dose intravenous methylprednisolone ( 1 g / day ) were initiated , followed by oral prednisolone at 0.5 mg / kg / day . the patient was discharged in april 2013 , and returned to full - time housekeeping with no notable neurological deficits . she is currently continuing treatment with low - dose oral prednisolone under active follow - up . the original technique of diagnostic biopsy of the pronator teres and a motor branch of the median nerve was described at academic meetings [ 5 , 6 ] and reported in a retrospective review of 20 patients as a safe and useful procedure to differentiate between motor neuropathy and motor neuron disease in patients with weakness of the upper limbs . a biopsy of the motor branch of the pronator teres muscle nerve may be considered a valuable diagnostic option in selected cases with neuropathy affecting the upper limb , when performed in cooperation with neurologists and orthopedic surgeons .
histological evaluation of a peripheral nerve is often the final diagnostic work - up for a neuropathy of unknown origin , and a distal sensory nerve is usually biopsied . here 
 , we report the case of a female patient with painful unilateral neuropathy in the upper arm . according to the histological evaluation of the pronator teres motor branch 
 , vasculitis seemed to be the most probable cause of the condition , and steroid therapy improved the patients symptoms . 
 a biopsy of the motor branch of the pronator teres muscle nerve may be considered a valuable diagnostic option in selected cases with neuropathy affecting the upper limb , when performed in cooperation with neurologists and orthopedic surgeons .