We present electrodiagnostic data on 30 patients with inclusion body myositis (IBM) in order to better delineate its electrophysiological features. Comprehensive electromyography (EMG) and nerve conduction studies (NCS) were performed in all cases. Twelve patients had single fiber electromyography (SFEMG). EMG showed abundant short-small motor unit potentials (MUP) with fibrillations and positive sharp waves in 56.6% of patients, and a mixed pattern of large and small MUP in 36.7%. In 6.7%, only "neurogenic" features were seen. NCS were slow in 33.3%. SFEMG revealed a mildly abnormal jitter and a slightly increased fiber density. IBM demonstrates a heterogeneous EMG profile. A pattern of large and small MUP is highly suggestive of IBM but is seen in only about one third of cases. Key words: inclusion body myositis electrodiagnosis electromyography single fiber electromyography nerve conduction studies MUSCLE & NERVE 13:949-951 1990


Inclusion body myositis (IBM) is a recently described inflammatory myopathy characterized by rimmed vacuoles, filamentous inclusions, slowly progressive proximal and/or distal weakness, prominent muscle atrophy, high incidence of hyporeflexia, male predominance, and poor response to corticosteroids, which is being reported with increasing fre uency in the last Previous reseveral years.2,3,5-7,9- 11.13,20-%25 ports have focused on its clinicopathological aspects.2-6~9~1 1,17,20,22

We present electrophysiological data on 30 IBM patients in order to better delineate the electrodiagnostic features of this rare disorder. PATIENTS AND METHODS

We studied 30 biopsy-proven cases of IBM: 23 males and 7 females (mean age = 59.7 years). All had compatible clinical features as outlined above. Studies were performed using a DISA 1500 electromyograph (Dantec Corporation, Santa Clara, CA). Motor, sensory, and mixed nerve conduction

From the Department of Neurology, University of Alabama at Birmingham, Birmingham, Alabama. Presented in part at the Annual Meeting of the American Association of Electromyography and Electrodiagnosis, San Diego, California, October, 1988. Address correspondence to Dr. Oh, Department of Neurology, University of Alabama at Birmingham, UAB Station, Birmingham, AL 35294. Accepted for publication November 5, 1989.

CCC 0148-639W90/0100949-03 $04.00 8 1990 John Wiley B Sons, Inc.

EMG in Inclusion Body Myositis

studies (NCS) were done following conventional methods." Nerves tested included the median, ulnar, peroneal, posterior tibial, and sural. Skin temperature was controlled at 2 31°C. Needle electromyography (EMG) was performed using monopolar electrodes. Weak muscles in at least two limbs were examined. A minimum of 4 quadrants were explored in each muscle and at least 10 motor unit potentials (MUP) were measured on the screen. MUP were considered to be of shortduration/small-amplitude (SS) if their duration was 18 msec in duration. Single fiber electromyography (SFEMG) of the extensor digitorum communis (EDC) was obtained in 1 1 patients, and of the rectus femoris in one, following previously published techniques. l9 Jitter and blocking were electronically measured using a DISA 15G22 single fiber unit. SFEMG was considered abnormal if one of the following was present: jitter >40.4 ksec (>60 psesc for the rectus femoris), >lo% of potentials pairs show blocking or a mean consecutive difference (MCD) >54 ~ s e c . ' Fiber ~ density was considered increased if >Z." RESULTS

EMG was abnormal in all 30 cases. Three distinct patterns (PTN) were observed: PTN 1-fibrillation potentials/positive sharp waves (F/PSW) and SS MUP in 17 cases (56.6%).


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PTN 2-F/PSW and a mixed pattern of SS and LH MUP in 11 (36.7%). PTN 3-F/PSW, normal and LH MUP in 2 (6.7%). In PTN 2 patients, SS and LH MUP were recorded from the same muscle in some, but in others, certain muscles showed SS MUP while others evidenced LH MUP. The interference pattern (IP) at maximal voluntary effort was full in 58.8% of PTN 1 patients and 36.4% of PTN 2 cases. A reduced IP was seen in 41.2% of PTN 1 individuals, 63.6% of PTN 2, and in both PTN 3 subjects. All 30 cases showed an excess of polyphasic MUP. NCS showed mild slowing in 33.3% of cases. When PTN 1 patients are excluded, the incidence of NCS abnormalities rises to 43.3%. Only 2/17 PTN 1 patients showed slowing of conduction velocity. SFEMG (12 cases) demonstrated a mildly abnormal jitter in 58.3% (mean = 46.5 psec). Fiber density (FD) was modestly increased (mean = 2.6). Blocking was not prominent (2/12 2 10%). Overall, SFEMG was abnormal and FD increased in 83% of patients. Table 1 summarizes the distribution of muscle weakness according to EMG pattern. Involvement of distal musculature was more frequent in PTN 2 and PTN 3 patients. DISCUSSION

IBM reveals a heterogeneous electrodiagnostic profile. Previous reports have described a “myopathic” pattern with FIPSW, a mixed pattern of “myopathic” and “neurogenic” featUreS,2-7,Y-11,13,20-22,25 or, rarely, patients with only “neurogenic” changes.7217In our experience, a mixed pattern of LH and SS MUP accompanied by abnormal spontaneous activity (PTN 2) is highly suggestive of IBM. Unfortunately, this pattern is seen in only one third of cases, the most common pattern being one of FIPSW and SS MUP, similar to polymyositis.l 2 Some authors

have described LH MUP intermixed with SS MUP (PTN 2) in chronic polymyositi~.’~~’~ Lozt and associates found a PTN 2 EMG to be nonspecific for IBM because they saw this pattern with similar frequency in patients with chronic polymy~sitis.’~ In our experience, this is extremely rare. So far, almost every patient we have seen with a PTN 2 EMG has had IBM proven by biopsy. Other entities reported to show mixed “myopathic” and “neurogenic” EMG features include X-linked humeroperoneal neuromuscular disease,24 familial “rimmed vacuole myopathy” sparing the quadriceps,’ and sporadic distal myopathy.z3 Interestingly, there are strong similarities between IBM, familial “rimmed vacuole m o athy”,’ and several types of distal myopathy.R J 1 8 2 3 Some of these cases might be, in fact, variants of IBM. Two of our cases (PTN 3) showed pure “neurogenic” features. Patients with this EMG pattern and normal NCS might erroneously be thought to have motor neuron disease. In our series, NCS abnormalities were not infrequent. In the cases with “neurogenic” features (PTN 2 and PTN 3) these were more prevalent. Thus, the presencse of unexplained peripheral neuropathy in a patient with electrodiagnostic evidence of myopathy should suggest the possibility of IBM. Initial reports im lied that NCS abnormalities were rare in IBM~”5,6~”.1’.13.”0.22 but recent series have reported up to a 50% incidence.“ Eisen and coworkers provided the only previous report of SFEMG changes in IBM.7 They reported 7 cases with strikingly high fiber density (mean = 6.3) and a moderately abnormal jitter (mean = 83 psec). This led them to believe that IBM had a substantial neurogenic c ~ m p o n e n t . ~ We found less striking SFEMG abnormalities. Our values were in the range seen with chronic myopathic proces~es.’~ All of Eisen and colleagues’ patients had prominent distal ~ e a k n e s sIn . ~contrast, most of our SFEMG patients had proximal involvement. Since the SFEMG is usually performed in the EDC (a distal muscle), it will be expected to

Table 1. Distribution of muscle weakness by EMG pattern Distribution of weakness (no. of cases) EMG pattern

Mainly proximal

Mainly distal









0 3 1

20 66.7%

4 13.3%

Totals Percentage


EMG in Inclusion Body Myositis

1 0 4 13.3%




0 1 1

2 6 7%


Total patients 15

13 2 30

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be more abnormal in patients with distal involvement. IBM possesses a rich electrophysiological spectrum. A mixed pattern of SS/LH MUP is very suggestive, but not specific, of IBM. NCS abnormalities seem to be more frequent than previously thought. Many of the electrodiagnostic features in

IBM are suggestive of a myopathic process. However, the frequent finding of abnormal NCS, LH MUP, and reduced MUP recruitment suggest a neurogenic component as well. More studies are needed to determine which portions of the motor unit apparatus are chiefly involved in this condition.

REFERENCES 1. Argov Z, Yarom R: “Rimmed vacuole myopathy” sparing the quadriceps: An unique disorder of Iranian Jews. J Neurol Sci 1984;64:33-43. 2. Calabrese LH, Mitsumoto H, Chou SM: Inclusion body myositis presenting as treatment-resistant polymyositis. Arthritis Rheum 1987;39:397-403. 3. Carpenter S, Karpati G, Heller I, Eisen A: Inclusion body

myositis: A distinct variety of idiopathic inflammatory myopathy. Neurology 1978;28:8- 17. 4. Chad D, Good P, Adelman L, Bradley WG, Mills J: Inclusion body myositis associated with Sjogren’s syndrome. Arch Neurol 1982;39:186- 188. 5. Danon MJ, Perurena OH, Ronan S, Manaligod JR: Inclusion body myositis associated with systemic sarcoidosis. Can J Neurol Sci 1986;13:334-336. 6. Danon MJ, Reyes MG, Perurena OH, Masdeu JC, Manaligod JR: Inclusion body myositis: A corticosteroid-resistant idiopathic inflammatory myopathy. Arch Neurol 1982; 39~760-764. 7. Eisen A, Berry K, Gibson G: Inclusion body myositis (IBM): Myopathy or neuropathy? Neurology (Cleveland) 1983;33:1109-1114. 8. Fukuhara N, Kumamoto T, Tsubaki T: Rimmed vacuoles. Acta Neuropathol (Berl) 1980;51 :229- 235. 9. Gutmann L, Govindan S, Riggs JE, Schochet SS: Inclusion body myositis and Sjogren’s syndrome. Arch Neurol 1985;42:1021- 1022. 10. Hartlage P, Rivner M, Henning W, Levy R: Electrophysiologic and prognostic factors in inclusion body myositis. Abstract. Muscle Nerve 1988;11:984. 1 1 . Ketelsen UP, Beckmann R, Zimmermann H, Sauer M: Inclusion body myositis: A “slow-virus” infection of skeletal musculature? Klin Wschr 1977;55:1063- 1066. 12. Kimura J: Electrodiagnosis in Diseases of Nerve and Muscle: Principles and Practice. Philadelphia, F.A. Davis, 1983. 13. Lazaro RP, Barron KD, Dentinger MP, Lava NS: Inclusion body myositis: Case reports and a reappraisal of an under-

EMG in Inclusion Body Myositis

recognized type of inflammatory myopathy. Mount Sinai J Med 1986;53:137- 144. 14. Lozt BP, Engel AG, Nishino H, Stevens JC, Litchy WJ: Inclusion body myositis: Observations in 40 patients. Brain 1989;112:727-747. 15. Matsubara S, Tanabe H: Hereditary distal myopathy with filamentous inclusions. Acta Neurol Scandinav 1982;65:363368. 16. Mechler F: Changing electromyographic findings during the chronic course of polymyositis. J Neurol Scz 1974;23:237-242. 7. Mikol J, Felten-Papaiconomou A, Ferchal F, Perol Y, Gautier B, Haguenau M, Pepin B: Inclusion-body myositis: Clinicopathological studies and isolation of an Adenovirus type 2 from muscle biopsy specimen. Ann Neurol 1981;11:576-581. 8. Oh SJ: Clinical Electromyography. Newe Conduction Studies. Baltimore, University Park Press, 1984. 19. Oh SJ: Electromyography: Neurornuscular Transmission Studies. Baltimore, Williams & Wilkins, 1988. 20. Riggs JE, Schochet S, Gutmann L, McComas CF, Rogers JS: Inclusion body myositis and chronic immune thrombocytopenia. Arch Neurol 1984;41 :93- 95. 21. Ringel SP, Kenny CE, Neville HE, Giorno R, Carry MR: Spectrum of inclusion body myositis. Arch Neurot 1987;44:1154- 1157. 22. Sawchak JA, Kula RW, Sher JH, Shafiq SA, Clark LM: Clinicopathologic investigations in patients with inclusion body myositis (IBM). Neurology 1983;33 (suppl 2):237. 23. Vaccario ML, Scoppetta C, Bracaglia R, Uncini A: Sporadic distal myopathy. J Neurol 1981;224:291-295. 24. Waters DD, Nutter DO, Hopkins LC, Dorney ER: Cardiac features of an unusual X-linked humeroperoneal neuromuscular disease. N Engl J Med 1975;293:10171022. 25. Yood RA, Smith TW: Inclusion body myositis and systemic lupus erythematosus. 1 Rheumatol 1985;12568-570.


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Electrophysiological spectrum of inclusion body myositis.

We present electrodiagnostic data on 30 patients with inclusion body myositis (IBM) in order to better delineate its electrophysiological features. Co...
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