Forty patients with polymyositis or dermatomyositis underwent detailed electromyographic evaluation. T h e paraspinal muscles of all patients were examined, as were several extremity muscles. The distribution of fibrillation potentials (FPs) in different muscles is discussed. FPs were most frequent in paraspinal muscles. We conclude that, for any patient suspected of having a myopathy, electromyographic examination should include the paraspinal muscles. MUSCLE & NERVE




Myopathies are characterized elcctromyographic;illy by motor unit action potentials (kfL'APs) of short duration, low amplitude, an increased number of phases, and recruitment of many motor units with mild contraction (early recruitment). Lambert et aI2' drew attention to the frequent occurrence of spontaneous potentials in polymyositis. T h e presence of fibrillation potentials (FPs) and positive sharp waves, along with bizarre high-frequency discharges, has been well documented in subsequent electromyographic (EMG) studies of pol ymyositis and dermat.omyositis.".'".'".'"FPs have also been observed in other primary muscle dis-

From the Department of Neurology (Drs. Slreib and Wilbourn) and the Department of Palhology (Dr. Mitsumoto). Cleveland Clinic Foundation, OH. This material was presented in part at the 1llh World Congress of Neurology, Amsterdam, September 11-16. 1977. Address reprint requests to Dr. StreLb at tiis current address: Department of Neurology. University of Nebraska Medical Center. 42nd and Dewey Ave., Omaha, NE 68105. Received for publication June 20, 1978; revised manuscripl accepted for publication November 6. 1978. 0148639xxm)1/0014$XI.CO'O e 1979 Houghton Mtfiin Professional PLblishers



Potentials in Myositis

eascs-e.g., in muscular dystrophy:*'.'**' parricularly the Duchenne form, and in acid maltasc deficiency myopathy.14 FPs may be observed in any disorder in which muscle fiber degeneration occurs-for example, in t r i c h i n ~ s i s h,fost . ~ ~ metabolic myopthies and so-called nonprogressive myopathies, however, d o not reveal abnormal spontaneous electrical activity in resting m i ~ s c l e-l'hus, .~~~~ the elcctroniyographic dcrnonstration of FPs is of major diagnostic importance in a patient with a myopathy . MarinacciTgnoted the presence of FPs in the paraspinal muscles of a patient with polymyositis. Engel et all4 reported increased muscle fiber irritability, spontaneous electrical activity, and myotonic discharges in patients with acid maltase deficiency myopathy. These abnormalities occurred most strikingly in thc trunk muscles. In a previous study, we found FPs to more frequent in the paraspinal niuscles than in the limb musclcs of patknts with polymyositis.28T h e present rctrospective study was designed to review the incidence of FPs of patients with dermatornyositis and polymyositis. We were particiilarly interested in comparing the distribution of FPs in the trunk muscles of these patients with that in the proxinial and more distal extremity muscles.


JanlFeb 1979

MATERIALS AND METHODS Patient Selection. All

patients referred to the EMG laboratory at Cleveland Clinic for possible myopathy from 1974 to mid-1977 were identified. All those included for further analysis were older than 18 years of age and exhibited definite muscle weakness as well as elevated serum CK enzyme levels at the time of EMG examination. In addition, all patients had had a muscle biopsy arid a final clinical diagnosis of polymyositis or dermatomyositis. Electrophysiologic examination included motor nerve conduction studies of at least one nerve of the upper and one nerve of the lower extremity, determination of sural and digital nerve action potentials, and needle electrode examination of the paraspinal and eight or more limb muscles. Not included in the study were patients whose primary diagnosis was a collagen-vascular disorder, and who developed muscle weakness in the course of their illness. Also excluded were patients with lumbosacral nerve root pain, previous lumbar Iaminectomies, and symptoms or signs of a peripheral neuropathy or a radiculopathy. Those with abnormal nerve conduction studies (except for decreased compound muscle action potentials or isolated median neuropathies) and those who had had an electromyographic diagnosis of polyradiculopathy or motor neuron disease were also eliminated from the study. The EhtG work protocols of the remaining patients were then reviewed for the presence of FPs and for determination of MUAP patterns (see below). Most examinations were performed by two of the authors (EWS and AJW). Standard concentric needle electrodes with a platinum core and a leading-off area of 0.076 inm2 were used throughout. Multiple areas of each muscle, including superficial and deep layers, were explored through one or more needle insertions. The paraspinal muscles were examined at the midlumbar and high lumbar areas or, less commonly, at the lower thoracic area. Most patients were examined at more than one trunk level using suggested anatomic landmarks.’* FPs were defined as the presence of low-amplitude and short-duration potentials or positive waves in the resting muscle. The discharge rate had to be regular and had to persist for more than 1 sec following needle electrode movement. Such activity had to be recorded in at least three areas outside the endplate zone. Short runs of positive waves following needle electrode movement, posElectromyographic Technique.

Fibrillation Potentials in Myositis

itive waves waxing and waning in frequency, bizarre high-frequency discharges, and uncertain findings were not included in this analysis. N o attempt was made to quantitate FPs. The MUAP pattern was characterized as consistent with a myopathy or as “myopathic” if increased proportions of polyphasic potentials o r short-duration potentials were present along with early recruitment of MUAPs at minimal to moderate muscle contraction. These changes had to be present in more than one area of an examined muscle, but not in all muscles of a given patient. The parameters of the MUAPs were determined by inspection of the oscilloscope during minimal, moderate, and maximal muscle contraction. N o quantitative measurements were made of the If u A Ps . Muscle Biopsies. Muscle biopsies were taken from either upper or lower extremities; no specimens were taken from the paraspinal muscles. One of us (HM)reviewed the biopsies at the time of the study without knowledge of clinical, electromyographic, or previous histologic diagnosis. Paraffin sections had been prepared on all biopsies, and frozen sections for histochemistry had been prepared on most, These biopsies were diagnosed as typical of polymyositis or dermatomyositis if signs of myopathic changes-including degeneration and regeneration of muscle fibers, together with perivascular and/or interstitial round-cell infiltrationwere present. If one of these features was absent, the biopsy was considered “nondiagnostic.”


Forty patients were included in the final analysis; 23 were female and 17 male. Twenty-seven cases were diagnosed as polymyositis and 13 as dermatomyositis. One patient was 19 years old; 5 patients were in the fourth, 9 in the fifth, 12 in the sixth, 10 in the seventh, arid 3 in the eighth decades of life. All 40 patients had symmetric proximal muscle weakness, elevated serum CK levels with a range of 170-7,700 (mean 1,630; normal less than 140), and abnornial EMG studies. ‘I‘wenty-six patients had typical muscle biopsies. I n 12, one or more of the typical pathologic features were missing, although none of the biopsies was normal. Tissue from 2 patients was not available for review, although the pathology reports indicated typical changes of inflammatory myopathy. ‘l’wo patients had associated diabetes inellitus but no symptoms or signs of a radiculopathy or polyneuropathy. T w o suffered from associated ad-


Jan/Feb 1979


enocarcinoma of the prostate, and two had possible mild hypothyroidism. ,411 of these six patients had typical muscle biopsy features. T h e duration of symptoms ranged from 1 to 108 months. In 25 patients, the duration of the illness was less than 12 months; in 10 patients, it was less than 36 months; and in 4 patients, the illness lasted longer than 36 months. In 1 patient, the length of the illness was not known. Twenty-eight patients were untreated at the time of the study. Nine had been on adequate corticosteroid treatment for at least several weeks. In 3 patients, the exact mode of therapy could not be determined.

All 40 patients had “myopathic” M U h P s (according to the previously delineated criteria). The distribution of FPs in the paraspinal muscles is compared to that in the upper-extremity muscles in table 1, and to that in the lower-extremity muscles in table 2. Different muscles were examined with various frequencies, as can be seen from the tables. It should be reiterated that, in each patient, at least eight and usually more muscles had been examined. In 14 ( 3 5 % ) of the patients, FPs were present in all examined muscles. In another 14 (357c), FPs were found in more than one-half but not all of the muscles examined. In 8 (20%)),fewer than half of the muscles revealed FPs; and in 4 (10%) of the patients, FPs Electromyographic Findings.


Table 1. Distribution of fibrillation potentials in polymyositis and dermatomyositis paraspinal versus upper-extremity muscles Muscles examined Paraspinal lnfrasprnatus Deltoid Biceps Triceps Brachioradi all s First dorsal interosseous

No tests performed

40 36

36 38

24 33 34

No positive (Yo)

37 (93) 27 (75: 26 (72) 29 (76)

16 (67) 26 (79) 16 (47)

Table 2. Distribution of fibrillation potentials in polymyositis and dermatomyositis paraspinal versus lower-extremity muscles Muscles examined Paraspinal Gluteus lliacus Vastus lateralis Gastrocnemius Anterior tihiatis


No tests performed

No positive (%)

40 30 33

37 (93) 22 (73) 25 (76) 25 (64)

39 24 40

Fibrillation Potentials in Myositis

13 (54) 28 (70)

were present in only one or two muscles. Of these 4 cases, 1 patient had FPs only in the trunk muscles, 2 patients had FPs in the trunk and in one extremity muscle, and in 1 patient the FPs were confined to one limb muscle. In all 40 patients, FPs were found in at least one of the examined muscles, DISCUSSION

The electrotnyographic findiIigs of various studies on polymyositis and dermatotnyositis are compared in table 3. Whercas there is agreement among these studies with respect to the MUAP pattern, the frequency of FPs varies from a low of 455613 to a high of 100% (in our study). Our patient group is similar to those of other series with regard to sex and age distribution. All patients fulfilled the criteria of definite or probable polymyositisidermatomyositis by the recently proposed criteria of Bohan et a1.’ On the other hand, our population included many acute and subacute cases. All had signillcant serum CK elevation and many had aldolasc elevation at the time of thc EMG study; further, about 75% of the patients had not received immunosuppressive or corticosteroid therapy at the time of evaluation. This could have contributed to the higher incidence of FPs in our study, as Rohan et a12 have shown that such therapy decreased the spontaneous electrical activity on sequential EMG examinations. However, other factors also play a role. We feel that the high frequency of FPs found in our patients results from the sampling of‘ more muscles, particularly the paraspinal muscles. Our data support this view. In 70% ofthe patients studied, FPs were present in more than one-half of the examined muscles. I n 10%’of the patients, FPs were present in only one or two muscles. Again, in all cases, at least eight extremity muscles had been routinely examined. V i l p p ~ i l aexamined ~~ one proximal muscle routinely; if the EMG findings were negative, an additional one or two muscles wcre explored. The other studies listed in table 3 d o not indicate which or how many muscles were studied. Desmedt arid Borenstein” recently pointed out the almost invariable occurrence of FPs in the deltoid muscles of patients with Duchenne muscular dystrophy. This contrasts to the smaller number of FPs found in another study of Duchenne d p ~ t r o p h y .Des~ niedt feels that a deliberate search of sampling sites for FPs was the cause of this increased yield.9 I t is generally agreed that the weakness found in polyrnyositis, as well as that in most other pri-


JaniFeb 1979

Table 3. Electromyographic findings in polymyositis and dermatornyositis Study Bohan e! aI2 Devere and Bradley13 K aeser' Larnbert et alzl Aiddoch and Morgan-HughesZ6 Vilpputaz9 Present study

No Patients

"Myopathic" MUAP pattern

FPs (sDontaneous activity)

151 118

90% 89% >90% 100%

74% 45% 60%- 70% 80%

100% 100% 100%

60% 55% 100%



20 21 40

rriary muscle disorders, principally affects the girdle and proximal muscles.3"That distal muscles are involved in the disease process to a lesser degree has been clearly shown i n the early stages of rnuscular In a study of patients with Duchenne dystrophy. Smith et alZ7found F-Ps in proximal muscles in 34 of 41 patients. The hand muscles showed FPs i n only 40% of these cases. Desmedt and Borenstein" also commented on an increased incidence of FPs in proximal muscles, in comparison to distal muscles, in Ducherine dystrophy . In a previous study,2Xit was observed that FPs occurred in the paraspinal muscles in 58% of 17 patients with myopathy. In none of these cases were FPs limited to the extremity muscles. Eight of the patients had been diagnosed as having an active myositis. All 8 had FPs in the paraspinal muscles, whereas only 5 had FPs in the extremity muscles. Bohan et al' state that 2 of their patients exhibited typical EMG findings only in the paraspinal muscles. Our present study reveals similar results: the frequency of FPs was highest in the most proximal muscles (i,e., the trunk muscles) and lowest in the first dorsal intcrosseous and gastrocnemius muscles. Furthermore, it should be noted that when FPs were found in only one or two muscles, the paraspinal muscles were negative only once. All patients had FPs in a t least one niuscle; but in 30% of patients, FPs were presenl in fewer than onehalf of the muscles examined. Occasional potentials resembling FPs can be found in normal rnusc1e.j Short runs of positive waves arid other signs of muscle fiber irritability are occasionally observed in patients without neuroniuscular illness. We have excluded such activity and uncertain findings. Opinions dif'fer as to how to define "definite" or "pathologic" FPs. We believe that our requirements are rather conservative and acceptable, as indicated by other author^,^,'^ and

Fibrillation Potentials in Myositis

that t.his did not influence the frequency of FPs in out- study. Signs of denervation (e.g., positive sharp waves and FPs, as d e h e d above) can readily be found in muscles innervated by the anterior and posterior primary ramus of' the spinal nerve when these most proximal parts of the motor unit are diseased. FPs in the paraspirial muscles occur in vcrtebral disc disease,' spinal metastases,'" spinal nerve root avulsion,6 diabetic ratiiciilopathies,'" and motor neuron Patients with symptoms or evidence of such illnesses were excluded from our study and could riot have been responsible for the high frequency of FPs found in the trunk muscles. Involvement of the terminal nerve axons was initially thought to be the cause of FPs in" Segmental necrosis of a muscle fiber leads to functional denervation of part of the fiber, a s a result of separation of that part of the fiber from the motor oridplate. This phenomenon is more likely to be the cause of those myopathic FPs'O-a contention that is further supported by signs of reinnervation in later stages of polymyositis and der-matomyositis, as documented by conventional EMG t e c h n i q i ~ e SFE;MG,l7 ,~~ and morphology.i , ' i The distribution of FPs in superficial muscles was not systematically compared to that in deeper compartments of the trunk muscles. In our experience, the FPs in the paraspinal muscles of patients with rnyositis are easy to detect in the superficial layers, and one need not explore the deep multifidus compartment as is necessary in the radicdopathies." This facilitates the examination, and only an occasional adult patient is unable to relax sufficiently for the observation of electrical activity in the resting muscle. MUAP changes are often striking in the trunk muscles. However, most patients are unable to activate individual units at minimal contraction, which makes this muscle less reliable for analysis of the M U A P pattern.


Jan/Feb 1979


Our study was not prospective, and our patients ma); have been a more selected group in comparison to those of other series. However, two important conclusions seem justified. First, the incidence of FPs during toutine clinical electro-

myography in patients with a definite or probable diagnosis of pol ymyositis increases with the number of inuscles sampled. Second, the paraspinal muscles give the highest yield, followed by the proximal extremity muscles.

REFERENCES I. Blom S, Lemperg R: EMG analysis of the lumbar muscu-


3. 4.



7. 8. 9.

10. 1 1.


lature in patients operated on for lumbar disc disease.J Seziruszcrg 26:25-30, 1967. Aohan A, Peter JB, Bowmarl RL, Pearson CM: A computer-assisted analysis of 153 patients with polyiriyositis and dennatomyositis. Mpdzrrnp (Baltimore) 56:255-286, 1977. Buchthal F: Electrophysiological abnormalities in metabolic mvopathies and neuropathies. Acta Neurol Scuntl 46:Sxppl 43: 129-1 76, 1970. Buchthal F: Diagnostic significance of the inyopathlc EMG. In 1)esineclt JE (Editor): Pathogeneslc of Hirmau .tlir.sor~/ar Dystrophirs. Amsterdam, Exerpta Medics, 1977, pp 205218. Buchthal F, Rosenfalck 1’: Spontaneous electrical activity of human rnuscle. Elcctrorncrph Clin Nruruphysiol 20:321336, 1966. Bufalini C , Pescatori G: Posterior cervical EMG in the diagnosis and prognosis of brachial plexus in,jury.,] BonrJomt Surg (Brit) 51:627-631, 1969. Coers C;, Telerman-l’oppet N, Gerard J M : Terminal inriervation ratio in neuromuscular disease. l,lirh Neurul 29:21.5-222, 1973. Conrad B, Sindrrmann F, Prochazka VJ: I n t e n a l analysis of rcpetitivc denervation potentials of human skeletal musc1v.J Nrurul ,Veurusurg Prychintry 3.5:834-840, 1972. Desmedt J E : Pnthogrncsrs u/ Human ~WicscuhrDystrophier. ilmsterdarn. Exerpta Medica, 1977, p 239. Desrnedt JE, Borenstein S: Relationship of spontaneous fibrillation potentials to muscle fiber segmentation in human muscular dystrophy. .2:ature 258:531-534, 1975. 1)csniedt J E , Borenstein S: Regeneration in Duchenne muscular dystrophy. Arch Xeurol 35542-650, 1976. Desmedt JE, Emeryk B: Disorder of muscle contraction processes in sex-linked (Duchennej muscular dystrophy with correlative t-lectromyographic study- of myopathic involvement in small hand muscles. Ant J Med 453853-872, 1968.

13. Devere R, Bradley WG: Polymyositis: its presentation, motbidity and mortality. Brain 98:637-666, 1975. 14. Engel AG, G m e z MR, Seybold ME, Lambert EH: The spectrum and diagnosis of acid maltase deficiency. ,Xreurology (Minneap) 23:95-106, 1973.


Fibrillation Potentials in Myositis

15. Essleri E, Isch I;, Larnbert EH, Liberson WT, Simpson JA: Terminology of electromyography. Electruriice,!dr Clrn .Vruruplyiol 26:224-226, 1969. 16. IIausmanowa-Petrusewicz J , Ryniewicz B: Electromyographic findings in the so-called non-progressive myopathies.1 Nrul-ol 21 1:241-251, 1976. 17. Henriksson KG, StHlbei-gE: I‘he terminal inner\ ation pattern in polymyositis: a histochemical and SFEMG study. ,tfuFrlr U ; \ ‘ P ~ P P 1:3-13, 1978. 18. Jonsson B: Topography of the lurnbar part of the erector spinae muscle. z ‘4fzat Entwzck/-G~cch 130:177-191, In70. 19. Kaescr HE: 1)crmatomyositis iind polymyositis. Schziriz Arch .“leiirul Neuruchir P.\ychintr 116:281-289, 1973. 20. I.aAan M M , Grant A€:: Occult spinal metastases-early EMC; manifestation. .4?-Ch P h y .Wed R ~ h a b i (523223-226, 1971. 21. 1.ambert EH, Sayrc GP, Earon 1.M: Electrical activity ot muscle in polymyositis. T m m .4m Sriirol 14ss0c 79:64-69, 1954. 22. Longstreth GF, Newcomer AD: Abdominal pain caused by diabetic radiculopathy. Ann I n l r n ~:Mrd 86:166-168. 1977. 23. Marinacci A.4: Electromyography in the diagnosis of polyinyositis. Electrorryogmphj 5 2 2 5 - 2 8 6 , 1965. 24. Mechler F: Changing electromyographic fitidirigs during the chronic roursc of polymyositis.,f :Venru/ Sri 23:235-242, 1974.

25. Richardson AT: Clinical and electromyographic aspects of polymyositis. Pruc R Soc ,Cfed 49: 111-1 14. 1956. 26. Riddoch D, Morgan-Hughes 3.4: Prognosis in adult p l y myositis.J Srurul Sci 26:71-80, 1975. 27. Smith HL, Amick LD, Johnson WW: Detection 01 subcliriical and carrier states in Duchennc muscular dystrophy. J Pedzal 69:67-79, 1966. 28. Sti-eib E, Daube JR: Electromyography of paraspinal m u s cles. lX‘~~~rulogy (bfinneap) 25:386. 1975 (abstr). 29. Vilppula A: Muscular disorders in some collagen diseases. .4cta M r d Scarid [Suppl] 43:129-176, 1970. 30. Walton J N , Adams RD: Pu4nryositr.s. Edi~iburgh,ChurchillLivingstone, 1958. 31. Waylonis GW, Jonnson EW: T h e EMG in acute trichinosis-report of 4 rases. Arch Phys ,21~dRPhnbil 4.5:177-183. 1964.


JaniFeb 1979

Spontaneous electrical muscle fiber activity in polymyositis and dermatomyositis.

Forty patients with polymyositis or dermatomyositis underwent detailed electromyographic evaluation. T h e paraspinal muscles of all patients were exa...
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