Differential diagnosis of myotonic disorders

The presence of myotonia and paramyotonia on clinical examination and of myotonic discharges during electrodiagnostic (EDX) studies are important for the diagnosis of certain neuromuscular conditions. The increased muscle activity of myotonia produces muscle stiffness that improves with repeated activity. Paramyotonia produces a similar symptom, but the stiffness paradoxically increases with activity. Myotonic discharges are easily recognized on EDX testing because of the waxing and waning discharges. Myotonic dystrophy and myotonia congenita share both clinical and electrodiagnostic myotonia. Paramyotonia congenita and hyperkalemic periodic paralysis are associated with clinical paramyotonia and electrical myotonia. Acid maltase deficiency often produces myotonic potentials without clinical evidence of myotonia or paramyotonia. The differential diagnosis of these myotonic disorders is discussed.     

Phenotypic variability and molecular genetics in proximal myotonic myopathy

Introduction: Myotonic dystrophy type 2 (DM2) is an autosomal dominant inherited disorder with (CCTG)n repeat expansion in intron 1 of the CNBP gene. Methods: We studied the first 16 Greek DM2 patients who had undergone thorough evaluation. Results: The age at diagnosis ranged from 38 to 69 years. The initial symptoms were proximal weakness, myalgias, and myotonia. Clinical myotonia was elicited in 10 patients, whereas electromyographic myotonic discharges were observed in almost all patients. Subcapsular cataract was frequently present, but cardiac arrhythmias were rare. Conclusions: In this study of Greek DM2 patients, proximal weakness was the most common initial symptom. Myalgias were also reported in a few patients, yet myotonia was not a major complaint. Although DM2 is considered relatively benign, there are patients who may be affected severely. Thus, a high index of suspicion must be maintained to make a timely diagnosis, especially in those of reproductive age. Muscle Nerve 51 :686–691, 2015     

Absent,unrecognized, and minimal myotonic discharges in myotonic dystrophy type 2

The purpose of this study was to describe the frequency of absent, unrecognized, or minimal myotonic discharges (MDs) in myotonic dystrophy type 2 (DM2). We performed a retrospective review of needle electromyography (EMG) data prior to genetic diagnosis in 49 DM2 patients at the Mayo Clinic. MDs were not reported on first or repeat EMG studies (n = 8) and not found in archived recordings of 4 patients (8%); archived EMG recordings (n = 4) confirmed the absence of MDs (n = 2), including 1 patient with normal insertional activity in all muscles, and misinterpretation of MDs as slow fibrillation potentials (n = 1) and complex repetitive discharge (CRD) activity (n = 1). Eight (16%) patients had minimal classic MDs with diffusely increased insertional activity, including waning‐only MDs in all patients in this group with archived EMG recordings (n = 5). Diffuse MDs were found in 33 (67%) patients. Absent or minimal MDs do not exclude DM2. Over‐reliance on diffuse MDs in patients who present with myopathy may lead to delay in genetic diagnosis of DM2. Muscle Nerve, 2010     

Myotonic dystrophy type 2 with focal asymmetric muscle weakness and no electrical myotonia

Genetically proven myotonic dystrophy type 2 (DM2) was found in a 61‐year‐old woman with creatine kinase (CK) elevation and only isolated weakness of one triceps. There was no clinical or electrical myotonia. Electromyography (EMG) showed only scattered fibrillation potentials and short duration motor unit potentials. Muscle biopsy showed nonspecific myopathic features and highly atrophic fibers with nuclear clumps. DM2 should be considered in patients with focal proximal weakness and abnormal EMG without myotonic discharges. Muscle Nerve 39: 383–385, 2009     

Quantitative analysis of the "warm-up" phenomenon in myotonic dystrophy type 1

To quantitate improvement in hand-grip myotonia and muscle strength (i.e., the "warm-up" phenomenon) in myotonic dystrophy type 1 (DM1), six successive, standardized maximum voluntary isometric contractions (MVICs) were recorded on 2 separate days using a computerized isometric hand-grip myometer in 25 genetically confirmed DM1 patients and in 17 normal controls. An automated computer program placed cursors along the declining (relaxation) phase of the MVICs at 90%, 50%, and 5% of peak force (PF) and calculated relaxation times (RTs) between these points. Mean 90% to 5% RT (a measure of myotonia) rapidly declined from 2.5 s in MVIC 1 to 0.8 s in MVIC 6 (warm-up = 1.7 s) in DM1; in controls, it remained 0.4 s for all six MVICs (warm-up = 0). In DM1, 70% of warm-up occurred between MVIC 1 and 2, almost exclusively in the terminal 50% to 5% phase of muscle relaxation. Day 1 warm-up was highly correlated with the severity of myotonia, and with day 2 warm-up. Improvement in myotonia was not accompanied by either transient paresis or improvement in PF. We conclude that, with this testing paradigm: warm-up of myotonia in DM1 can be reliably measured; is proportional to severity of myotonia; occurs rapidly, being most prominent between the first and second grips; mainly results from shortening of the terminal phase of muscle relaxation; and is not accompanied by significant warm-up in force output.     

Oculomotor involvement in myotonic dystrophy type 2

Oculomotor function has not been studied in patients with myotonic dystrophy type 2 (DM2). We report the presence of rebound nystagmus in seven of eight patients with DM2 in the absence of a structural brainstem or cerebellar lesion. The rebound nystagmus observed in these patients is very suggestive of ocular myotonia, and examination of patients using video-Frenzel goggles may be a useful method for diagnosing myotonia of the extraocular muscles.     

Proximal myotonic myopathy

Three Swedish patients with proximal muscle weakness, myotonia and lack of CTG expansion on genetical analysis are presented. Clinical neurological and neurophysiological examination and muscle biopsy were performed. There was an indication of autosomal dominant inheritance in 2 of the 3 patients. The main symptoms and clinical findings in the 3 patients were weakness of the proximal muscles, myotonia, muscle stiffness, muscle pain and muscle atrophy. Neurophysiological examination showed myotonic bursts and muscle biopsy snowed a variation of fibre sizes, an increased number of muscle fibres with centralized nuclei and scattered atrophic muscle fibres. Laboratory data showed elevated CK, GT and LD in 1 patient. Before genetical analysis was performed, all 3 patients had been diagnosed as atypical cases of myotonic dystrophy. However, the symptoms, clinical signs, laboratory data, electrophysiological and muscle biopsy findings were compatible with proximal myotonic myopathy (PROMM).     

强直性肌营养不良1型患者的临床特点及CTG重复次数分析

目的 总结强直性肌营养不良1型患者的临床、神经电生理和遗传学特点。方法 收集3例强直性肌营养不良1型患者的临床症状、肌电图、肌肉病理及基因检测结果。结果 3例患者（男性1例）均为成年起病,慢性病程。临床表现为四肢远端无力和肌强直;2例患者存在眼外肌或面肌无力;1例伴随前额脱发;2例存在心脏传导紊乱如阵发性室性心动过速、左前束支传导阻滞、右束支传导阻滞;2例出现脑白质病变。3例强直性肌营养不良1型患者DMPK基因3'非翻译区的突变CTG重复次数分别为104、150、299,均大于50次。3例患者肌电图检查所检肌肉均可见肌强直放电。结论 强直性肌营养不良1型患者肌无力主要出现在远端肌群,心脏传导紊乱和脑白质病变是其多系统受累的显著表现。肌电图可以发现临床下肌强直放电,是最敏感的筛查手段。     

强直性肌营养不良1型患者的临床特点及CTG重复次数分析

目的总结强直性肌营养不良1型患者的临床、神经电生理和遗传学特点。方法收集3例强直性肌营养不良1型患者的临床症状、肌电图、肌肉病理及基因检测结果。结果 3例患者(男性1例)均为成年起病,慢性病程。临床表现为四肢远端无力和肌强直; 2例患者存在眼外肌或面肌无力; 1例伴随前额脱发; 2例存在心脏传导紊乱如阵发性室性心动过速、左前束支传导阻滞、右束支传导阻滞; 2例出现脑白质病变。3例强直性肌营养不良1型患者DMPK基因3’非翻译区的突变CTG重复次数分别为104、150、299,均大于50次。3例患者肌电图检查所检肌肉均可见肌强直放电。结论强直性肌营养不良1型患者肌无力主要出现在远端肌群,心脏传导紊乱和脑白质病变是其多系统受累的显著表现。肌电图可以发现临床下肌强直放电,是最敏感的筛查手段。     

Muscle phenotype in patients with myotonic dystrophy type 1

Introduction: The pathogenesis of muscle involvement in patients with myotonic dystrophy type 1 (DM1) is not well understood. In this study, we characterized the muscle phenotype in patients with confirmed DM1. Methods: In 38 patients, muscle strength was tested by hand‐held dynamometry. Myotonia was evaluated by a handgrip test and by analyzing the decrement of the compound muscle action potential. Muscle biopsies were assessed for morphological changes and Na⁺‐K⁺ pump content. Results: Muscle strength correlated with a decline in Na⁺‐K⁺ pump content (r = 0.60, P < 0.001) and with CTG expansion. CTG expansion did not correlate with severity of myotonia, proximal histopathological changes, or Na⁺‐K⁺ pump content. Histopathologically, we found few centrally placed nuclei (range 0.2–6.9%). Conclusions: The main findings of this study are that muscle weakness correlated inversely with CTG expansion and that central nuclei are not a prominent feature of proximal muscles in DM1. Muscle Nerve 47:409‐415, 2013     

强直性肌营养不良临床特点分析

目的分析强直性肌营养不良(DM)的临床特点,以提高对DM疾病的认识及诊断水平。方法对21例DM患者的临床资料进行回顾性总结与分析。结果 21例患者均为慢性起病,以双手无力,活动不灵活起病多见,其中5例有家族史,部分病例伴有心脏、眼部、内分泌及中枢神经系统等其他多系统损害。19例行肌电图检查提示肌源性损害,其中16例发现有肌强直电位。10例行肌活检,主要表现为部分肌纤维萎缩,变性、坏死肌纤维,核内移及肌浆块形成,部分萎缩纤维内可见无结构胞浆体。1例强直性肌营养不良蛋白激酶(DMPK)基因CTG重复序列分析发现拷贝数超过正常范围。结论 DM是一种主要累及肌肉系统,以肌强直、肌无力和肌萎缩为主要临床表现并伴有多系统损害的疾病。综合评估多系统损害并结合肌肉的电生理学及病理学检查,有助于提高对DM的认识;在有条件的医疗机构可以开展DM基因诊断,对DM确诊很有意义。     

强直性肌营养不良症的临床与肌肉病理学特点

目的探讨强直性肌营养不良症(DM)的临床及肌肉病理学的特点。方法对6例DM患者的临床资料进行回顾性分析。结果6例患者均呈慢性病程,以肌无力、肌强直和肌肉萎缩为主要表现,多伴有脱发、白内障、心脏传导阻滞等多系统损害。肌电图检查结果为肌源性损害,6例均可见肌强直电位发放。病理学检查见肌纤维核内移、核袋及核链现象,部分患者可见肌质块及肌纤维分布异常。结论DM是一种以肌无力和肌强直为主要表现的多系统损害的遗传性疾病;特征性病理改变为肌纤维核内移、核链以及肌质块、肌纤维分布异常。     

Hyper-CK-emia as the sole manifestation of myotonic dystrophy type 2

A 49-year-old man had an 8-year history of persistent, isolated elevation of serum creatine kinase (hyper-CK-emia) without muscle symptoms, and no electromyographic evidence of myotonia; his muscle biopsy showed features reminiscent of myotonic dystrophy (DM), with morphometric findings consistent with those described in DM type 2 (DM2). Genetic studies excluded mutations in the DM type 1 (DM1) gene, but revealed a CCTG repeat expansion in the ZNF9 gene, which is associated with DM2. Our data suggest that in asymptomatic patients with persistent hyper-CK-emia, DM2 should be considered in the differential diagnosis.     

CTG-repeat length in distal and proximal leg muscles of symptomatic and non-symptomatic patients with myotonic dystrophy: relation to muscle strength and degree of histopathological abnormalities

Myotonic dystrophy (DM) is an autosomal dominant, multisystemic disorder with a variable phenotypic expression including muscle weakness and myotonia. The muscle wasting is most marked in distal limbs and in facial and neck muscles, although proximal limb muscles become affected as the disease progresses. The CTG-trinucleotide-repeat expansion associated with myotonic dystrophy is usually larger in muscle tissue than in leukocytes. It is unclear whether the repeat length itself bears any relation to the differences in the degree of weakness and atrophy between different muscles. We therefore analysed CTG-repeat lengths in blood and in proximal (m. vastus lateralis) and distal (m. tibialis anterior) muscles of patients with DM (n = 4) and non-symptomatic carriers of the mutant DM allele (n = 2) using conventional Southern blot hybridization. Muscle strength and histopathological abnormalities were evaluated for each muscle. In patients with clinical symptoms, the degree of paresis and morphological abnormalities was markedly more pronounced in m. tibialis anterior than in m. vastus lateralis. In these individuals, the CTG-repeat length was larger in muscles than in leukocytes, whereas in the two non-symptomatic carriers no difference could be detected. Furthermore, there was no clear difference in the repeat length between the two muscles in any of the patients. In conclusion, the selective muscular weakness and atrophy in DM do not seem to be related to differences in CTG-repeat length between different muscles.     

Evoked myotonia can be “dialed‐up” by increasing stimulus train length in myotonic dystrophy type 1

It is unknown how evoked myotonia varies with stimulus frequency or train length, or how it compares to voluntary myotonia in myotonic dystrophy type 1 (DM1). First dorsal interosseous (FDI) tetanic contractions evoked by trains of 10–20 ulnar nerve stimuli at 10–50 HZ were recorded in 10 DM1 patients and 10 normals. For comparison, maximum voluntary handgrip contractions were also recorded. An automated computer program placed cursors along the declining (relaxation) phase of the force recordings at 90% and 5% of peak force (PF) and calculated relaxation times (RTs) between these points. For all stimulus frequencies and train lengths, evoked RTs were much shorter, and evoked PFs were much greater in normals than in DM1. In normals, evoked RT was independent of stimulus frequency and train length, while in DM1 RT was longer for train lengths of 20 stimuli (mean: 9 s in DM1; 0.20 in normals) than for 10 stimuli (mean: 3 s in DM1, 0.19 in normals), but it did not change with stimulus frequency. In both groups PF increased greatly as stimulus frequency rose from 10–50 HZ but only slightly as train length rose from 10–20 stimuli. Voluntary handgrip RT (mean: 1.9 s) was less than evoked FDI RT (mean: 9 s). In DM1, evoked RT can be “dialed up” by increasing stimulus train length. Evoked myotonia testing utilizing a stimulus paradigm of at least 20 stimuli at 30–50 HZ may be useful in antimyotonic drug trials, particularly when grip RT is normal or equivocal. Muscle Nerve, 2010     

Computerized hand grip myometry reliably measures myotonia and muscle strength in myotonic dystrophy (DM1)

The aim of this study was to develop a reliable, sensitive, quantitative measure of grip myotonia and strength and to determine whether CTG repeat length is correlated with grip myotonia and with muscle strength in myotonic dystrophy type 1 (DM1). Three maximum voluntary isometric contractions (MVICs) of the finger flexors (i.e., handgrip) were recorded on 2 successive days using a computerized handgrip myometer in 29 genetically confirmed DM1 patients and 17 normals. An automated computer program calculated MVIC peak force (PF) and relaxation times (RTs) along the declining (relaxation) phase of the force recordings at 90%, 75%, 50%, 10%, and 5% of PF. Patients also underwent quantitative strength testing (QST) manual muscle testing (MMT). The patients had longer grip RTs and lower PFs than normals. RT (90% to 5%) was above the normal mean +2.5 SD in 25 (86%) patients. In DM1, prolongation of RT was mainly in the terminal (50% to 5%), rather than the initial (90% to 50%) phase of relaxation. PFs and RTs for each patient were reproducible on consecutive days. RTs were positively correlated with leukocyte CTG repeat length, whereas measures of muscle strength, such as PF, QST, and MMT, were negatively correlated with repeat length. We conclude that computerized handgrip myometry provides a sensitive, reliable measure of myotonia and strength in DM1 and offers a method to assess natural history and response to treatment.     

The diagnosis and treatment of myotonic disorders

Myotonia is a defining clinical symptom and sign common to a relatively small group of muscle diseases, including the myotonic dystrophies and the nondystrophic myotonic disorders. Myotonia can be observed on clinical examination, as can its electrical correlate, myotonic discharges, on electrodiagnostic testing. Research interest in the myotonic disorders continues to expand rapidly, which justifies a review of the scientific bases, clinical manifestations, and numerous therapeutic approaches associated with these disorders. We review the pathomechanisms of myotonia, the clinical features of the dystrophic and nondystrophic myotonic disorders, and the diagnostic approach and treatment options for patients with symptomatic myotonia. Muscle Nerve 47: 632–648, 2013     

Myotonic dystrophy type 2 and related myotonic disorders

Abstract. The myotonic dystrophies are a group of dominantly inherited disorders characterized by muscle wasting, myotonia, cataracts, hypogonadism and other system manifestations. Myotonic dystrophy type 1 (DM1) results from an unstable expansion of a CTG repeat in 3 UTR of the DM protein kinase (DMPK) gene on chromosome 19q 13.3. Myotonic dystrophy type 2 (DM2) is caused by an unstable expansion of a CCTG tetraplet repeat in intron 1 of the zinc finger 9 (ZFN9 gene) on chromosome 3q 21.3. However, the clinical diagnosis of DM2 is more complex than that of DM1, and conventional molecular genetic methods used for diagnosis of DM1 are not helpful for DM2. We here describe the detailed clinical, laboratory and biomolecular tests to identify DM2 and related myotonic disorders. At present, foci of accumulated noncoding CCTG repeat RNA (ribonuclear inclusions) in the cell nuclei are thought to interfere with the regulation and expression of several genes at the basis of multisystemic aspects of myotonic dystrophy type 2.     

Comparative efficacy of repetitive nerve stimulation, exercise, and cold in differentiating myotonic disorders

The decremental response of the compound muscle action potential (CMAP) to provocative tests is not characterized in genetically verified myotonic disorders. We therefore studied the relationship between decremental responses and mutation type in 10 patients with recessive myotonia congenita (rMC), two with paramyotonia congenita (PMC), nine with myotonic dystrophy type 1 (DM1), four with DM2, and 14 healthy people. CMAPs were measured at rest, just after a short exercise test (SET), and during short, 5- and 10-HZ, repetitive nerve stimulation (RNS) trains at 32 degrees C and at 20 degrees C. The degree of decrement was not related to the severity of clinical myotonia. Controls and PMC patients had similar responses when warm, but with cooling PMC patients had a persistent decrement of CMAPs. In the rMC patients the decremental responses were related to the type of mutation of the CLCN1 gene, as a decrement was encountered in the T268M, R894X, IVS17+1 G>T, K248X, and 2149 del G, but not with the IVS1+3 A>T, F167L, or dominant A313T mutations. In DM1 patients there was no relationship between decrement and CTG repeats. The degree of partial inexcitability in myotonic muscle membrane therefore depends on the mutation type rather than degree of clinical myotonia. RNS at 10 HZ is more sensitive than SET for demonstrating abnormalities in rMC patients when warm; differences are less marked when cold, which is useful to diagnose PMC. Provocative tests are therefore useful in myotonias to demonstrate muscle inexcitability, which depends on the chloride or sodium channelopathy.