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 Table of Contents  
Year : 2021  |  Volume : 16  |  Issue : 4  |  Page : 427-436

Myositis mimics

Department of Clinical Immunology and Rheumatology, Sanjay Gandhi Postgraduate Institute of Medical Sciences, Lucknow, Uttar Pradesh, India

Date of Submission07-Sep-2020
Date of Acceptance20-Oct-2020
Date of Web Publication08-May-2021

Correspondence Address:
Dr. Latika Gupta
Department of Clinical Immunology and Rheumatology, Sanjay Gandhi Postgraduate Institute of Medical Sciences, Lucknow, Uttar Pradesh
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/injr.injr_250_20

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Proximal muscle weakness in children, as well as adults, can be the presenting feature of a wide range of diseases including but not limited to the idiopathic inflammatory myopathies, muscle dystrophies, metabolic, endocrine, and drug-induced myopathies. Certain diagnostic clues such as selective muscle involvement in inherited dystrophies, cramping on exertion in metabolic myopathies, diffuse muscular pain in endocrinal myopathies, and relevant drug history along with a lack of appropriate treatment response to steroids should be considered. Awareness of other possibilities and a keen eye to identify the mimics may avoid diagnostic delays and allow timely treatment. In this review, we cover some of the salient features of the various myositis mimics and the pertinent investigation findings as well as gray areas which may confound recognition of the underlying etiology. Later, we discuss key features of certain mimics while providing an overview of the differential diagnosis and clinical management for treating clinicians.

Keywords: Calpainopathy, dermatomyositis, drug, dysferlinopathy, dystrophy, inflammatory, inherited, metabolic, mimics, myositis

How to cite this article:
Ganguly S, Chatterjee R, Zanwar A, Gupta L. Myositis mimics. Indian J Rheumatol 2021;16:427-36

How to cite this URL:
Ganguly S, Chatterjee R, Zanwar A, Gupta L. Myositis mimics. Indian J Rheumatol [serial online] 2021 [cited 2023 Feb 5];16:427-36. Available from:

  Introduction Top

Proximal muscle weakness may manifest as a symptom or sign in varied systemic illnesses. Since disorders of the muscle can be of varied etiopathology, in the rheumatology clinic, it is important to consider differentials that deviate from the usual gamut of inflammatory myopathy. Although the diagnosis of inflammatory myopathy is rewarding as it generally portends a good recovery, we must remember that in terms of sheer numbers, inherited myopathies tend to outweigh them.[1],[2] Thus, one is expected to encounter a hereditary myopathy as frequently as, if not more than an idiopathic inflammatory myopathy (IIM). Moreover, muscle symptoms may be the presenting feature of common illnesses such as hypothyroidism and diabetes mellitus, as well as other endocrinopathies, including adrenal disorders and disorders of the parathyroid.[3],[4],[5],[6] In this review, we outline some basic principles to keep in mind while approaching a patient presenting with muscle weakness. Following a brief overview of the classic inflammatory myopathies, the salient points of dystrophies, namely facioscapulohumeral dystrophy (FSHD), dysferlinopathies, limb-girdle muscle dystrophies (LGMD), calpainopathies, and metabolic myopathies, are discussed. Finally, we address some of the endocrine causes of muscle weakness that may present to the rheumatology clinic along with some miscellaneous causes such as infections and drug toxicities which may confound the diagnosis. It is important to remember that while treating a patient of inflammatory myopathy may be rewarding, the risks of prescribing immunosuppressive medication in myositis mimics, while in good faith may be equally harmful.

  Approach to the Patient Top

Age at onset

The age of onset is an important clue to the diagnosis. In children, most myopathies are inherited, the only inflammatory disorder being juvenile dermatomyositis (JDM), which comes on with florid cutaneous signs, ranging from the classic heliotrope and Gottron's rash to periungual desquamation, inflammatory edema, calcinosis, or cutaneous vasculitis resulting in ulcers.[7] Juvenile polymyositis (JPM) is less common, and malar rash, linear extensor erythema, photosensitivity, and lack of periungual capillary abnormalities have been described as predictors of JPM over JDM.[8] Muscular dystrophies typically present in infancy or childhood and must be ruled out in cases nonresponsive to therapy. Most of these do not have a rash at presentation, unlike jDM, and are diagnosed as JPM. In the young individual, slow progression of weakness suggests noninflammatory myopathy rather than myositis, the latter typically being subacute in onset (weeks to months). Calpainopathies such as LGMD2A and FSHD can present with onset in the second to third decade. The most common LGMD type 2A results from mutated protease calpain 3 (CAPN3).[9] Neurological mimics such as myasthenia gravis and motor neuron disease (MND) usually have a later onset ranging from sixth to the seventh decade.[10],[11] A child complaining of muscle weakness after a period of viral-like illness also points to possible viral myositis, which is more common in this age group, although it can also be seen in adults.

Type of onset

The onset of the disease is subacute for inflammatory myositis while the inherited myopathies exhibit a chronic course (months to years). The rapidity of progress is informative as the former tends to progress over months, whereas the latter spans a time frame of years.[9],[12] An exception to this would be inclusion body myositis (IBM) which has a protracted course. On the flip side, certain IIMs, such as immune-mediated necrotising myositis (IMNM) and inherited muscle disorders such as the dysferlinopathies such as Miyoshi myopathy, may present with an acute onset and rapid progress (days).[13],[14] Often, when the onset in inherited myopathies is acute, it will be preceded by an index event such as rigorous exercise or trauma, mostly reported in dysferlinopathies, specially Miyoshi myopathy.[15] A recent entity added to the compendium of muscle disorders is statin-induced myopathy. Onset can often be acute to subacute. It ranges from myalgia with no rise in muscle enzymes or a mild elevation in serum creatinine kinase (CK) levels to significant proximal muscle weakness, which may or may not be associated with autoantibodies. Statin-associated myalgia can be seen in up to 10% of the cases.[16] The presence of pure muscle weakness in these individuals calls for a meticulous review of drugs taken. On rare occasions, such incidents can be precipitated by the unsuspected intake of CYP3A4 inhibitors such as clarithromycin, calcium channel blockers, fibrates, and protease inhibitors with resultant halted metabolism of statin and consequent rhabdomyolysis.[16]

Pattern of involvement

The pattern of muscle involvement is a good discriminant of the underlying diagnosis. The inflammatory myositis is typified by the symmetry of onset of proximal muscle weakness. The exception once more is IBM, which may be asymmetric and involve distal weakness similar to the hereditary myopathies. Individual peculiarities such as the early finger flexor weakness of IBM or the calf weakness of Miyoshi type myopathy are more informative than just noting proximal muscle weakness as this is a finding common to most myositis mimics.[13],[17] Numerous signs have been described in literature based on a characteristic pattern of muscle group involved in various dystrophies and will be dealt with in examination [Table 1].[18],[19],[20],[21],[22],[23]
Table 1: Characteristic muscle involvement and named signs in myositis mimics

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Other features

Atrophy is rarely seen in the IIMs at the time of presentation except for IBM.[13] Muscle pain is another feature that is uncommon in IIM.[14] It is a common feature of the metabolic myopathies, more specifically exercise-induced myalgia seen in glycogen storage disorders, more characteristically in McArdle's disease.[24] It may also be observed in the hereditary myopathies. Pain preceding muscle weakness may occasionally be seen in FSHD and Miyoshi myopathy.[17] Other rheumatological causes such as polymyalgia rheumatica may also present with pain and stiffness of the shoulder and hip girdle that is disproportionate to weakness and lead to confusion in the clinic.

While evaluating adults with symmetrical proximal muscle weakness, increased fatiguability and a characteristic history of increased weakness toward the end of the day which nearly disappears next morning needs to be noted which will lead us to myasthenia gravis.[10] MND may present with a history suggestive of gradual onset proximal along with extremity weakness, leading to difficulty in writing, turning locks, or raising arms overhead. This should prompt the clinician to look for fasciculations on examination.[11]

Family history

For every child or adolescent who presents to the clinic with a history of proximal muscle weakness, it is prudent to delve deeper into the family history, degree of consanguinity, and prior births of offspring which culminated in early life deaths.

Another underrecognized group of illness manifesting with proximal muscle weakness are the mitochondrial myopathies, which may run in families with a constellation of muscle, neurologic involvement, ocular involvement, or cardiac issues such as conduction blocks, which can also be added clues.[25] Such patients give a history of exercise-induced exacerbation, but are not associated with cramping and myalgia, unlike the glycogenoses.

Family history may not always be present as dystrophies may also result from de novo mutations.[25] Most of the dystrophies that mimic IIM are autosomal recessive in inheritance.[9],[26] History of isolated mother to offspring transmission is typical of the mitochondrial myopathies.

Drug history

Drugs used may be another potentially missed trigger, which need to be identified by a meticulous history. Glucocorticoid-induced myopathy can be associated with preferential weakness and wasting of the proximal muscles of the lower limb and the leg along with other tell-tale sequelae of steroid toxicity such as buffalo hump, moon facies, cataract, and purple striae.[27] Cocaine, colchicine, antimalarial drugs, and chronic alcohol use may cause a gradual onset proximal muscle weakness. Chronic alcohol myopathy is seen in malnourished addicts with concomitant Wernicke encephalopathy or chronic liver disease. Cocaine, on the other hand, may lead to acute onset muscle weakness due to vasoconstriction induced muscle ischemia, which occurs soon after beginning use. Long-term hydroxychloroquine usage (HCQ) may also result in chronic symmetric proximal myopathy. There is variable recovery after stopping HCQ.[28] On the other hand, colchicine myotoxicity may present with a gradual onset proximal muscle weakness of lower limb more than upper limb with a relatively asymptomatic neuropathy, which is more common in chronic kidney disease.[29] The usage of penicillamine has reportedly been associated with the onset of reversible DM in the past.[30]

Clues on examination

In any patient who presents with weakness, the examination should first localize the weakness to the muscles by confirming predominant proximal weakness associated with intact deep tendon reflexes (DTR). Absence of reflexes in the presence of proximal or distal weakness may be seen in chronic inflammatory demyelinating polyneuropathy (CIDP).[31] The next point to consider would be whether there is a differential weakness in the agonists and antagonists for the same range of motion. Differential weakness is a finding typically present in inherited myopathies and precludes a diagnosis of inflammatory myositis on most occasions. The presence of upper and lower motor neuron signs of involvement along with variable reflex involvement and possible dysphagia and dysarthria may be indicative of an underlying MND.[11]

Face and neck

Examination of the muscles of the face, an area that often stands ignored, can preempt a misclassification of a hereditary myopathy as inflammatory myositis. Patients may be unaware of facial atrophy, though it may be obvious to the examiner in FSHD. The weakness can at times be minimal, so much so to have the patient caught unawares as well. They might then recall, in retrospect, difficulty in the usage of straw and gradual acquired loss of the ability to whistle. Simple tests to examine the orbicularis oris, oculis, and zygomatics with the closure of the eye, blowing air in the cheek, and grinning can elucidate the above. Although asymmetry is not typical of FSHD, it could be encountered at times.[32] Men with mitochondrial myopathies could also manifest weakness of the ocular musculature seen as ophthalmoplegia, typical for chronic progressive ophthalmoplegia, and Kearns–Sayre syndrome.[24] Often, patients with prominent muscle weakness but only mild eye involvement land up in the rheumatology clinic.

It is extremely important to check for neck muscle weakness and pharyngeal weakness as these may have direct treatment implications. Although it occurs late in inflammatory myositis, they generally respond well to therapy if measures are taken to prevent aspiration pneumonitis and adequate respiratory support is provided.

Upper limb

In the upper limb, it is prudent to examine the scapular movements. FSHD and at times calpainopathies can manifest as weak shoulder shrug and winging of the scapula. There is selective wasting of the trapezius, proximal one-fourth of deltoid, biceps and triceps with preservation of infraspinatus, deltoid, and brachioradialis. This leads to a peculiar appearance of multiple hills seen during examination when the patient adopts a particular posture, also known as the Poly Hill sign in FSHD.[9],[18],[32] In LGMD2a, there is the presence of weak biceps with sparing of the triceps. Dysferlinopathies have upper limb involvement at later stages of the disease with atrophy of biceps and deltoid. The calf head sign is sometimes observed in Miyoshi myopathy wherein on flexing the elbow and abducting shoulders and viewing from behind, a prominent deltoid (resembles calves head), upper border of trapezius (resembles horn), the upper medial part of infraspinatus (resembles ear), prominent spinous process of the scapula (upper border of calf's neck) are seen.[20] Selective involvement of biceps fibers leading to a nodular swelling known as the Biceps Lump as been noted.[33]

For IBM, asymmetrical onset and the presence of weak forearm flexors and quadriceps wasting can guide the diagnosis. A common mistake is to test for grip strength without ruling out finger flexor weakness. Since lumbricals and interossei are typically spared in IBM, it is important to specifically check for muscle power of the flexor digitorum profundus, as the flexor digitorum profundus is preferentially involved in IBM. This is demonstrated in fist sign wherein a normal person when asked to form a fist can normally, flex his distal interphalangeal joints (DIP), effectively burying all his fingernails in the palm but a patient of IBM with FDP weakness is unable to flex his DIPs effectively while forming a fist. In late stages, atrophy of forearm flexors leads to a scalloped appearance. Forearm extensors are often spared, and the arm muscles are not as weak as the distal musculature. Involvement of the neck extensors and the paraspinal muscles leads to a head drop with “camptocormia” or “bent spine syndrome.” Tibialis anterior weakness in the foot can cause difficulty in ankle dorsiflexion.[13]


Asymmetrical involvement of the abdominal muscles is also seen in FSHD with sparing of the upper abdominal muscles, leading to the Beevor sign.[34] Involvement of the abdominal muscles and external obliques in calpainopathies leads to hernias, often reaching the surgeon's office.[9]

Pelvic girdle and lower limb

Akin to most IIMs, the patients with calpainopathies can have proximal pelvic girdle muscle weakness. However, remarkably weak hip adductors in the face of spared abductors are the clue. There is involvement of posterior compartment of the thigh with sparing of gracilis and sartorius. In the calf, there is the involvement of soleus and medial head of gastrocnemius with sparing of lateral head. These patients can have Achilles tendon shortening and resultant heel contractures. In the late stages, foot drop can be seen.[9]

Of the various LGMDs, dysferlinopathies and specifically, LGMD 2b run the most risk of being misdiagnosed as inflammatory myositis. These patients with LGMD2b manifest with predominant proximal lower limb involvement, whereas patients with Miyoshi myopathy are typically young adults with distal lower limb predominance. There is the involvement of hamstrings and hip flexors such as iliopsoas and quadriceps. Wasting of vastus lateralis and rectus femoris in their upper and lower part with sparing of the middle section leads to its prominence on contraction or the Diamond on Quadriceps sign. Gastrocnemius involvement leads to marked calf atrophy and inability to stand on tiptoes.[34]

Reflexes, sensory system

Once the type of muscle involvement and the groups of muscles predominantly, involved are ascertained, the presence of intact DTR is an important clue. Absent DTR's may be seen in CIDP and other cases of neuropathy and in some cases of MND. Sensory involvement like paresthesias may also be present in CIDP.[31]

Other systems

Since mitochondrial myopathies can present with a multisystemic involvement, a cardiovascular and a central nervous system (CNS) examination should also be included, and electrocardiogram and liver function tests need to be evaluated.[25]


Muscle enzymes

Normal muscle enzymes essentially rule out PM, dysferlinopathies, and calpainopathies. However, the elevation is not of much diagnostic avail. They can be normal to high depending on the muscle mass remaining in the presenting individual. They may be normal to mildly raised in FSHD. Very high CK values suggest necrosis of the muscle, seen in IMNM which can be idiopathic or statin related. On the other hand, most Mitochondrial myopathies do not exhibit much rise in enzymes.[14]


Electrodiagnostic studies are used in any patient with muscle weakness primarily to rule out neuromuscular conditions such as MND, motor neuropathies, and neuromuscular disorders. All myopathies have common features of the increased insertional activity, positive sharp waves, early recruitment, and small motor unit action potentials. These features cannot differentiate between hereditary myopathies and IIM. However, electromyography (EMG) can be used to localize the distribution of involvement and to select sites for biopsy. Involvement of paraspinal muscles on the EMG is suggestive of metabolic myopathies such as Pompe's disease.

Muscle biopsy

The advances in the evaluation of autoantibodies have reduced the role of a muscle biopsy in the diagnosis of inflammatory myositis.[35] In most patients with a myositis-specific antibody positivity and typical cutaneous manifestations, a muscle biopsy is no longer sought for DM, immune-mediated necrotizing myopathies (IMNM) and antisynthetase syndromes. It is still required in PM and IBM to rule out dystrophies. Inflammatory infiltrates on the biopsy are not specific for inflammatory myositis. Dysferlinopathies, congenital dystrophies, and FSHD can all have patchy inflammatory infiltrates with the presence of both atrophy as well as hypertrophy of muscle fibers.[36] While atrophy is common in inflammatory myositis, hypertrophy is usually not seen. MHCI upregulation is not specific for any one class of myositis or for dystrophies and may be seen in all of these. It has been argued that MHC II upregulation in conjunction with MHC I upregulation is more specific for the diagnosis of inflammatory myositis as this is not typically seen in those with dystrophies.[37],[38] The sensitivity of MHC II upregulations may be low (25%–60%), but it has a high specificity for the diagnosis of inflammatory myositis (90%–100%).[37],[38] Special immunochemical stains of sarcolemmal proteins and stains for metabolic products such as glycogen may help distinguish hereditary myopathies and metabolic myopathies, respectively[34] [Table 2].
Table 2: Muscle biopsy findings of the common myositis mimics

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Myositis-specific antibodies (MSAs) are an excellent tool for clinching the diagnosis of IIM. Still, the difference in assays and limited availability hinders its way to coming in day-to-day practice. Also, a negative test does not exclude the possibility of IIM as it is present in ~50% of cases only. Myositis-associated antibodies, which are widely available, have less specificity for IIM. However, the presence does point toward inflammatory pathology. MSA are specific for myositis with a prevalence of <0.1% in the general population, while MAAs can often be seen in as many as 0.5%–1% of the general population.

Magnetic resonance imaging with T1 and STIR sequences

T1 images are used to identify morphological changes, such as the extent of muscle atrophy or fatty infiltration. T2 and STIR images are useful in picking up muscle oedema. Magnetic resonance imaging can be used to localize the best site for a muscle biopsy as well as pick up subtle inflammation at sites which are not picked up on physical examination. The pattern of muscle involvement may also give a clue to the underlying diagnosis with many of the inherited dystrophies selectively affecting a certain group of muscles. IBM can have asymmetrical involvement of proximal muscles, especially quadriceps sparing the rectus femoris and posterior compartment and distal muscle involvement including gastrocnemius with fatty infiltration along with a striking predilection for FDP. A unique “melting appearance” due to the fatty infiltration may be seen.[39] LGMD2b can have diffuse and severe involvement of the shoulder girdle and relative sparing of the quadriceps, sartorius, and gracilis muscles at the pelvic level. Hyperintensities may be present on STIR images even if T1 images are normal. In calpainopathies, there is the involvement of posterior compartment of thigh including adductors and semimembranous and posterior calf excluding lateral head of the gastrocnemius and tibialis anterior and posterior. This pattern should prompt the physician to go ahead for genetic testing for calpainopathies even if Western blot was normal.[40]

A detailed history with examination is paramount to suspect mimics. Salient points in history and examination should be looked for and are listed in [Figure 1]. [Table 3] summarizes the clinical signs of the common mimics discussed above.
Figure 1: Key point on when to suspect a myositis mimic

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Table 3: Common mimics and clues to identify them

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Idiopathic inflammatory myopathies

In 2017, EULAR published new classification criteria for the inflammatory myositis. These use a weighted scoring system to assign a probability of labelling a particular case as JDM, DM, amyopathic DM, PM, or immune-mediated necrotizing myopathies (IMNM). The salient features of the subgroups of inflammatory myositis are given in [Table 3]. They also released a web calculator to aid in the application of these classifications. This can be accessed at

Commonly encountered myositis mimics

Muscular dystrophies

Facioscapulohumeral dystrophy

This typically presents in adulthood with slowly progressive asymmetric involvement of the shoulder girdle with relative sparing of the deltoid. The sparing of the deltoid is not seen in any other myopathy. These patients also often have marked facial atrophy without being symptomatic for the same. Scapular winging is prominent on examination. It is also one of the few myositis mimics that had an autosomal dominant inheritance. On investigation, creatine phosphokinase (CK) shows the mild elevation and a markedly raised CK precludes a diagnosis of FSHD. Muscle biopsy may reveal patchy inflammatory infiltrates of T and B cells without any muscle invasion which distinguishes this from PM.[32]


This form of muscular dystrophy may be the most difficult to differentiate from inflammatory myositis due to its subacute onset with relatively rapid progression with markedly elevated CK levels. These are autosomal recessive in inheritance. Two important types are limb-girdle muscular dystrophy type 2B and Miyoshi Myopathy. The former is associated with proximal muscle weakness involving the shoulder and hip girdles which progresses slowly. Miyoshi myopathy typically has a distal onset with early calf involvement and rapid progression. Later, in the disease course, proximal muscle groups may also be involved. Both of these occur in late adolescence to early adulthood. Patients often undergo muscle biopsy, which shows inflammatory infiltrates of mononuclear cells and macrophages with evidence of muscle necrosis. The most common misdiagnosis is of PM, and patients often undergo genetic testing after a therapeutic trial of corticosteroids fails.[34]


A proximal myopathy that initiates in childhood with early involvement of the hip girdle. Shoulder girdle occurs late in the disease course as does distal involvement. JDM must always be ruled out by checking for any subtle dermatological signs and genetic testing if the need arises. Another close differential is FSHD due to the presence of scapular winging with the notable exception that facial muscles are spared. There are no distinctive features on muscle biopsy but immunohistochemistry for calpain 3, when available is negative.[9]

Metabolic myopathies

Metabolic myopathies may occur when there are perturbations in the energy source either in glycogen or lipid metabolism and present with a characteristic history of intermittent weakness. An approach to such patients is shown in [Figure 2].
Figure 2: Flowchart showing an approach to metabolic myopathies

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Myophosphorylase deficiency (McArdle's Disease)

The primary cause of this disease is an inability to utilize muscle stores of glycogen. It presents in adolescents and young adults with a characteristic history of exercise-induced fatigue, cramping, and myoglobinuria, which recovers on rest. This second wind phenomenon occurs due to the ability of the muscle to utilize circulating glucose for energy needs. Recurrent attacks may precipitate acute kidney injury due to myoglobinuria. With age, the weakness tends to remain persistent in the form of proximal muscle weakness, thereby leading to greater chances of being confused with inflammatory myositis. There are intramuscular glycogen deposits on muscle biopsy and immunohistochemistry when available will show lack of muscle glycogen phosphorylase.[24]

Acid maltase deficiency (Pompe's disease)

This manifests with proximal muscle weakness akin to PM and has infantile, juvenile, and adult forms depending on the extent of enzyme activity that is present. Infantile forms are severe and are invariably fatal. Adult forms may rarely have cardiac defects in the conduction system as well as left ventricular hypertrophy and resultant cardiomyopathy. These patients may also have ventilatory defect due to diaphragmatic involvement. There are characteristic periodic acid–Schiff positive vacuoles on muscle biopsy due to the inability to break down glycogen.[24]

Carnitine palmitoyltransferase deficiency

This deficiency leads to the inability to metabolize fatty acid oxidation which is required when there is sustained periods of low-intensity exercise especially in states, in which glucose levels are low such as in fasting state. This is in contrast to the weakness and muscle pain that occurs immediately after rapid bursts of activity and exercise in myophosphorylase and acid maltase deficiency. Myoglobinuria may be present depending on the extent of physical activity undertaken. Immunohistochemistry on muscle biopsy specimens for the enzyme carnitine palmitoyltransferase II may be helpful.[24]

Mitochondrial myopathies

Another underrecognized group of illness manifesting with proximal muscle weakness are the Mitochondrial myopathies, typically passed on from the mother to offspring. However, de novo mutations may occur as well. Multisystem involvement like ocular involvement, conduction block, liver and CNS dysfunction can be added clues. The extraocular muscle involvement present in these myopathies is never seen in inflammatory myositis. Such patients give a history of exercise-induced exacerbation, but are not associated with cramping and myalgia, unlike the glycogenoses. There may be mild elevations of CK, but the hallmark is lactic acidosis. Biopsy reveals typical ragged red fibers on Gomori Methenamine silver staining or COX-negative ragged blue fibers on succinate dehydrogenase staining.[25]

Endocrine myopathies

Osteomalacia is an important differential to consider, especially in middle-aged women of Asian descent and can present with diffuse aches and pains associated with mild proximal muscle weakness. Typical laboratory results would show a low to normal calcium level, normal phosphorus level, elevated alkaline phosphatase and a low Vitamin D level. The associated risk of fracture and the rapid improvement with adequate dietary supplementation of calcium and Vitamin D makes this a rewarding diagnosis for both the patient and the physician.[41]

A thyroid disorder, both hypo as well as hyperthyroidism, may cause proximal muscle weakness mimicking an inflammatory myopathy. Weight loss, alopecia and lethargy may be red herrings in a patient with thyroid illness that can reinforce a false impression of a rheumatic disease. Clues in the history of associated weight gain or loss and heat or cold intolerance must be looked for even in the absence of a visible goiter. Classic thyroid eye signs may occasionally be the only manifestation of thyroid illness accompanying proximal myopathy. In hypothyroidism, patients typically have a proximal myopathy with delayed relaxation of DTR. Creatine kinase is generally elevated, and the EMG may show myopathic changes in some but not all patients. Hence, a thyroid profile is indicated in any patient who presents with proximal myopathy. The weakness typically recovers over months after starting thyroid replacement. Hyperthyroidism may also result in a proximal myopathy that is generally functional with normal muscle enzymes. There is rapid correction with correction of the hyperthyroid state.[4],[5]

Endogenous or exogenous Cushing's disease may be a cause of proximal myopathy due to excess glucocorticoids. Recognition is fairly simple based on the presence of typical morphological features of Cushing's such as abdominal striae, fat redistribution with central adiposity, peripheral wasting, a buffalo hump and moon facies. Steroid-induced myopathy typically has lack of CK elevation and may reveal type 1 fiber atrophy on muscle biopsy. However, steroid-induced myopathy rarely occurs at exogenous corticosteroid doses of <10 mg/day prednisone equivalent.[27]

Rapidly progressive pain and weakness involving the muscles around the hip girdle may be a feature of diabetic amyotrophy. The features that help differentiate this entity are the severe radicular pain disproportionate to the weakness and the striking atrophy of the hip flexor, extensor and adductor compartments that may have progressed over weeks. There is an associated history of uncontrolled sugars. Occasionally, people with diabetes who have recently been started on therapy and had rapid control of sugars after prolonged years of uncontrolled diabetes may present similarly.[42]

Miscellaneous causes

In a tropical country like India, one must keep an eye out for infectious causes of myositis. Suppurative myositis due to Staphylococcus aureus and rare causes of microsporidia myositis have been described.[43] Rare cases of sarcoidosis presenting with myositis as part of extrapulmonary manifestations with evidence of noncaseating granulomas have also been described.[44]

  Conclusion Top

There is an entire gamut of causes which can present to the rheumatologist with proximal muscle weakness. It is up to the astute clinician to discern the minute clinical signs which give the mimics away. A structured and detailed history and examination would allow the physician to identify minor clues that suggest the underlying diagnosis. Since these cases are rare, systematic and collaborative data collection on the clinical presentation and outcomes of noninflammatory myopathies may provide a clear comparison with existent literature in IIM.[45],[46],[47] An open dialogue with the pathologist and radiologist regarding possible diagnosis may be useful to approach challenging cases, bringing the clinician a step closer to diagnosis.

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Conflicts of interest

There are no conflicts of interest.

  References Top

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  [Figure 1], [Figure 2]

  [Table 1], [Table 2], [Table 3]


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