American Academy of Neurology (ANN) guidelines for genetic testing for suspected<unk> . Not all laboratories follow this workflow.FSHD can be presumptively diagnosed in many cases based on signs, symptoms, and/or non-genetic medical tests. Genetic testing can provide definitive diagnosis. In the absence of an established family history of<unk> , diagnosis can be difficult due to the variability in how<unk>  manifests. Genetic testing Genetic testing is the gold standard for<unk>  diagnosis, as it is the most sensitive and specific test available. Commonly,<unk> 1 is tested for first. A shortened D4Z4 array length (EcoRI length of 10 kb to 38 kb) with an adjacent 4qA allele supports<unk> 1. If<unk> 1 is not present, commonly<unk> 2 is tested for next by assessing methylation at 4q35. Low methylation (less than 20%) in the context of a 4qA allele is sufficient for diagnosis. The specific mutation, usually one of various SMCHD1 mutations, can be identified with next-generation sequencing (NGS). Assessing D4Z4 length Measuring D4Z4 length is technically challenging due to the D4Z4 repeat array consisting of long, repetitive elements. For example, NGS is not useful for assessing D4Z4 length, because it breaks DNA into fragments before reading them, and it is unclear from which D4Z4 repeat each sequenced fragment came. In 2020, optical mapping became available for measuring D4Z4 array length, which is more precise and less labor intensive than southern blot. Molecular combing is also available for assessing D4Z4 array length. These methods, which physical measure the size of the D4Z4 repeat array, require specially prepared high quality and high molecular weight genomic DNA (gDNA) from serum, increasing cost and reducing accessability to testing. Diagram showing restriction enzyme sites used to differentiate between D4Z4 repeat arrays of 4q and 10q. Restriction fragment length polymorphism (RFLP) analysis was the first genetic test developed and is still used as of 2020, although it is being phased out by newer methods. It involves dicing the DNA with restriction enzymes and sorting the resulting restriction fragments by size using southern blot. The restriction enzymes EcoRI and BlnI are commonly used. EcoRI isolates the 4q and 10q repeat arrays, and BlnI dices the 10q sequence into small pieces, allowing 4q to be distinguished. The EcoRI restriction fragment is composed of three parts: 1) 5.7 kb proximal part, 2) the central, variable size D4Z4 repeat array, and 3) the distal part, usually 1.25 kb. The proximal portion has a sequence of DNA stainable by the probe p13E-11, which is commonly used to visualize the EcoRI fragment during southern blot. The name "p13E-11" reflects that it is a subclone of a DNA sequence designated as cosmid 13E during the human genome project. Considering that each D4Z4 repeat is 3.3 kb, and the EcoRI fragment contains 6.9 kb of DNA that is not part of the D4Z4 repeat array, the number of D4Z4 units can be calculated. :D4Z4 repeats = (EcoRI length - 6.9) / 3.3 Sometimes 4q or 10q will have a combination of D4Z4 and D4Z4-like repeats due to DNA exchange between 4q and 10q, which can yield erroneous results, requiring more detailed workup. Sometimes D4Z4 repeat array deletions can include the p13E-11 binding site, warranting use of alternate probes. Assessing methylation status Methylation status of 4q35 is tradationally assessed after<unk> 1 testing is negative. Methylation sensitive restriction enzyme (MSRE) digestion showing hypomethylation has long been considered diagnostic of<unk> 2. Other methylation assays have been proposed or used in research settings, including methylated DNA immunoprecipitation and bisulfite sequencing, but are not routinely used in clinical practice. Bisulfite sequencing, if validated, would be valuable due to it being able to use lower quality DNA sources, such as those found in saliva. Auxiliary testing MRI showing asymmetrical involvement of various muscles in<unk>  Other tests can support the diagnosis of<unk> , although they are all less sensitive and less specific than genetic testing. Nonetheless, they can rule out similar-appearing conditions. * Creatine kinase (CK) blood level is often ordered when muscle damage is suspected. CK is an enzyme found in muscle, leaking into the blood when muscles become damaged. In<unk> , CK level is normal to mildly elevated, never exceeding five times the upper limit of normal. * Electromyogram (EMG) measures the electrical activity in the muscle. EMG can show nonspecific signs of muscle damage or irritability. * Nerve conduction velocity (NCV) measures the how fast signals travel along a nerve. The nerve signals are measured with surface electrodes (similar to those used for an electrocardiogram) or needle electrodes. * Muscle biopsy is the surgical removal and examination of a small piece of muscle, usually from the arm or leg. Microscopy and a variety of biochemical tests are used for examination. Findings in<unk>  are nonspecific, such as presence of white blood cells or variation in muscle fiber size. This test is rarely indicated. * Muscle MRI is sensitive for detecting muscle damage, even in mild cases. Because of the particular muscle involvement patterns of<unk> , MRI can help differentiate<unk>  from other muscle diseases, directing genetic testing. Differential diagnosis Included in the differential diagnosis of<unk>  are limb-girdle<unk>  dystrophy (especially calpainopathy), scapuloperoneal myopathy, mitochondrial myopathy, Pompe disease, and polymyositis. Calpainopathy and scapuloperoneal myopathy, like<unk> , present with scapular winging. Features that suggest<unk>  are facial weakness, asymmetric weakness, and lack of benefit from immunosuppression medications. Features the suggest an alternative diagnosis are contractures, respiratory insufficiency, weakness of muscles controlling eye movement, and weakness of the tongue or throat.
Facioscapulohumeral muscular dystrophy
Limb–girdle muscular dystrophy
Becker muscular dystrophy
Ullrich congenital muscular dystrophy
Fukuyama congenital muscular dystrophy
Spinal muscular atrophy
Duchenne muscular dystrophy
Jokela type spinal muscular atrophy