Pattern Recognition Approach to Myopathic Disorders
After answering the six key questions obtained from the history and neurological examination outlined previously, a myopathic disorder can be classified into one of six distinctive patterns of muscle weakness. Although there are certainly exceptions to any rule concerning the distribution of involved muscles, using this classification system allows the practitioner to be able to substantially restrict the differential diagnosis. The final diagnosis can then be confirmed based on information from a selective number of laboratory studies.
The most common pattern of muscle weakness in myopathies is symmetric weakness affecting predominantly the proximal muscles of the legs and arms, or the so-called “limb-girdle” distribution. The distal muscles are usually involved, but to a much lesser extent. Neck extensor and flexor muscles are also frequently affected. This pattern of weakness is seen in most hereditary and acquired myopathies and, therefore, is the least specific in arriving at a particular diagnosis.[12,14]
The distal pattern of weakness predominantly involves the distal muscles of the upper or lower extremities (anterior or posterior compartment muscle groups) ( Table 14 ). Depending on the diagnosis and severity of disease, proximal muscles may also be affected. The involvement is usually, although not invariably, symmetric. Selective weakness and atrophy in distal extremity muscles is more commonly a feature of neuropathies; therefore, a careful sensory and reflex examination must always be performed in patients presenting with this phenotype.
The scapuloperoneal pattern of weakness affects the parascapular muscles of the proximal arm and the anterior compartment muscles of the distal lower extremity, or the so-called “scapuloperoneal” distribution ( Table 14 ). The scapular muscle weakness is usually characterized by scapular winging. Weakness can be very asymmetric. These patients often relate a history of prominent “sloped shoulders” or “poor posture” and may have difficulty climbing a rope, throwing a ball, or swinging a golf club. When this pattern is associated with facial weakness, it is highly suggestive of a diagnosis of FSH dystrophy. Asymmetric facial weakness in FSH dystrophy results in an odd, twisted smile with dimpling at the corner of the mouth and a depressed and “flat” appearance to the patient's face. Other hereditary myopathies that are associated with a scapuloperoneal distribution of weakness include scapuloperoneal dystrophy, Emery-Dreifuss dystrophy, LGMD 1B (laminopathy), LGMD 2A (calpainopathy), and LGMD 2C-F (sarcoglycanopathies), LGMD 2I (FKRP), congenital myopathies, phosphorylase deficiency, and acid maltase deficiency.
The distal arm/proximal leg pattern is associated with distal arm weakness involving the distal forearm muscles (wrist and finger flexors) and proximal leg weakness involving the knee extensors (quadriceps). Patients commonly complain of falls caused by their “knees giving out” and difficulty walking down stairs. They may also complain of difficulty picking up coins, winding their wrist watch, or knitting. The facial muscles are usually spared. Involvement of other muscles is extremely variable. In addition, the weakness is often asymmetric between the two sides, which is uncommon in most myopathies. This pattern is essentially pathognomonic for inclusion body myositis. The pattern may also represent an uncommon presentation of myotonic dystrophy; however, unlike inclusion body myositis, muscle weakness is symmetric.
Myopathies presenting with predominant involvement of ocular and/or pharyngeal muscles represent a relatively limited group of disorders ( Table 15 ). The eye involvement principally results in ptosis and ophthalmoplegia, which usually, although not always, occurs without symptoms of diplopia. Facial weakness is not uncommon and questioning may elicit a history of having a “funny smile” or having difficulty blowing up balloons, drinking through a straw or clearing food caught between the lips and gums. Extremity weakness is extremely variable, depending on the diagnosis.
The combination of ptosis, ophthalmoplegia without diplopia, and dysphagia should suggest the diagnosis of oculopharyngeal dystrophy, especially if the onset is in middle age or later. Ptosis and ophthalmoplegia without prominent pharyngeal involvement is a hallmark of many of the mitochondrial myopathies. Ptosis and facial weakness without ophthalmoplegia is a common feature of myotonic dystrophy and fascioscapulohumeral dystrophy.
The prominent neck extensor weakness pattern is characterized by severe weakness of the neck extensor muscles. The term “dropped head syndrome” has been used in this situation ( Table 16 ). Involvement of the neck flexors is variable. Extremity weakness depends on the diagnosis and may follow one of the previously outlined phenotypic patterns. For example, a patient with a limb-girdle pattern of weakness may also have significant neck extensor involvement. Isolated neck extension weakness represents a distinct muscle disorder called isolated neck extensor myopathy (INEM). Prominent neck extensor weakness is also common in two other neuromuscular diseases: amyotrophic lateral sclerosis and myasthenia gravis.
Semin Neurol. 2008;28(2):228-240. © 2008 Thieme Medical Publishers
Cite this: Clinical Approach to Muscle Diseases - Medscape - Apr 01, 2008.