NEUROMUSCULAR DISORDERS
Neuromuscular disorders refer to a large heterogeneous group of diseases involving lower motor unit at various sites: (a) anterior horn cells, (b) peripheral nerves, (c) myoneural junction, and (d) muscles.
All neuromuscular disorders are characterized by generalized or localized flaccid weakness, hypotonia, and hyporeflexia, with/without sensory or autonomic involvement. The term floppy child or infant is usually reserved for a child with generalized hypotonia due to any cause, discussed later (Ch 18.19).
Clinical differentiation between neuropathies and myopathies is not always easy, though some important clues may be obtained on history, examination and basic investigations (Table 18.39).
Some useful clinical indicators for different pattern of weakness in neuromuscular disorders in children are as follows:
• Proximal muscle weakness manifests with difficulty in getting up from chair, going up/ down stairs, combing the hair.
• Distal muscle weakness manifests with difficulty in wearing slippers, buttoning/unbuttoning and holding the objects in hand.
• Truncal weakness presents with difficulty in turning on bed or sitting from supine position
| TABLE 18.39: General D/D myopathies and neuropathies* | ||
| Myopathies | Neuropathies | |
| Family history | Common | Uncommon |
| Muscle weakness | Proximal | Distal (except SMA) |
| Muscle mass | Pseudohypertrophy | Atrophy |
| Deep reflexes | Depressed | Absent |
| Sensory involve- | Absent | Present |
| ment | ||
| CPK levels | Abnormal | Near normal |
| EMG | Myopathic pattern | Neuropathic pattern |
| Muscle biopsy | Diagnostic | Non-specific |
* based on usual features, though exceptions are common
• Bulbar weakness leads to difficulty in swallowing, nasal regurgitation, soft cry, dysphonia and recurrent aspirations.
• Facial muscle weakness manifests with difficulty in blowing balloons, sucking through straw and closing the eyes.
Important investigations in neuromuscular disorders include:
• Serum CPK levels: CPK is a lysosomal enzyme, released from damaged or degenerating muscle cells (N: lt;160 IU/L). These levels are markedly elevated (in thousands) in myopathies, though mild elevation is not uncommon in neuropathies.
• Nerve conduction velocity (NCV) is abnormal in neuropathies, only if gt;80% of nerve fibers are damaged. As NCV is slower in infancy than in older children, interpretation is difficult in this age group.
• Electromyography (EMG), as measured by insertion of a needle into the belly of partially involved muscle (atrophic or normal muscles should not be used for EMG) helps to differentiate neuropathies from primary myopathies, but does not identify the specific cause.
Normally, the resting muscle is silent on EMG, while a triphasic record of motor unit action potential (MUAP) is obtained on its contraction. Important EMG abnormalities include:
± Fasciculations or fibrillations in denervation lesions, e.g. spinal muscular atrophy;
± Large amplitude, long duration MUAPs in peripheral neuropathies;
± Low amplitude, short duration MUAPs in myopathies;
• Muscle biopsy is useful in suspected myopathies, which helps to differentiate primary myopathy from neuropathy as well as to identify the specific cause and enzyme deficiency.
• Nerve biopsy is indicated in probable neuropathic disorders. Sural nerve, just behind the lateral malleolus is preferred as it is a purely sensory nerve, supplies only a small area over lateral aspect of foot and regenerates rapidly in 90% cases even if injured.
18.18.1