Chapter 8 Ultrasound of Motor Neuron Disease
Video 8.1:
Video 8.2:
ALS (also called Charcot’s or Lou Gehrig’s disease) is the most commonly encountered motor neuron disease in modern clinical practice, with an incidence between 1.5 and 2.5 per 100,000 per year.1 It is a neurodegenerative condition in which upper and lower motor neurons are progressively lost, and a disease for which there is no cure. There are variants of ALS in which only upper (primary lateral sclerosis) or only lower (progressive muscular atrophy) motor neurons are affected. SMA is also a neurodegenerative disease, but in contrast to ALS it is inherited (autosomal recessive) and typically affects children. There are four subtypes of SMA, and the divisions are based on the age at onset.
Imaging in Motor Neuron Disease
Historically motor neuron diseases have been diagnosed clinically, with electrodiagnostic studies used to support the diagnosis. Imaging studies are rarely supportive in this regard, although magnetic resonance imaging (MRI) of the brain and cervical spine is often used to rule out ALS mimics.2 More recently, advanced central nervous system imaging techniques, such as functional MRI, MR tractography, and positron-emission tomography (PET) have demonstrated changes in those with ALS.3–5 Although these techniques have provided insight into the pathophysiology of motor neuron disease, they are not well suited for diagnostic applications. Neuromuscular imaging has been explored in motor neuron disease, but the field is not well developed. In theory, peripheral nervous system imaging could be beneficial in motor neuron disease because it could aid in diagnosis, advance our understanding of the pathophysiology, and serve as a surrogate marker of disease progression in therapeutic trials and clinical practice. Some of these applications have been preliminarily explored using neuromuscular ultrasound.
Nerve Ultrasound in Motor Neuron Disease
Imaging studies focusing on peripheral nerves in individuals with motor neuron disease are exceedingly rare. Autopsy studies have shown that nerve roots are atrophic in those with ALS,6 but there is a surprising lack of reports confirming root atrophy through imaging in living individuals. There is also a dearth of studies in which specific emphasis is placed on imaging peripheral nerves in those with motor neuron disease.
There is one study in which neuromuscular ultrasound was used to examine the nerves of those with ALS.7 In this report, high-resolution ultrasound (Biosound MyLab 25, Esaote Group, Genoa, Italy; with 18MHz transducer) was used to image the median nerve at the mid-arm (Fig. 8.1) and the sural nerve above the lateral malleolus (Fig. 8.2) in 20 individuals with ALS and 20 age- and gender-matched controls. The mean cross-sectional area of the median nerve was slightly smaller in the ALS patients than the controls, whereas the sural nerve area did not differ between these two groups (Fig. 8.3). However, the absolute difference in mean median nerve area between the ALS patients (10.5 mm2) and controls (12.7 mm2) was not large, and the mean median nerve area in the ALS patients was actually slightly larger than that seen in previous studies of healthy controls. The larger median nerve cross-sectional area in controls in this study may be a result of a higher mean age compared with previous control populations, because nerve area does increase slightly with increasing age. This study was limited in that only two nerves were examined, no nerve root imaging was included, and a distal motor nerve was not assessed. However, this report did demonstrate a subtle difference, with the median nerve cross-sectional area being smaller in those with ALS than in an age- and gender-matched control groups.
Muscle Ultrasound in Motor Neuron Disease
Diagnosis in SMA
Some of the first studies in the field of neuromuscular ultrasound were performed to assess the diagnostic accuracy of the technique in pediatric neuromuscular disorders. In 1982, Heckmatt and associates subjectively compared muscle ultrasound findings between 60 children with neuromuscular disorders and 60 healthy controls. A clear difference was noted between the two groups, and those with SMA were found to have increased muscle echogenicity, muscle atrophy, and increased depth of subcutaneous tissue (Fig. 8.4).8 This study was followed by a comparison between 20 boys with Duchenne’s muscular dystrophy and 10 children with SMA, and it was found that ultrasonographic muscle changes correlated with changes seen on muscle biopsy, and even those with minimal clinical findings had easily detectable changes of increased muscle echogenicity9 (Box 8.1 lists the neuromuscular ultrasound findings in SMA).
