Motor neuron disease
|Motor neuron disease|
|Synonyms||motor neurone disease|
Commonly, the following disorders are counted among motor neuron diseases:
- Amyotrophic lateral sclerosis (ALS)
- Hereditary spastic paraplegia (HSP)
- Primary lateral sclerosis (PLS)
- Progressive muscular atrophy (PMA)
- Progressive bulbar palsy (PBP)
- Pseudobulbar palsy
- Spinal muscular atrophy (SMA)
- Spinobulbar muscular atrophy (SBMA)
Motor neuron diseases affect both children and adults. While each motor neuron disease affects patients differently, they all cause movement-related symptoms, mainly muscle weakness. Most diseases seem to occur randomly without known causes, but some forms are inherited. Studies into these inherited forms have led to discoveries of various genes (e.g. SOD1) that are thought be important in understanding how the disease occurs.
Symptoms of motor neuron diseases can be first seen at birth or can come on slowly later in life. There is no definitive test to diagnose these diseases. Therefore, a combination of various information (e.g. symptoms, blood tests, imaging studies) is used to reach the diagnosis. Most diseases worsen over time, with some diseases shortening one’s life expectancy (e.g. ALS) while others do not. Currently, there no approved treatments for the majority of motor neuron disorders and care is mostly symptomatic.
In the United States, the term is often used to denote ALS or Lou Gehrig's Disease, the most common disorder in the group. In the United Kingdom, the term is spelled motor neurone disease and is sometimes used for the entire group, but can also refer specifically to ALS.
While MND refers to a specific subset of similar diseases, there are numerous other diseases of motor neurons that are referred to collectively as "motor neuron disorders", for instance disease belonging to spinal muscular atrophies. However, they are not classified as "motor neuron diseases" by the tenth International Statistical Classification of Diseases and Related Health Problems (ICD-10), which is the definition followed in this article.
Motor neuron disease describes a collection of clinical disorders, characterized by progressive muscle weakness and the degeneration of the motor neuron on electrophysiological testing. As discussed above, the term "motor neuron disease" has varying meanings in different countries. Similarly, the literature inconsistently classifies which degenerative motor neuron disorders can be included under the umbrella term "motor neuron disease". The four main types of MND are marked (*) in the table below.
All types of MND can be differentiated by two defining characteristics:
- Is the disease sporadic or inherited?
- Is there involvement of the upper motor neurons (UMN), the lower motor neurons (LMN), or both?
Sporadic or acquired MNDs occur in patients with no family history of degenerative motor neuron disease. Inherited or genetic MNDs adhere to one of the following inheritance patterns: autosomal dominant, autosomal recessive, or X-linked. Some disorders, like ALS, can occur sporadically (85%) or can have a genetic cause (15%) with the same clinical symptoms.
|Type||UMN degeneration||LMN degeneration|
|Sporadic amyotrophic lateral sclerosis (ALS)*||Yes||Yes|
|Primary lateral sclerosis (PLS)*||Yes||No|
|Progressive muscular atrophy (PMA)*||No||Yes|
|Progressive bulbar palsy (PBP)*||Yes||Yes, bulbar region|
|Pseudobulbar palsy||Yes, bulbar region||No|
|Familial amyotrophic lateral sclerosis (ALS)*||Yes||Yes|
|Spinal muscular atrophy (SMA)||No||Yes|
|Spinobulbar muscular atrophy (SBMA)||No||Yes|
|Hereditary spastic paraplegia (HSP)||Yes||No|
Signs and symptoms
Signs and symptoms depend on the specific disease, but motor neuron diseases typically manifest as a group of movement-related symptoms. They come on slowly, and worsen over the course of more than three months. Various patterns of muscle weakness are seen, and muscle cramps and spasms may occur. One can develop shortness of breath with exertion, shortness of breath while lying down (orthopnea), or even respiratory failure if breathing muscles become involved. Bulbar symptoms, including difficulty speaking (dysarthria), difficulty swallowing (dysphagia), and excessive saliva production (sialorrhea), can also occur. Sensation, or the ability to feel, is typically not affected. Emotional disturbance (e.g. pseudobulbar affect) and cognitive and behavioral changes (e.g. problems in word fluency, decision-making, and memory) are also seen. There can be lower motor neuron findings (e.g. muscle wasting, muscle twitching), upper motor neuron findings (e.g. brisk reflexes, Babinski reflex, Hoffman's reflex, increased muscle tone), or both.
Motor neuron diseases are seen both in children and in adults. Those that affect children tend to be inherited or familial, and their symptoms are either present at birth or appear before learning to walk. Those that affect adults tend to appear after age 40. The clinical course depends on the specific disease, but most progress or worsen over the course of months. Some are fatal (e.g. ALS), while others are not (e.g. PLS).
Patterns of weakness
Various patterns of muscle weakness occur in different motor neuron diseases. Weakness can be symmetric or asymmetric, and it can occur in body parts that are distal, proximal, or both. According to Statland et al., there are three main weakness patterns that are seen in motor neuron diseases, which are: 
- Asymmetric distal weakness without sensory loss (e.g. ALS, PLS, PMA)
- Symmetric weakness without sensory loss (e.g. SMA, PMA, PLS)
- Symmetric focal midline proximal weakness (neck, trunk, bulbar involvement; e.g. ALS, PBP, PLS)
Lower and upper motor neuron findings
Motor neuron diseases are on a spectrum in terms of upper and lower motor neuron involvement. Some have just lower or upper motor neuron findings, while others have a mix of both. Lower motor neuron (LMN) findings include muscle atrophy and fasciculations, and upper motor neuron (UMN) findings include hyperreflexia, spasticity, muscle spasm, and abnormal reflexes.
Pure upper motor neuron diseases, or those with just UMN findings, include HSP and PLS.
Pure lower motor neuron diseases, or those with just LMN findings, include SMA, spinobulbar muscular atrophy (SBMA), PMA.
Motor neuron diseases with both UMN and LMN findings include both familial and sporadic ALS.
Within primary motor neuron diseases, some are inherited. However, most cases are sporadic and their causes are usually not known.
Autosomal dominant: familial ALS, distal SMA, familial ALS with frontal lobe demential
X-linked: spinal and bulbar muscular atrophy
In sporadic motor neuron diseases, it is thought that environmental, toxic, viral, or genetic factors may be involved.
Associated risk factors
In adults, men are more commonly affected than women.
Differential diagnosis can be challenging due to the number of overlaping symptoms shared between several motor neuron diseases. Frequently, the diagnosis is based on clinical findings (ie. LMN vs. UMN signs and symptoms, patterns of weakness), family history of MND, and a variation of tests, many of which are used to rule out disease mimics, which can manifest with identical symptoms.
Please refer to individual articles for the diagnostic methods used in each individual motor neuron disease.
- Genetic testing: Hereditary MNDs can be diagnosed with high certainty using genetic testing.
- Cerebrospinal fluid (CSF) tests: Analysis of the fluid from around the brain and spinal cord could reveal signs of an infection or inflammation.
- Magnetic resonance imaging (MRI): An MRI of the brain and spinal cord is recommended in patients with UMN signs and symptoms to explore other causes, such as a tumor, inflammation, or lack of blood supply (stroke).
- Electromyogram (EMG) & nerve conduction study (NCS): The EMG, which evaluates muscle function, and NCS, which evaluates nerve function, are performed together in patients with LMN signs.
- For patients with MND affecting the LMNs, the EMG will show evidence of: (1) acute denervation, which is ongoing as motor neurons degenerate, and (2) chronic denervation and reinnervation of the muscle, as the remaining motor neurons attempt to fill in for lost motor neurons .
- By contrast, the NCS in these patients is usually normal. It can show a low compound muscle action potential (CMAP), which results from the loss of motor neurons, but the sensory neurons should remain unaffected.
- Tissue biopsy: Taking a small sample of a muscle or nerve may be necessary if the EMG/NCS is not specific enough to rule out other causes of progressive muscle weakness, but it is rarely used.
There are no known curative treatments for the majority of motor neuron disorders. Please refer to the articles on individual disorders for more details.
The table below lists life expectancy for patients who are diagnosed with MND.
|Type||Median survival time |
from start of symptoms
|Amyotrophic lateral sclerosis (ALS)||2–5 years|
|Primary lateral sclerosis (PLS)||8–10 years|
|Progressive muscular atrophy (PMA)||2–4 years|
|Progressive bulbar palsy (PBP)||6 months – 3 years|
|Spinal muscular atrophy (SMA)||Varies widely by age of onset|
|Spinobulbar muscular atrophy (SBMA)||Normal life span|
|Hereditary spastic paraplegia (HSP)||Normal life span|
- Neilson S, Rose FC. Motor Neurone Disease: The 'at Your Fingertips' Guide. Class Publishing Ltd. p. 2. ISBN 9781859590478. Retrieved 4 August 2016.
- Ince PG, Clark B, Holton J, Revesz T, Wharton SB (2008). "Chapter 13: Diseases of movement and system degenerations". In Greenfield JG, Love S, Louis DN, Ellison DW. Greenfield's neuropathology. 1 (8th ed.). London: Hodder Arnold. p. 947. ISBN 978-0-340-90681-1.
- "Motor Neuron Diseases Fact Sheet: National Institute of Neurological Disorders and Stroke (NINDS)". www.ninds.nih.gov. Archived from the original on 13 April 2014. Retrieved 7 November 2010.
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- Shaw PJ (August 2005). "Molecular and cellular pathways of neurodegeneration in motor neurone disease". Journal of Neurology, Neurosurgery, and Psychiatry. 76 (8): 1046–57. doi:10.1136/jnnp.2004.048652. PMC 1739758. PMID 16024877.
Many doctors use the terms motor neuron disease and ALS interchangeably.
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- "A Look at Upper Motor Neuron Diseases Hereditary Spastic Paraparesis and Primary Lateral Sclerosis" (PDF). sp-foundation.org. Archived from the original (PDF) on 25 March 2017. Retrieved 25 March 2017.
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- "Different types of MND". Irish Motor Neurone Disease Association. Retrieved 2018-12-12.
- D'Amico A, Mercuri E, Tiziano FD, Bertini E (November 2011). "Spinal muscular atrophy". Orphanet Journal of Rare Diseases. 6 (1): 71. doi:10.1186/1750-1172-6-71. PMC 3231874. PMID 22047105.
- "Hereditary Spastic Paraplegia Information Page | National Institute of Neurological Disorders and Stroke". www.ninds.nih.gov. Retrieved 2018-12-12.
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