Motor neuron disease


Motor neuron diseases or motor neurone diseases are a group of rare neurodegenerative disorders that selectively affect motor neurons, the cells which control voluntary muscles of the body. They include amyotrophic lateral sclerosis, progressive bulbar palsy, pseudobulbar palsy, progressive muscular atrophy, primary lateral sclerosis, and monomelic amyotrophy, as well as some rarer variants resembling ALS.
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 of these 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 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. Most of these diseases worsen over time; while some, such as ALS, shorten one's life expectancy, others do not. Currently, there are no approved treatments for the majority of motor neuron disorders, and care is mostly symptomatic.

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 have difficulty breathing with climbing stairs, difficulty breathing when lying down, or even respiratory failure if breathing muscles become involved. Bulbar symptoms, including difficulty speaking, difficulty swallowing, and excessive saliva production, can also occur. Sensation, or the ability to feel, is typically not affected. Emotional disturbance and cognitive and behavioural changes are also seen. There can be lower motor neuron findings, upper motor neuron findings, 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, while others are not.

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:
  1. Asymmetric distal weakness without sensory loss
  2. Symmetric weakness without sensory loss
  3. Symmetric focal midline proximal weakness

    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 findings include muscle atrophy and fasciculations, and upper motor neuron findings include hyperreflexia, spasticity, muscle spasm, and abnormal reflexes.
Pure upper motor neuron diseases, or those with just UMN findings, include PLS.
Pure lower motor neuron diseases, or those with just LMN findings, include PMA.
Motor neuron diseases with both UMN and LMN findings include both familial and sporadic ALS.

Causes

Most cases are sporadic and their causes are usually not known. It is thought that environmental, toxic, viral, or genetic factors may be involved.

DNA damage

, also referred to as TDP-43, is a critical component of the non-homologous end joining enzymatic pathway that repairs DNA double-strand breaks in pluripotent stem cell-derived motor neurons. TDP-43 is rapidly recruited to double-strand breaks where it acts as a scaffold for the recruitment of the XRCC4-DNA ligase protein complex that then acts to repair double-strand breaks. About 95% of ALS patients have abnormalities in the nucleus-cytoplasmic localization in spinal motor neurons of TDP43. In TDP-43 depleted human neural stem cell-derived motor neurons, as well as in sporadic ALS patients’ spinal cord specimens there is significant double-strand break accumulation and reduced levels of NHEJ.

Associated risk factors

In adults, men are more commonly affected than women.

Diagnosis

Differential diagnosis can be challenging due to the number of overlapping symptoms, shared between several motor neuron diseases. Frequently, the diagnosis is based on clinical findings, 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.

Classification

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:
  1. Is the disease sporadic or inherited?
  2. Is there involvement of the upper motor neurons, the lower motor neurons, 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 or can have a genetic cause with the same clinical symptoms and progression of disease.
UMNs are motor neurons that project from the cortex down to the brainstem or spinal cord. LMNs originate in the anterior horns of the spinal cord and synapse on peripheral muscles. Both motor neurons are necessary for the strong contraction of a muscle, but damage to an UMN can be distinguished from damage to a LMN by physical exam.

Tests

There are no known curative treatments for the majority of motor neuron disorders. Please refer to the articles on individual disorders for more details.

Prognosis

The table below lists life expectancy for patients who are diagnosed with MND. Please refer to individual articles for more detail.
TypeMedian survival time
from start of symptoms
Amyotrophic lateral sclerosis 2–5 years
Primary lateral sclerosis 8–10 years
Progressive muscular atrophy 2–4 years
Progressive bulbar palsy 6 months – 3 years
Pseudobulbar palsyNo change in survival

Terminology

In the United States, the term motor neuron disease is often used to denote amyotrophic lateral sclerosis, the most common disorder in the group. In the United Kingdom, the term is spelled motor neurone disease and is frequently 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 the diseases belonging to the spinal muscular atrophies group. However, they are not classified as "motor neuron diseases" by the 11th edition of the International Statistical Classification of Diseases and Related Health Problems, which is the definition followed in this article.