Generality
Motor Neuron Diseases is the generic name given to a group of neurodegenerative conditions that specifically affect the good health and functioning of motor neurons.
Motor neurons are nerve cells responsible for controlling voluntary muscles, glands, parts of the heart and smooth muscles.
The best known motor neuron diseases are: ALS, primary lateral sclerosis, progressive muscle atrophy, progressive bulbar palsy and pseudobulbar palsy.
Currently, doctors do not know the exact causes of all these pathologies.
The list of symptoms of a motor neuron disease is highly variable and depends on what tasks the motor neurons involved perform. For example, in ALS, motor neuron involvement is total, therefore, the patient complains of difficulties in various areas, from walking to from speech to breathing and chewing ability.
Diagnosis of motor neuron disease requires several tests, as there is no specific diagnostic test.
For the moment, there are no specific treatments, only symptomatic treatments.
What is motor neuron disease?
Motor neuron disease is the medical term for any progressive degenerative neurological condition that specifically affects nerve cells known as motor neurons or motor neurons.
In medicine, a progressive degenerative neurological condition, such as a general neuronal disease, is called a neurodegenerative disease.
WHAT ARE AND WHAT FUNCTIONS DO MOTONEURONS PERFORM?
Originating in the brain or spinal cord, motor neurons are the special nerve cells that, with their network of axons, control voluntary muscles, glands, heart and smooth muscles.
The human nervous system comprises two classes of motor neurons: upper motor neurons (or first motor neurons) and lower motor neurons (or second motor neurons).
These two classes of motor neurons are comparable to two runners participating in a relay race in pairs. In fact, the upper motor neurons represent the first athlete (i.e. the one who starts the race with the baton), as they are the processing and starting points of the nerve signals intended for the voluntary skeletal muscles, glands, etc. The lower motor neurons, on the other hand, represent the second athlete (i.e. the one who, having received the baton from the first athlete, completes the race), as they are the reception and delivery centers of nerve signals coming from the upper motor neurons.
In essence, therefore, the upper motor neurons are the creators of those nerve signals, whose final delivery to the voluntary muscles, glands, etc. it is up to the lower motor neurons.
Recovering the comparison with the two relay runners, the point of delivery of the baton between the upper motor neurons and the lower motor neurons is the spinal cord.
Important functions depend on the good health of human motor neurons, such as breathing, walking, swallowing, speaking, holding objects, the release of glandular secretions, etc.
Types
Under the heading “motor neuron disease” there are various pathologies, all clearly associated with the “involvement of motor neurons.
Pathologies belonging to the group of motor neuron diseases include:
- Amyotrophic lateral sclerosis, better known as ALS. It is the most common and famous motor neuron disease;
- Primary lateral sclerosis, also known by the acronym SLP;
- Progressive Muscle Atrophy, or AMP;
- Progressive bulbar palsy, also known as PBP;
- Pseudobulbar paralysis;
- Spinal muscular atrophy, also called by the acronym SMA. Its inclusion in the list of motor neuron diseases has always been uncertain and a reason for continuous debates by the medical-scientific community.
Of these types of motor neuron diseases, some are more serious than others.
By far, the most serious motor neuron disease is ALS.
Table. Epidemiology of some types of motor neuron diseases.
IMPORTANT CLARIFICATION ON THE TERMINOLOGY "MOTONEURON DISEASE"
In medicine, the term “motor neuron disease” can be used as the name of a group of neurodegenerative diseases - which are those indicated above - or as one of the many synonyms of ALS. There is no "more correct" meaning than another; however, in this article, the first option was chosen.
Causes
Motor neuron disease arises as a result of degeneration and atrophy of motor neurons. The degeneration and atrophy of motor neurons progressively affects their state of health and their ability to control voluntary muscles, glands, etc.
Today, despite numerous researches on the subject, the causes of the processes of degeneration and atrophy of motor neurons (therefore the causes of a generic motor neuron disease) remain a mystery, except for the pathological forms of hereditary genetic character (eg: SMA, some subtypes of ALS etc.).
SLA
In ALS, motor neuron degeneration and atrophy affect both upper and lower motor neurons.
PRIMARY LATERAL SCLEROSIS
In primary lateral sclerosis, motor neuron degeneration and atrophy affect only the upper motor neurons.
PROGRESSIVE MUSCULAR ATROPHY
In progressive muscular sclerosis, the process of degeneration and atrophy of the motor neurons affects only the lower motor neurons.
PROGRESSIVE BULBAR PARALYSIS
In progressive bulbar palsy, most scientific evidence shows that motor neuron degeneration and atrophy predominantly affect the lower motor neurons.
PSEUDOBULBAR PARALYSIS
In pseudobulbar palsy, the process of degeneration and atrophy of the motor neurons concerns the upper motor neurons of the brainstem, which are part of the so-called corticobulbar tract.
SMA
In SMA, motor neuron degeneration and atrophy target some lower motor neurons.
Symptoms, signs and complications
Possible symptoms and signs of motor neuron disease include:
- Sense of weakness and compliance scattered in various parts of the body (eg: trunk, arms, hand, jaw, legs, feet, hips, etc.);
- Problems with chewing and swallowing, and spasticity of the tongue;
- Problems with walking, problems with balance and presence of spasticity of the limbs;
- Tremors in the arms
- Inability to grip with the hands;
- Recurrent muscle cramps and fasciculations
- Difficulty keeping your head up
- Difficulty maintaining an upright posture
- Slurred speech (dysarthria)
- Loss of reflexes
- Respiratory difficulties.
For the avoidance of doubt, it is important to specify that the extent of the symptomatological picture depends on the type of motor neuron disease present. For example, ALS includes all the clinical manifestations listed above, while pseudobulbar paralysis includes only the clinical manifestations that have as their object the ability to swallow, the ability to chew, tongue and speech.
COMPLICATIONS
As a result of a progressive and inevitable degeneration of motor neurons, any motor neuron disease involves, in more or less long times, the onset of serious complications, some of which have a fatal outcome.
The complications in question correspond, in fact, to a marked worsening of the various symptoms mentioned above.
Therefore, walking difficulties turn into a "complete inability" to use the leg (patients in this state, in fact, need a wheelchair); dysarthria becomes complete lack of speech (some patients use special electronic devices to communicate); chewing and swallowing problems worsen to the point that the patient needs medical support for artificial nutrition; breathing difficulties become total inability breathing (patients need continuous spontaneous breathing support); and so on.
Each motor neuron disease has very different times and modalities of symptomatology evolution.
For example, in the case of ALS, the final stage of the disease occurs 1-3 years after the onset of the first symptoms; in progressive bulbar paralysis, already after 6 months; in primary lateral sclerosis, after one or two tens of years; in AMP, after 2-4 years; etc.
MAIN CAUSE OF DEATH
In most patients with life-threatening neuronal disease (eg, ALS or progressive bulbar palsy), death occurs from severe respiratory failure or severe pneumonia.
Diagnosis
Currently, there is no highly specific diagnostic test, which makes it possible to identify with certainty a given motor neuron disease.
Therefore, to diagnose motor neuron disease, doctors have to resort to different tests, some of which are also aimed at excluding pathologies with similar symptoms (differential diagnosis).
Tests for diagnosing motor neuron disease include:
- The physical examination;
- Electromyography and nerve conduction velocity tests;
- Laboratory tests on blood and urine;
- MRI and / or CT scan of the brain and spinal cord (for differential diagnosis);
- Muscle biopsy (for differential diagnosis);
- Lumbar puncture (for differential diagnosis).
Therapy
At the present time, any motor neuron disease is an incurable condition, in the sense that there are neither treatments capable of stopping the progressive degeneration of motor neurons nor, much less, treatments capable of healing degenerated motor neurons.
The only treatments that people with motor neuron disease can rely on are symptomatic-type therapies (or symptomatic therapies). As it can be understood, symptomatic therapy is a therapy aimed at relieving one or a series of symptoms, in order to improve the patient's quality of life.
Symptomatic therapies for motor neuron diseases vary according to the symptoms present: for example, if a motor neuron disease impairs walking and chewing and swallowing skills, the patient will need support to walk or move (e.g. crutches or chair on wheels) and nutrition support (e.g. artificial nutrition)
The effectiveness of symptomatic therapies depends on the severity of the motor neuron disease present: there are motor neuron diseases whose symptoms are more treatable than in other cases.
Prognosis
Motor neuron diseases cause irreversible and incurable damage to the nervous system, severely affect the quality of life of patients and, in certain forms, are lethal.
All this explains why, in the presence of a generic motor neuron disease, doctors usually outline an unfavorable prognosis.
Some exceptional cases see the patient survive for about ten years.
A minority of patients live with the disease for about twenty years.
