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MOTOR NEURON DISEASE

 

A motor neuron disease, also spelled motor neurone disease (MND) is any of several neurological disorders that selectively affect motor neurons, the cells that control voluntary muscles of the body. They include amyotrophic lateral sclerosis (ALS), primary lateral sclerosis, progressive muscular atrophy, progressive bulbar palsy, pseudobulbar palsy, and spinal muscular atrophy. They are neurodegenerative in nature and cause increasing disability and eventually, death.

 

Technically the term “motor neuron disease” includes several diseases including amyotrophic lateral sclerosis (ALS), primary lateral sclerosis, progressive muscular atrophy, progressive bulbar palsy, pseudobulbar palsy, and spinal muscular atrophy.

In the United States the term is often used interchangeably with ALS. In the United Kingdom it is spelled “motor neurone disease” (MND) and is the usual term for ALS, and is sometimes used for the group of motor neuron diseases.[

Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig’s disease and motor neurone disease (MND), is a specific disease that causes the death of neurons which control voluntary muscles.Some also use the term “motor neuron disease” for a group of conditions of which ALS is the most common. ALS is characterized by stiff muscles, muscle twitching, and gradually worsening weakness due to muscles decreasing in size. This results in difficulty in speaking, swallowing, and eventually breathing.

The cause is not known in 90% to 95% of cases. About 5–10% of cases are inherited from a person’s parents. About half of these genetic cases are due to one of two specific genes. The diagnosis is based on a person’s signs and symptoms with testing done to rule out other potential causes.

The disorder causes muscle weakness and atrophy throughout the body due to the degeneration of the upper and lower motor neurons. Individuals affected by the disorder may ultimately lose the ability to initiate and control all voluntary movement, although bladder and bowel function and the muscles responsible for eye movement are usually spared until the final stages of the disorder.

Cognitive and/or behavioural dysfunction is present in up to half of individuals with ALS. Around half of people with ALS will experience mild changes in cognition and behaviour, and 10 – 15% will show signs of frontotemporal dementia. Repeating phrases or gestures, apathy, and loss of inhibition are frequently reported behavioural features of ALS. Language dysfunction, executive dysfunction, and troubles with social cognition and verbal memory are the most commonly reported cognitive symptoms in ALS; a meta-analysis found no relationship between dysfunction and disease severity. However, cognitive and behavioral dysfunctions have been found to correlate with reduced survival in people with ALS and increased caregiver burden; this may be due in part to deficits in social cognition.About half the people who have ALS experience emotional lability, in which they cry or laugh for no reason.

Sensory nerves and the autonomic nervous system are generally unaffected, meaning the majority of people with ALS maintain hearing, sight, touch, smell, and taste.[

The start of ALS may be so subtle that the symptoms are overlooked. The earliest symptoms of ALS are muscle weakness and/or muscle atrophy. Other presenting symptoms include trouble swallowing or breathing, cramping, or stiffness of affected muscles; muscle weakness affecting an arm or a leg; and/or slurred and nasal speech. The parts of the body affected by early symptoms of ALS depend on which motor neurons in the body are damaged first.

In limb onset ALS people first experience awkwardness when walking or running or even tripping over or stumbling may be experienced and often this is marked by walking with a “dropped foot” which drags gently on the ground. Or if arm-onset, difficulty with tasks requiring manual dexterity such as buttoning a shirt, writing, or turning a key in a lock may be experienced.

In bulbar-onset ALS, initial symptoms will mainly be of difficulty speaking clearly or swallowing. Speech may become slurred, nasal in character, or quieter. There may be difficulty in swallowing and loss of tongue mobility. A smaller proportion of people experience “respiratory-onset” ALS, where the intercostal muscles that support breathing are affected first.

Over time, people experience increasing difficulty moving, swallowing (dysphagia), and speaking or forming words (dysarthria). Symptoms of upper motor neuron involvement include tight and stiff muscles (spasticity) and exaggerated reflexes (hyperreflexia) including an overactive gag reflex. An abnormal reflex commonly called Babinski’s sign also indicates upper motor neuron damage. Symptoms of lower motor neuron degeneration include muscle weakness and atrophy, muscle cramps, and fleeting twitches of muscles that can be seen under the skin (fasciculations) although twitching is not a diagnostic symptom and more of a side effect so twitching would either occur after or accompany weakness and atrophy

Although the order and rate of symptoms varies from person to person, the disease eventually spreads to unaffected regions and the affected regions become more affected. most people eventually are not able to walk or use their hands and arms, lose the ability to speak and swallow food and their own saliva, and begin to lose the ability to cough and to breathe on their own.

The rate of progression can be measured using an outcome measure called the “ALS Functional Rating Scale Revised (ALSFRS-R)”, a 12-item instrument administered as a clinical interview or self-reported questionnaire that produces a score between 48 (normal function) and 0 (severe disability); it is the most commonly used outcome measure in clinical trials and is used by doctors to track disease progression. Though the degree of variability is high and a small percentage of people have a much slower disorder, on average, people with ALS lose about 0.9 FRS points per month. A survey-based study amongst clinicians showed that they rated a 20% change in the slope of the ALSFRS-R as being clinically meaningful.

Disorder progression tends to be slower in people who are younger than 40 at onset, are mildly obese, have disorder restricted primarily to one limb, and those with primarily upper motor neuron symptoms. Conversely, progression is faster and prognosis poorer in people with bulbar-onset disorder, respiratory-onset disorder, and frontotemporal dementia.

Difficulty in chewing and swallowing makes eating very difficult and increases the risk of choking or of aspirating food into the lungs. In later stages of the disorder, aspiration pneumonia can develop, and maintaining a healthy weight can become a significant problem that may require the insertion of a feeding tube. As the diaphragm and intercostal muscles of the rib cage that support breathing weaken, measures of lung function such as vital capacity and inspiratory pressure diminish. In respiratory-onset ALS, this may occur before significant limb weakness is apparent. Most people with ALS die of respiratory failure or pneumonia

Although respiratory support can ease problems with breathing and prolong survival, it does not affect the progression of ALS. Most people with ALS die between 2 and four years after the diagnosis.] Around half of people with ALS die within 30 months of their symptoms beginning, and about 20% of people with ALS live between 5 years and 10 years after symptoms begin. Guitarist Jason Becker has lived since 1989 with the disorder, while physicist Stephen Hawking has survived for more than 50 years, but they are considered unusual cases.

Most people with ALS die in their own home, with their breath failing while they sleep; people rarely choke to death.

No test can provide a definite diagnosis of ALS, although the presence of upper and lower motor neuron signs in a single limb is strongly suggestive. Instead, the diagnosis of ALS is primarily based on the symptoms and signs the physician observes in the person and a series of tests to rule out other diseases. Physicians obtain the person’s full medical history and usually conduct a neurologic examination at regular intervals to assess whether symptoms such as muscle weakness, atrophy of muscles, hyperreflexia, and spasticity are worsening.

Differential diagnosis

Because symptoms of ALS can be similar to those of a wide variety of other, more treatable diseases or disorders, appropriate tests must be conducted to exclude the possibility of other conditions. One of these tests is electromyography (EMG), a special recording technique that detects electrical activity in muscles. Certain EMG findings can support the diagnosis of ALS. Another common test measures nerve conduction velocity (NCV). Specific abnormalities in the NCV results may suggest, for example, that the person has a form of peripheral neuropathy (damage to peripheral nerves) or myopathy (muscle disease) rather than ALS. While a magnetic resonance imaging (MRI) is often normal in people with early stage ALS, they can reveal evidence of other problems that may be causing the symptoms, such as a spinal cord tumor, multiple sclerosis, a herniated disk in the neck, syringomyelia, or cervical spondylosis.

Based on the person’s symptoms and findings from the examination and from these tests, the physician may order tests on blood and urine samples to eliminate the possibility of other diseases, as well as routine laboratory tests. In some cases, for example, if a physician suspects the person may have a myopathy rather than ALS, a muscle biopsy may be performed.

Viral infectious diseases such as human immunodeficiency virus (HIV), human T-cell leukaemia virus (HTLV), Lyme disease, syphilis and tick-borne encephalitis can in some cases cause ALS-like symptoms. Neurological disorders such as multiple sclerosis, post-polio syndrome, multifocal motor neuropathy, CIDP, spinal muscular atrophy, and spinal and bulbar muscular atrophy can also mimic certain aspects of the disease and should be considered.

ALS must be differentiated from the “ALS mimic syndromes” which are unrelated disorders that may have a similar presentation and clinical features to ALS or its variants.[64] Because of the prognosis carried by this diagnosis and the variety of diseases or disorders that can resemble ALS in the early stages of the disease, people should always obtain a specialist neurological opinion, so alternative diagnoses are clinically ruled out. Benign fasciculation syndrome is another condition that mimics many of the symptoms of ALS, but is accompanied by normal EMG readings and no major disablement

However, most cases of ALS are readily diagnosed and the error rate of diagnosis in large ALS clinics is less than 10%. In one study, 190 people who met the MND/ALS diagnostic criteria, complemented with laboratory research in compliance with both research protocols and regular monitoring. Thirty of these people (16%) had their diagnosis completely changed during the clinical observation development period.In the same study, three people had a false negative diagnosis, myasthenia gravis (MG), an autoimmune disease. MG can mimic ALS and other neurological disorders leading to a delay in diagnosis and treatment. MG is eminently treatable; ALS is not.[68] Myasthenic syndrome, also known as Lambert-Eaton syndrome, can mimic ALS and its initial presentation can be similar to that of MG.