Motor Neuron Disease generally refers to the diagnosis of Amyotrophic Lateral Sclerosis (ALS), or Lou Gehrig's Disease.  In the UK, Motor Neurone Disease refers to both ALS specifically (the most common form of disease) and to the broader spectrum of motor neuron diseases, including Primary Lateral Sclerosis (PLS) and Hereditary Spastic Paraplegia (HSP). In children, MND refers to inherited diseases such as Spinal Muscular Atrophy (SMA).

In order to move a voluntary muscle, usually two motor nerves are involved, an upper motor neuron (UMN), that connects the brain movement areas and the spinal cord, and a lower motor neuron (LMN), that connects the spinal cord with muscles.  Motor Neuron diseases are classified based on the major site of motor neuron abnormality. Common MNDs include Amyotrophic Lateral Sclerosis, which affects both upper and lower motor neurons; Primary lateral sclerosis, a disease of the upper motor neurons; Primary Muscular Atrophy, a disease of the lower motor neurons; and Hereditary Spastic Paraplegia, a disease that generally affects upper motor neurons.

Amyotrophic Lateral Sclerosis (ALS)

Amyotrophic Lateral Sclerosis (ALS) is a rare disease, occurring in about 2 out of 100,000 people.  People with ALS experience progressive weakness in muscles responsible for movement, speaking, swallowing, and breathing.  These symptoms are due to dysfunction and death of motor neurons, disconnecting them from their target muscles.  Muscles that are disconnected from their parent motor neuron become small (“atrophy”) and exhibit spontaneous twitching (“fasciculations”).  People with ALS also may develop stiffness, called “spasticity”, which is also do to the death of motor neurons. Sensory functions – such as sight, smell and taste – remain intact, and significant pain or numbness are not typical.  About 25% of people with ALS will also develop abnormalities in cognition and behavior termed dementia.

The current treatment of ALS is focused on managing symptoms and maintaining strength and the best possible quality of life, as there is no cure for the disease.

In the United States, approximately 5,000 people every year will be diagnosed with ALS, and about 25,000 people currently live with the disease.  The average age of ALS onset is approximately 55 years, although the distribution is broad enough that ALS can be seen in teenagers and adults 80 years and older.

Most cases of ALS occur in the absence of a family history of the disease (sporadic ALS), though five to ten percent of patients inherit the disease (this is known as familial ALS). In most cases of familial ALS, the disease is passed on in an “autosomal dominant pattern;” this means that you only need to get the abnormal gene from one parent in order for you to inherit the disease.  However, having only a parent with the disease, and no other family members, does not necessarily increase the risk of disease in a child.  In families with ALS, specific disease causing gene mutations have been identified, and can be tested for in a simple blood test.  The most common gene mutations causing ALS are in the genes C9ORF72 and superoxide dismutase 1 (SOD 1).  How these mutations cause disease is the focus of intense research.   

Primary Lateral Sclerosis (PLS)

Primary Lateral Sclerosis (PLS) is a rare form of motor neuron disease affecting the upper motor neurons only, resulting in increasing muscles stiffness (“spasticity”) and weakness.   PLS progresses more slowly than ALS, and unlike ALS, PLS does not cause muscles wasting, as spinal motor neurons or lower motor neurons stay intact.  However, some people who initially appear to have PLS will, with time, develop weakness and muscle loss, transforming the diagnosis to ALS.

PLS does not usually run in families and the age of onset is generally between 35 and 66 years of age.  The treatment of PLS is focused on symptom relief, as nothing is available at this time to prevent, slow, stop, or reverse the disease.

Hereditary Spastic Paraplegia (HSP)

Hereditary Spastic Paraplegia (HSP) is a group of rare, inherited neurological disorders whose primary symptoms are progressive muscle weakness and increased muscle stiffness (spasticity) that usually starts in the legs.  Approximately 30 different types of HSP are defined by different gene mutations, and it is estimated that there are 10,000-20,000 people in the United States living with the disorder.

HSP is identified by difficulty walking due to increasingly weak and stiff muscles. Initial symptoms may include difficulty with balance, and, as the disease progresses, canes, walkers, or wheelchairs may be required. Other common symptoms of HSP are hyperactive tendon reflexes, involuntary muscular contractions and relaxations (known as clonus), and congenital foot problems such as pes cavus (high arched foot). While HSP usually runs in families, even within the same family the severity of symptoms and the exact age of symptom onset can differ. Treatment of HSP is focused on symptom relief, as no treatments are available at this time to prevent, stop, slow, or reverse the disease.