COMMON TYPES OF NEURO DISEASES - PART 3 - DEMYELINATING DISEASES

In continuation on our series on Neurological Disease, here is the next segment on Demyelinating Diseases.

Demyelinating diseases are uncommon conditions in which the myelin sheath is attacked, resulting in neurodegeneration and eventual disability. The myelin sheath surrounds axons in the brain and peripheral nervous system. Axons are nerve cells that extend neurons. While neurons coordinate sensory and motor information in the brain, the axons deliver that information to where it needs to go; either between neurons or outside the brain to organs and tissues within the body.

The myelin sheath is a very important part of axonal function as it coordinates optimal electrical conductance. It ensures that neuro-impulses are transmitted at such rapid speeds that our perceptions and movements appear instantaneous. Myelin conductance is known as saltatory conduction and is a lot more efficient than normal neuronal transmission. The myelin sheath also acts as an insulator for the large amounts of electrical energy that nerves transmit; making sure nothing is lost in the process.

Some demyelinating diseases attack the peripheral nervous system while others attack the central nervous system. The myelin sheath in either systems are not identical, being governed by distinctly unique neural cells. Oligodendrocytes (a type of nerve cell known as a glial cell) reside in the CNS and have many arm-like appendages that wrap around multiple axons within the brain. These wrappings form the myelin sheath and these cells constantly ensure that the myelin sheath remains salient. Schwann cells reside in the peripheral nervous system and are simpler than oligodendrocytes in structure, with individual cells forming a single wrapping of myelin across the axon segments they cover.

Types of Demyelinating Disorders

Demyelinating diseases are very rare occurrences, with an average incidence of less than 0.01% amongst the population. The most common demyelinating disorders occur within the central nervous system, destroying myelin in the brain and spinal cord. Smaller percentages occur within the peripheral nervous system.

Multiple Sclerosis

Multiple sclerosis (MS) is perhaps one of the most common demyelinating diseases to affect young people, aged between 20 and 40 years on average. It is an autoimmune condition in which the myelin sheath is attacked by faulty immune cells, which infiltrate the brain due to a weakening of the blood-brain barrier. Sites of demyelination show tiny lesions with an ischemic component. Damaged axons eventually become unable to re-myelinate. Brain inflammation and vascular abnormalities also contribute towards the demyelination, which may be episodic, progressive or both.

Neurological symptoms of MS are often unique to the individual, depending on the brain areas being demyelinated. Common sites include the corpus callosum (connecting left and right brains), temporal lobes, occipital lobe, prefrontal cortex and the spinal cord[1]. The demyelination, whether slow (progressive) or spontaneous (episodic) tends to cause severe physical disability after a decade or two and patients often become confined to a wheelchair or suffer severe cognitive deficit.

Amyotrophic Sclerosis

Amyotrophic sclerosis is another demyelinating disease with an entirely unknown cause. Sometimes this condition is referred to as “Lou Gehrig’s Disease,” after a famous baseball player who contracted the disease. It is speculated that multiple unique factors can cause the condition, including autoimmune attack of the myelin, genetic mutations, protein aggregation (as seen in frontotemporal dementia), excessive excitotoxicity and more. The end result is neurodegeneration of axons, glial cells and the myelin sheath.

Those with amyotrophic sclerosis suffer severe symptoms of motor neuron degeneration, including muscle wasting, spasticity, hyper-reflexivity, and chronic weakness. It typically begins in one limb, which becomes weak, flails uncontrollably and starts to waste, amongst other symptoms. Eventually it spreads neatly throughout the body over time until the patient is completely disabled.[2]

Other Primary Demyelination Diseases

All demyelinating diseases tend to result in physical, cognitive or visual disability. Each reveals a different pattern of demyelination within the nervous system when checked on a brain scan. Symptoms are fairly similar when the same regions of the brain are demyelinated.

Examples of other primary demyelinating diseases of the central nervous system include:

• Acute Disseminated Encephalomyelitis (ADEM)
• Neuromyelitis Optica Spectrum Disorders
• Balo’s Concentric Sclerosis

Peripheral demyelination diseases include[3]:

• Guillain-Barre Syndrome
• Anti-Myelin Associated Glycoprotein (MAG) Neuropathy
• POEMS Syndrome (Polyneuropathy, Organomegaly, Endocrinopathy, M protein, and Skin changes)
• Charcot Marie Tooth disease

Secondary Demyelinating Disorders

Demyelination may be a secondary condition resulting from another completely unique primary disease. The following conditions can give rise to demyelination and constitute secondary demyelination disorders:

• Ischemic attacks[4]
• Heavy metal toxicity
• Neurotropic viral infections such as Lyme Disease and Syphilis
• Metabolic diseases that impact re-myelination, such as non-alcoholic fatty liver disease, hyperlipidemia and hypo- or hyper-cholesterolemia
• Autoimmune conditions
• Cardiovascular diseases

Symptoms

Symptoms of demyelination vary from condition to condition and ultimately depend on the sites in the brain or periphery that are being affected. Common symptoms include[5]:

• Lack of physical coordination and balance
• Reduced cognitive functions
• Limb spasticity
• Slowness in thinking and movements
• Headaches
• Fevers
• Sleep disturbances
• Emotional volatility
• Visual disturbances, such as blurred eyesight and eventual loss of sight
• Muscle wasting
• Accumulative disability
• Pain
• Cardiovascular issues such as arrhythmia and blood pressure irregularities
• Numbness
• Chronic fatigue
• Dizziness

Risk Factors

Risk factors for demyelinating diseases include:

• Dyslipidemia
• Autoimmunity
• HIV/AIDS
• Liver disease
• Alcoholism
• Excessive radiation exposure or injury
• Hypogonadism or low hormonal status
• Electrolyte imbalances
• Ischemia or oxygen loss
• Low nutritional status

Treatment Options

There are very limited treatment options for those with demyelinating diseases. If an autoimmune condition is suspected, as in multiple sclerosis, therapies may be employed that promote balanced or suppressed immune function such as interferon-beta infusions and chemotherapy. If a viral etiology is indicated, antiviral treatment is offered.

Future research aims to dissect potential underlying causes in order to treat demyelination diseases, with a focus on treating autoimmunity, warped lipid status, cellular metabolism and neurodegeneration.

Source:

• [1] https://www.ncbi.nlm.nih.gov/books/NBK554329/
• [2] https://www.ncbi.nlm.nih.gov/books/NBK556151/
• [3] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6433847/
• [4] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4581889/
• [5] https://www.healthline.com/health/multiple-sclerosis/demyelination#ms-and-demyelination

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