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Neurodegenerative
disorders |
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The neurodegenerative diseases are common
disorders in which there is degeneration of specific groups of neurons or brain
areas, causing characteristic clinical syndromes.
Motor neuron disease (amyotrophic lateral sclerosis) causes paralysis, due to
death of motor neurons in the motor cortex, brain stem and spinal cord}.
Motor neuron disease is a progressive neurodegenerative disease. It is mainly
seen in old age, a small number of patients presenting in middle age.
Disease
typically begins as mild weakness in one limb. There is then progression to
severe paralysis, with loss of swallowing and respiration leading to death in
2-3 years. The cause of the disease is unknown.
Different sub-types relate to the loss of different groups of motor neurons:
• Amyotrophic lateral sclerosis is the most common pattern, showing loss both of
cortical motor neurons and lower motor neurons in the spinal cord and brain
stem.
• Progressive bulbar palsy is common, shows loss of brain stem motor neurons
• Progressive muscular atrophy is uncommon and shows loss restricted to spinal
lower motor neurons.
• Primary lateral sclerosis is rare and shows loss of neurons restricted to the
motor cortex.
There is loss of motor neurons from cortex, brain stem and cord, and gliosis
with secondary degeneration of motor tracts. Inclusion bodies containing the
protein ubiquitin are found in surviving neurons.
Parkinson's disease results from loss of neurons from the substantia nigra.
Parkinson's disease is a movement disorder that mainly affects patients over the
age of 45 years. It is clinically characterized by disturbance of movement with
rigidity, slowness of voluntary movement (bradykinesis), and rest tremor.
Severity of disease is related to loss of the neuromelanin-containing nerve
cells from the substantia nigra in the midbrain; these cells normally produce
dopamine, their loss reducing the amount of dopamine in the basal ganglia.
Macroscopically, there is loss of pigment from the substantia nigra, which is
the result of death of the melanin-containing dopaminergic cells. The surviving
cells in the substantia nigra contain spherical inclusions termed Lewy bodies.
The cause of Parkinson's disease is unknown. The disease may be symptomatically
treated by administration of drugs that correct the neurotransmitter imbalance,
e.g. l-dopa. The natural history of the disease is for patients to develop
failure of response to treatment, with death eventually occurring from wasting
and poor nutritional intake.
In this section through the midbrain, pigmented neurons have been lost from the
substantia nigra (between lines), which is abnormally pale.
Lewy bodies are spherical inclusions seen in the melanin-containing neurons in
Parkinson's disease. Typically they have a hyaline core and a pale halo. They
are based on aggregated neurofilaments.
Huntington's disease is an autosomal dominant disease causing chorea and
dementia
Huntington's disease is a neurodegenerative disease causing choreiform movements
and dementia, with onset in middle life. The disease is an autosomal dominant
disorder with a prevalence of approximately 1 in 20,000; the gene has been
characterized at its location in the short arm of chromosome 4.
Macroscopically the brain shows atrophy of the caudate and putamen due to cell
loss and gliosis, and careful measurements have shown subtle loss of neurons
from the cerebral cortex.
The natural history of this disease is for affected individuals to develop
progressive cognitive decline, with increase in severity of the movement
disorder. Patients die as a result of their severe mental and physical
incapacity.
Alzheimer's disease is the most common cause of dementia
Alzheimer's disease is the most common neurodegenerative disease and the most
common cause of dementia.
The brain in Alzheimer's disease is smaller than normal and brain weight is
reduced, evident as shrinkage of gyri and widening of sulci of the cerebral
hemispheres.
Histologically, there are several main abnormalities seen in Alzheimer's
disease. Amyloid, composed of b(A4) protein, is deposited in the cerebral cortex
as spherical deposits termed senile plaques. Intraneuronal inclusions comprising
bundles of abnormal filaments termed {\B neurofibrillary tangles} develop in
cortical neurons.
Tangles are frequently flame-shaped and occupy much of the space within the
neuronal cytoplasm, being composed of a microtubule-binding protein called Tau
protein. Cortical nerve-cell processes become twisted and dilated (neuropil
threads) due to accumulation of the same filaments that form the tangles.
Neuronal loss ( 50%) is seen from the cortex, particularly in patients under the
age of 80 years. The amyloid is often also deposited in cerebral arteries,
causing an amyloid angiopathy.
Neuronal system atrophies are neurodegenerative diseases affecting several
neuronal groups
A number of uncommon neurodegenerative diseases show loss of neurons from
several nuclei in the brain, brain stem, cerebellum and spinal cord, and are
given names according to the pattern of involvement. There are large numbers of
such syndromes, each having different genetic and pathological causes.
Clinically, patients have symptoms and signs that relate to the neuronal groups
involved, including rigidity, chorea, cognitive decline and weakness.
Among the main types of multisystem atrophy are spinocerebellar degenerations, a
group of familial diseases with several inheritance patterns, which are
characterized by loss of cerebellar cortical neurons and degeneration of spinal
cord tracts. The best known of these is Friedreich's ataxia, a spastic
cerebellar ataxia with degeneration of posterior columns and corticospinal and
spinocerebellar tracts.
Cerebellar cortical atrophies are familial diseases in which the main
abnormality is loss of Purkinje cells from the cerebellar cortex, with lesser
involvement of other neuronal groups. Patients present with cerebellar ataxia,
usually in the third decade.
Olivopontocerebellar atrophies are familial diseases, many of which are
autosomal dominant. Patients have prominent cerebellar ataxia, extrapyramidal
rigidity and bulbar paresis, caused by loss of neurons from the cerebellar
cortex, substantis nigra and pons respectively. In many cases, inclusions are
seen in glial cells.
Idiopathic orthostatic hypotension (Shy-Drager syndrome) is a disorder dominated
by the presence of orthostatic hypotension. This may occur in isolation, or,
more usually, as part of a multiple-system atrophy involving the substantia
nigra, motor neurons and cerebellum. The basis of hypotension is loss of neurons
from the intermediolateral column in the thoracic spinal cord.
In cerebellar degeneration there is atrophy of the cerebellar folia. This case
was a familial cerebellar degeneration causing a severe ataxia. |
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