Diseases of the Central Nervous System Flashcards
NERVOUS SYSTEM INFECTIONS
Bacteria
Viruses
Fungi and Parasites
Prions
Bacteria - NERVOUS SYSTEM INFECTIONS
The most common bacterial disease is meningitis, meaning inflammation of the leptomeninges (arachnoid and pia).
- It can be produced by a variety of bacteria (e.g. meningococcus).
Clinical manifestations include fever, stiff neck, and clouding of consciousness.
Diagnosis is confirmed by examination of a sample of cerebrospinal fluid (CSF) obtained through a lumbar puncture.
- The CSF will show numerous inflammatory cells (polymorphonuclear leukocytes), increased protein and reduced glucose.
In developed countries, the incidence of certain pathogens has changed dramatically because of vaccination programs (esp. H flu, meningococcus).
Bacterial abscesses can also occur in the brain, where an infectious focus reaches parenchyma and is gradually walled off by fibrovascular granulation.
- The most common sources are cardiac (endocarditis, AV defects) and pulmonary (chronic infections) and common organisms are Strep and Staph species.
- Microscopically there is central necrosis bordered in turn by granulation tissue, a fibrous capsule and finally gliotic brain.
Viruses - NERVOUS SYSTEM INFECTIONS
Can cause meningitis, or encephalitis (inflammation of the brain parenchyma).
Viruses are often highly selective in the type of neuron and area of involvement (polio infects motor neurons, herpes infects sensory neurons).
HIV would be the most common cause of encephalitis globally.
Fungi and parasites - NERVOUS SYSTEM INFECTIONS
These organisms may produce meningitis, encephalitis or abscess.
They are more common in developing and tropical countries but are also an important cause of disease in immunosuppressed individuals (e.g. corticosteroids/cyclosporin, chemotherapy, AIDS).
Prions - NERVOUS SYSTEM INFECTIONS
Infectious variants of normal proteins causing Creutzfeldt-Jakob disease and bovine spongiform encephalopathy (mad cow disease).
The agents appear to consist exclusively of protein, and cause disease not by replication but by inducing a change in the conformation of a normal protein produced by the host.
Prions are resistant to many disinfection procedures, but can be destroyed by bleach or autoclaving.
The disease is characterized by a rapidly progressive dementia (months), and pathologically by the development of innumerable small vacuoles in grey matter, referred to as spongiform change.
INFANTILE: PERINATAL & CONGENITAL NERVOUS SYSTEM DISEASES
Perinatal Disease
Congenital diseases
- Prenatal injuries
- Malformations
Perinatal Disease
Hypoxia and acidosis around the time of birth may result in lesions in the basal ganglia and hemispheric white matter that give rise to the syndrome of cerebral palsy, i.e.: a non progressive syndrome of weakness and stiffness (spasticity) of the extremities, often accompanied by abnormal involuntary movements, with or without mental retardation.
Severely premature infants (2 or more months premature) are particularly at risk.
Prenatal injuries
Cerebral palsy and mental retardation can also be the result of injuries sustained in utero (nutritional, vascular, infectious, toxic).
Malformations
Congenital malformations of the CNS indicate abnormal development.
Some congenital malformations are known to be inherited or result from chromosomal abnormalities; the cause of the majority is unknown.
There is a large variety of malformations, ranging from anencephaly (absent cerebrum) to minor disorganization of the cerebral cortex.
A common malformation (myelomeningocele) involves the lumbar spine and cord.
- This malformation may result in motor deficits in the legs and incontinence.
TUMOURS
Tumours present clinically either by their focal effects, or by increasing intracranial pressure.
The focal effects of the tumour can produce a functional deficit (paralysis, visual field defect, etc.), or seizures.
Increased intracranial pressure is manifested by headaches (worse in the morning, aggravated by bending, coughing, or straining), vomiting, clouding of consciousness, ataxia and incontinence.
TYPES OF TUMOURS AFFECTING THE CNS
Primary, intrinsic
Primary extrinsic
Secondary
Primary, intrinsic tumours
generally derived from glial cells, (i.e. gliomas) subdivided into astrocytomas, oligodendrogliomas, and ependymomas.
Because they tend to infiltrate the surrounding normal nervous tissue, complete surgical resection is not usually possible.
The average life expectancy with the most malignant astrocytoma (grade 4) is less than 1 year.
Low grade gliomas are slow growing and may be survived for many years, even decades.
A recent advance in the classification and prognostication of primary brain tumours (and hopefully in the eventual development of more effective treatments) comes in the form of the molecular characterization of the tumour.
Primary extrinsic tumours
i.e. meningiomas, schwannomas, pituitary adenomas
These tumours compress the brain without invading it and are often resectable (and therefore curable).
Primary CNS lymphomas are a brain tumour in search of a category.
Like peripheral lymphomas, these are mostly (90%) of B cell lineage.
The latter tend to be multifocal, periventricular, arising in immunocompromised hosts and carrying a poor prognosis.
Microscopically, their perivascular and angioinvasive growth pattern is characteristic.
Secondary tumours
i.e. metastatic.
Most commonly from lung, breast, bowel or melanoma primary sources.
Unlike gliomas, metastases are well-defined from the adjacent brain tissue.
They tend to land at the junction of cortex and white matter and grow in a spherical shape.
They will often be surrounded by significant edema.
GROSS ANATOMY of the brain
The central nervous system (CNS), the brain and spinal cord, is wrapped in meninges and encased in bone (skull and vertebrae).
- The CNS contains grey matter, enriched with the cell bodies of nerve cells, and white matter, consisting mostly of myelinated axons of neurons.
- The peripheral nervous system (PNS) begins where the CNS ends and is largely made up of the nerves carrying impulses between the CNS and the rest of the body.
The brain consists of two cerebral hemispheres, two cerebellar hemispheres, and the brain stem.
- The brain stem, contiguous with the spinal cord, the cerebellum, and the cerebral hemispheres acts as a conduit of impulses between all these structures.
- In addition, it houses the cells that are the origin of the cranial nerves (innervating the head and neck), and structures responsible for the maintenance of consciousness and vegetative functions (the reticular activating system).
- The cerebellum is involved in coordination and equilibrium.
The cerebral hemispheres are covered by the cortex, divided by sulci into many gyri.
- Specific functions are associated with certain areas of the cortex.
- For example, vision is located in the occipital lobes.
- Deep to the cortex is an expanse of white matter, formed by the fibers conducting impulses within the CNS.
- Additional pairs of grey matter structures or nuclei are present at the base of the cerebrum, near the midline: the basal ganglia and thalami.
A wide range of brain weights (approximately 1250 to 1800g for males) allow normal intellectual function.
- Female brains are, on average 100-150g lighter (which leads me to conclude that the female brain is more efficient!).
The spinal cord has a diameter of approximately 1 cm.
- It contains neurons that control muscle and viscera.
- In addition, the cord serves as the conduit of motor and sensory impulses.
The brain and cord are wrapped in three membranes, collectively called the meninges.
- Outermost is the tough dura.
- The delicate inner membranes are the arachnoid and the pia respectively. The latter is in contact with the surface of the brain.
One fluid-filled cavity (lateral ventricle) lies at the depth of each cerebral hemisphere.
- Cerebrospinal fluid (CSF) produced there by the choroid plexus flows to the midline third and then fourth ventricles.
- CSF exits the fourth ventricle by small openings (foramina) to enter the subarachnoid space which covers the entire CNS.
- The subarachnoid space lies between the arachnoid and pia.
- CSF in the subarachnoid space is reabsorbed into the blood stream across arachnoid granulations which penetrate into venous channels.
- Any obstruction to the circulation of the CSF, whether congenital or acquired, results in hydrocephalus, that is, increased size of the ventricles.
How neurons work
Neurons and glia are the two principle cell populations in the CNS.
Neurons consist of a body (or soma), containing the nucleus and protein synthetic machinery and multiple processes (cytoplasmic extensions), that can be extraordinarily long (over 1 m).
- Most of the processes, called dendrites, collect impulses, whereas a single one, the axon, conducts impulses away from the neuron.
- The electrical impulse conducted by the axon results in the release of one of a variety of substances (neurotransmitters) at the point where the axon contacts a neuron, muscle, or other end organ.
- The specialized structure found at this point is called a synapse.
- The receiving neuron (post-synaptic) in turn develops (or is inhibited from developing) an electrical impulse in response to the release of neurotransmitter from the presynaptic terminal.
How neurons respond to injury
Neurons may respond to injury in a limited number of ways.
Acute neuronal injury is most commonly seen in association with hypoxic/ischemic insults, less common appearances include chromatolysis (swelling of neuronal cytoplasm especially in response to axonal injury) and by developing inclusions (cytoplasmic or nuclear) the latter in certain degenerative and infectious diseases.
