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Chapter: 11th Microbiology : Medical Microbiology

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Infections of the Nervous System

Some of the most devastating infectious diseases are those that affect the nervous system, especially the brain and the spinal cord.

Infections of the Nervous System

 

Some of the most devastating infectious diseases are those that affect the nervous system, especially the brain and the spinal cord. Damage to these areas can lead to deafness, blindness, learning disabilities, paralysis and death. Microbial infections of CNS are infrequent but often have serious consequences. In pre antibiotic times, they were almost always fatal. An infection of CNS can be life threatening condition, especially for children with weakened immune system. These infections need quick diagnosis and immediate treatment by an infectious disease specialist. Bacteria, Fungi and viruses are the most common causes of CNS infections.

 

Structure of Nervous System

The human nervous system is organized into two divisions: The Central Nervous System (CNS) and Peripheral Nervous System (PNS). The Central Nervous System (CNS) consists of brain and spinal cord. It controls most functions of the body and mind. The peripheral nervous system (PNS) consists of all the nerves that branch off from the brain and spinal cord. These peripheral nerves are the lines of communication between the CNS, the various parts of the body and the external environment (Figure 12.19).

Brain and spinal cord are covered by three layers of membranes called meninges. These layers are the outermost dura mater, the middle arachnoid mater, and the innermost pia mater. Between the pia mater and arachnoid membranes is a space called the subarachnoid space, in which there is cerebrospinal fluid (CSF) circulating.


 

Barriers of CNS

Dyes such as Trypan blue injected into the systemic circulation stain virtually all tissues, with the exception of the brain and spinal cord. This blood brain barrier excludes most macromolecules, microorganisms, immunocompetent cells and antibodies. Even pathogens that are circulating in the bloodstream usually cannot enter the brain and spinal cord because of blood brain barrier. Certain capillaries permit some substances to pass from the blood into the brain but restricts others. These capillaries are less permeable than others within the body and are therefore more selective in passing materials (Figure 12.20). The blood brain barrier (Figure 12.21) is due to the cellular configuration of cerebral capillaries, the choroid plexus and arachnoid cells. It acts as a natural barrier that prevents the invasion of microorganisms into the brain. If this is breached organisms enter the brain. The blood CSF barrier (Figure 12.22) (also calle brain CSF barrier) consists of endothelium with fenestrations, and tightly joined choroid plexus epithelial cells. It acts as a natural barrier that prevents the invasion of microorganisms into the meninges.



 

Routes through which microorganisms enter nervous system

·        Skull or bone fractures

·        Medical procedures

·        Peripheral nerves

·        Blood or lymph

 

Clinical Manifestations of Nervous System Infections

Some of the symptoms of nervous System infections are headache, fever, stiff neck, focal signs, seizures, confusion, weakness, hallucinations, stupor, coma, abnormal behavior and sleep disorder

 

Infections of Nervous System

·        Meningitis is an inflammation of the meninges (membrane covering the brain). Meningitis is a diffuse infection caused by a variety of different agents.

·        Encephalitis is defined as inflammation of the brain. Unlike an abscess, which is a localised area of bacterial or fungal growth, Encephalitis is usually due to viruses that produce more widespread intracellular infections.

·        Brain abcess is a focus of purulent infection and is usually due to bacteria. Brain abscesses develop from either a contiguous focus of infection (such as the ears, the sinuses, or the teeth) or hematogenous spread from a distant focus (such as the lungs or heart, particularly with chronic purulent pulmonary disease, subacute bacterial endocarditis, or cyanotic congenital heart disease). In many cases the source is undetectable.

 

Etiological agents of Meningitis

This can be caused by a wide range of microorganisms and can be classified as pyogenic and non pyogenic meningitis. In pyogenic meningitis infiltration of pus cells (neutrophils) will be seen. In Non pyogenic or aseptic meningitis infiltration of lymphocytes may be seen. Diseases of nervous system are listed in Table 12.10.




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