[Web Module] Central Nervous System Labs

What are three diagnostic procedures used to detect CNS infections?

-CT scan

-MRI scan

-Lumbar puncture

What is a CT scan?

Medical imaging tool that is used to detect pathological or physiological changes in living tissue.

CT scans use x rays to acquire images, making it a good tool for imaging dense tissue

What is a MRI scan?

Medical imaging tool that is used to detect pathological or physiological changes in living tissue.

MRI scans use non-ionizing radio frequency signals to acquire images. It is the best tool for analyzing non calcified tissue

What is a lumbar puncture?

An invasive diagnostic procedure where a needle is inserted into the subarachnoid space in the spinal column.

It is used to measure the pressure in the subarachnoid space and to obtain a CSF sample for chemistry, microbiology, and cytology

What is pharmacological therapy for a CNS infection influenced by?

Influenced by both the suspected pathogen and the blood brain barrier, which only allows specific molecules into the brain

Non polar, lipid soluble drugs are the best choice for CNS infections as they readily cross the blood brain barrier

What types of pharmacological agents are best for CNS infections?

Non polar and lipid soluble drugs - able to cross the blood brain barrier

What is the blood brain barrier?

A membrane that controls passage of substances from the blood into the CNS - protects the brain from toxins

What can increase permeability of the blood brain barrier?

Systemic inflammation can increase the permeability of the blood brain barrier, leading to pathogen entry and causing infection of the brain and meninges

What is the rationale of an IV normal saline infusion for hypotension?

Increases fluids -> leads to increased blood volume which increases blood pressure

What is a petechial rash?

Aka non-blanching petechial rash, it is a significant sign of an infection of a gram negative bacteria - which produces endotoxins that cause blood vessel damage, leading to small bleeds, and disseminated intravascular coagulation, which lead to widespread coagulation and eventually systemic hemorrhage as clotting factors become exhausted

What is disseminated intravascular coagulation?

A condition resulting in widespread and disordered coagulation due to abnormally elevated levels of thrombin, which is commonly induced by gram negative endotoxin production.

This can lead to ischemic tissue damage due to blood clots and result in systemic hemorrhage as clotting factors become exhausted

What is a irretrievable specimen?

A specimen which cannot be replaced without the clinical condition of the patient - includes CSF samples

What is a stat specimen?

A specimen which requires rapid results for appropriate management as it could identify a life-threatening infection

An example is collecting a CSF sample for suspected gram negative bacteria caused meningitis

Ideally these specimens should be transported within 1 hour of collection

When should CSF specimens be transported to the labratory?

A stat specimen - therefore should be transported within 1 hour of collection

What is the order of collecting the 4 required CSF samples?

Tube 1 - Cell count

Tube 2 - Chemistry (Glucose and protein)

Tube 3 - Microbiology (Gram staining and culturing)

Tube 4 - Cell count (Comparing with tube 1)

How should the patient be positioned during a lumbar puncture?

Patient should be kept motionless, laying on the side, with back arched forward so that head almost touches their knee

What antibiotic class are first line therapy for suspected Gram negative meningeal infections?

Third generation cephalosporins (cefotaxime, ceftriaxone)

What is centrifugation?

The process of using centrifugal forces to separate mixtures into a precipitate (sediment) and a supernatant (liquid)

What is latex agglutination?

A test used to determine the presence of antigens in bodily fluids - which can include saliva, urine, CSF, or blood

What occurs to the precipitate part of a specimen after centrifugation?

The sediment is used to prepare a Gram stain and to perform a culture

Gram staining can be used to make a presumptive diagnosis of bacterial meningitis

What occurs to the supernatant part of a specimen after centrifugation?

The supernatant is collected for latex agglutination, which is used to detect the presence of antigens in the liquid

Presence of meningeal antigens in the CSF can be used to make a presumptive diagnosis of bacterial meningitis

What is the importance of obtaining bacterial cultures for an infection?

-Determining antibiotic susceptibility

-Performing complete subtyping

-Examining the expression of antigens that should be included in future vaccines

-Determining the pathophysiology of the pathogen

What mediums should be used when performing a culture of a CSF specimen?

-Blood agar plate (support bacterial growth)

-Chocolate agar plate (support fastidious bacterial growth)

What is the purpose of a blood agar plate culture for diagnosing bacterial meningitis?

Contains sheep red blood cells in an agar base, which is sufficient to support the growth of most bacteria.

In the context of bacterial meningitis, it can be used to culture Streptococcus pneumoniae, and Neisseria meningitides

What is the purpose of a chocolate agar plate culture for diagnosing bacterial meningitis?

Contains cooked sheep's blood in an agar base, which increases the amount of nutrients available for the organism - allowing fastidious organisms to grow

In the context of bacterial meningitis, it can be used to culture Haemophilius influenza.

What is a fastidious bacterial organism?

A bacterial species that has complex nutritional or culture media requirements

What results from gram staining and culturing (blood agar + chocolate agar) are required for a presumptive diagnosis of Neisseria meningitidis for bacterial meningitis?

Gram staining - gram negative diplococci

Culturing - bacteria grows on both blood agar and chocolate agar

What results from gram staining and culturing (blood agar + chocolate agar) are required for a presumptive diagnosis of Streptococcus pneumoniae for bacterial meningitis?

Gram staining - gram positive diplococci

Culturing - bacteria grows on both blood agar and chocolate agar

What results from gram staining and culturing (blood agar + chocolate agar) are required for a presumptive diagnosis of Hemophilus influenza for bacterial meningitis?

Gram staining - gram negative coccobacilli

Culturing - bacteria grows on chocolate agar but not on blood agar (fastidious organism)

What are polymorphonuclear leukocytes?

Highly phagocytic granulocytes (aka neutrophils) that may appear on gram staining

What does a Neisseria meningitidis colony look like upon culturing?

Appear round, smooth, moist, glistening, and convex with an entire edge.

Some colonies may appear to coalesce with their neighbors

The growth is overall greyish and unpigmented

What are droplet precautions?

Isolation precautions designed to reduced droplet transmissions from infected agents - which includes contact of the mucous membranes or conjunctiva with the respiratory droplets of someone with the disease or is a carrier of the microorganism

Respiratory droplets can be generated from coughing, sneezing or talking and can only travel short distances (~3 ft)

Respiratory droplets includes wearing a surgical mask when entering the patient's room

What are the three types of meningococcal vaccines?

Men-C-C -> coveys protection against meningococcal serogroup C, given to infants 12 months of age

Men-C-ACYW -> conveys protection against meningococcal serogroup A, C, Y and W135, given to children in 7th grade

4CMenB -> conveys protection against meningococcal serogroup B, given to individuals with high risk conditions

What is the Men-C-C vaccine?

Meningococcal vaccine that conveys protection against the serogroup C bacterial strains

Given to infants 12 months of age

What is the Men-C-ACYW vaccine?

Meningococcal vaccine that conveys protection against the serogroup A,C, Y, and W135 bacterial strains

Given to children in 7th grade

What is the 4CMenB vaccine?

Meningococcal vaccine that conveys protection against the serogroup B bacterial strain

Given to individuals with high risk conditions

Which individuals are considered casual contacts with patients with Neisseria meningitis?

Individuals with no history of direct exposure to the patient's oral secretions during the 7 days before the onset of disease