There are two types of neurologic tests for the brain: those that examine the structure of the brain and those that examine its function. The CAT scan and MRI look at the structure of the brain, wheras the electroencephalogram (EEG), SPECT scan, PET scan, and evoked studies examine the function of the brain.
MRI and CAT Scans
The MRI and CAT scan slice the brain radiographically into slabs. The MRI does this with magnetic fields; the CAT scan uses x-rays. The MRI provides more detail than the CAT scan. Hence, brain damage seen on an MRI – as small as 1-2mm in size – may escape detection by a CAT scan. The CAT scan is superior to the MRI in detecting fresh blood in and around the brain, while the MRI is better at detecting the remnants of old hemorrhaged blood, called hemosiderin. CAT scans are often repeated to insure that a brain injury is not becoming more extensive, usually in the early stages of ER treatment.
Being so very sensitive, the MRI commonly detects clinically silent (asymptomatic) “brain damage” in the normal population. For example, as we age it is common for myelin in the white matter to degenerate. An MRI can detect this myelin degeneration as white matter hyperintensities. The MRI is also sensitive to cerebral atrophy (brain shrinkage), another normal phenomenon as we age. Excessive numbers of white matter hyperintensities or excessive atrophy signal a possible neurologic illness, or injury.
Midline Shift – Any mass inside or outside the brain can shift the brain to one side. When severe, the shift can involve important midline structures. The brain should return to its normal position after the cause of the shift is identified and corrected.
MRA (Magnetic Resonance Angiography)
MRA, or magnetic resonance angiography, is a means of visualizing the carotid and vertebral arterial systems in the neck and brain without having to inject contrast into the bloodstream. The resolution is not as good as with conventional arteriography, but the patient is spared the risks of catheterization and allergic reactions to the dye. (In conventional arteriography, a catheter is threaded from the femoral artery in the groin backward up the aorta into a carotid or vertebral artery in the neck, and then dye is injected up the catheter. As the dye flows into the brain, x-rays are taken of the cerebral vasculature.)
EEG (Electroencephalogram)
Monitors the brain’s electrical activity by means of wires attached to the patient’s scalp. These wires act like an antenna to record the brain’s electrical activity. The resting brain normally emits signals at a frequency of 8 to 13cps (cycles per second), called alpha activity; anything faster than this is called beta activity. Slower rhythms include theta activity (6-7cps) and delta activity (3-5cps).
Theta and delta activity occur in the normal brain as the patient descends into sleep. If the patient is awake, any slowing of electrical activity in a focal area of the brain may indicate a lesion there. Similarly, widespread slowing indicates a widespread disturbance of brain function, often due to a blood born insult like low blood sugar, drug intoxication, liver failure, etc. “Spiking” (sharp waves of electrical activity) discharges indicate an irritable area of cerebral cortex. If allowed to spread, the spikes can produce a seizure.
It is not uncommon for an EEG to be normal between seizures in patients with bonafide seizures. During a seizure, however, the EEG is almost invariably abnormal. Conversely, 15% of the population shows mild abnormalities on EEG, representing old head trauma, old strokes, migraine, viral infections, and most of the time for unknown reasons.
Quantitative EEG (QEEG, BEAM, Brain Mapping)
This test is performed in a way similar to EEG. Brain wave activity varies throughout the day depending on the state of alertness. Each area of the brain normally spends a characteristic amount of time in alpha, beta, theta and delta activity. Brain-mapping computers are now capable of creating a map of the brain’s electrical activity depicting how long each area of the brain spends in each of the basic rhythms. By comparing the patient’s map with that of a control population, it is possible to localize areas of focal slowing of electrical activity. Alone, a QEEG is insufficient to diagnose brain damage but in conjunction with other neurologic tests, QEEG can be confirmatory.
PET Scan (Positron Emission Tomography)
PET scanning (positron emission tomography) is based on the fact that the brain uses glucose for energy. By labeling a glucose molecule with a radioactive “tag” and then inhaling radioactive glucose and placing the patient’s head under a large geiger counter, one can identify abnormal areas of the brain that are underutilizing glucose. Because cyclotrons are needed to generate the radioactive gas, PET scanning is not widely available.
SPECT Scan (Single Photon Emission Computed Tomography)
SPECT scanning (single photon emission computed tomography) is similar to PET scanning in that a radioactive chemical is administered intravenously to the patient, but the radioactive chemical remains in the bloodstream and does not enter the brain. As a result, the SPECT scan maps the brain’s vascular supply. Because damaged brain tissue normally shuts down its own blood supply, focal vascular defects on a SPECT scan are circumstantial evidence of brain damage. The advantage of a SPECT scan over a PET scan is its ready availability and relatively cheap cost. Recent studies have demonstrated abnormal SPECT scans after head trauma when the CAT and MRI were normal, suggesting that the SPECT scan is more sensitive to brain injury then either CT or MRI scans. Because the radioactive chemicals used in SPECT and PET scans are carried to all parts of the body by vascular tree, SPECT scans and PET scans are used judiciously in patients of reproductive age.
Evoked Potentials
Evoked studies take advantage of the fact that each time a sensory system of the body – vision, hearing, touch – is stimulated, an electrical signal is generated in the brain. These electrical signals can be detected with electrical wires on the scalp.
Lumbar Puncture
A lumbar puncture (spinal tap) is used to analyze cerebrospinal fluid. An analysis of the fluid can help tell doctors, for example, if there is any bleeding in the brain or spinal cord areas.
Magnetic Resonance Spectroscopy (MRS)
This is an exciting new tool, used in conjunction with MRI, that detects the intra-cellular relationship of brain metabolites. Studies show that in an injured brain, the relationship between the amount of certain compounds in the brain changes in predictable ways, which can be picked up, non-invasively, by MRS. While MRS is in its early stages, it holds great promise in the “objectivication” of brain injury. This data can and should be captured on MRI within six weeks of injury.
Neurologic Testing, Conventional can help with the following
Key
| May do some good |
Glossary
CAT Scan
(Computerized Axial Tomography scan). A scanning procedure using X-rays and a computer to detect abnormalities of the body's organs.
MRI
Magnetic Resonance Imaging. A technique used in diagnosis that combines radio waves and magnetic forces to produce detailed images of the internal structures of the body.
Millimeter
(mm): A metric unit of length equaling one thousandth of a meter, or one tenth of a centimeter. There are 25.4 millimeters in one inch.
Asymptomatic
Not showing symptoms.
Myelin
A substance made of protein and lipid (fat) that protects the nerves, especially in the brain. The myelin sheath is a jacket of insulation around axons to help them conduct their electrical discharges quickly down the axon.
Seizure
While there are over 40 types of seizure, most are classed as either partial seizures which occur when the excessive electrical activity in the brain is limited to one area or generalized seizures which occur when the excessive electrical activity in the brain encompasses the entire organ. Although there is a wide range of signs, they mainly include such things as falling to the ground; muscle stiffening; jerking and twitching; loss of consciousness; an empty stare; rapid chewing/blinking/breathing. Usually lasting from between a couple of seconds and several minutes, recovery may be immediate or take up to several days.
Migraine
Not just a headache, but a disorder affecting the whole body, characterized by clearly defined attacks lasting from about 4 to 72 hours, separated by headache-free periods; progresses through five distinct phases. Prodrome: experienced by about 50% of migraineurs and starting up to 24 hours before the headache - changes in mood, sensory perception, food craving, excessive yawning, or speech or memory problems. Aura: experienced by about 15% and starting within an hour before the headache - disruption of vision (flashing lights, shimmering zigzag lines, blind spot) or sensation (numbness or 'pins and needles' around the lips or hand), or difficulty speaking. Headache: usually pulsating and occurring on one side of the head, it may occur on both sides of the head and alternate from side to side. Muscles in the neck and scalp may be tender; there may be nausea and the desire not to eat, move, see or hear. Resolution: the headache disappears and the body returns to normal. Resolution may occur over several hours during sleep or rest; an intense emotional experience or vomiting may also end the headache. Postdrome: After the headache stops, the sufferer feels drained, fatigued and tired. Muscles ache, emotions are volatile and thinking is slow.
Glucose
A sugar that is the simplest form of carbohydrate. It is commonly referred to as blood sugar. The body breaks down carbohydrates in foods into glucose, which serves as the primary fuel for the muscles and the brain.
Metabolite
Any product (foodstuff, intermediate, waste product) of metabolism.