Description of the House
Built in 1886, Surrey House is the oldest homestead property on the Gold Coast. Originally an 80-acre dairy, the old home was raised long ago and has been a medical centre for the past 60 years.
What is Neurophysiology?
Neurophysiology is the study of how nerve cells (neurones) receive and transmit information. Two types of phenomena are involved in processing nerve signals: electrical and chemical. Electrical events propagate a signal within a neurone, and chemical processes transmit the signal from one neurone to another neurone, or to a muscle cell.
A neurone is a long cell that has a thick central body containing the nucleus. It also has one long process called an axon and one or more short, bushy processes called dendrites. Dendrites receive impulses form other neurones (the main exceptions are sensory neurones, such as those that transmit information about temperature or touch, in which the signal is generated by specialised receptors in the skin.) These impulses are propagated electrically along the cell membrane to the end of the axon. At the end of the axon the signal is chemically transmitted to an adjacent neurone or muscle cell.
Live neurones are normally polarised at rest, ie, they have a negative electrical charge inside the cell membrane. This is due to the free movement of positively- charged potassium ions through the cell membrane and, at the same time, the retention of large, negatively-charged molecules within the cell. Positively-charged sodium ions are kept outside the cell by an active metabolic process. All cells have this charge difference, but when a suitable stimulus is applied to a nerve cell, a unique sequence of events occurs. First, potassium ions flow into the cell, reducing the negative charge (depolarisation). At a certain point (as a result of this) the properties of the membrane change and the cell becomes permeable to sodium, which rapidly enters the cell and causes a net positive charge inside the neurone. This is called the action potential.
Once this potential is reached at one area in the neurone, it travels down the axon by ion exchange at specific points, called Nodes of Ranvier. The size of the action- potential is self-limiting, because a high internal sodium concentration causes the pumping out first of potassium and then of sodium ions, restoring the negative charge inside the cell membrane, ie, the neurone is repolarised. This whole process takes less than one-thousandth of a second. After a very brief period, called the refractory period, the neurone can repeat this process.
When the above electrical signal reaches the end of an axon, it stimulates small presynaptic vesicles in the cell. These vesicles contain chemicals called neuro-transmitters, which are released into the sub-microscopic space between neurones (the synaptic cleft). These neuro-transmitters attach to specialised receptors on the surface of the adjacent neurone. This stimulus causes the adjacent cell to depolarise and propagate an action potential of its own. The duration of the stimulus from a neurotransmitter is limited by the breakdown of the chemicals in the synaptic cleft and their re-uptake by the neurone that produced them.
All of the above neural processes result in electrical signals being generated. Although the voltages are typically small (they are often at only a micro-volt level), it is possible with neurophysiology techniques to study brain, peripheral-nerve and muscle disorders by recording and analysing these electrical signals. This is achieved by using either electrodes on the surface of the skin or by inserting a fine needle into selected muscles. The resulting studies are called electroencephalograms, nerve conduction studies and electromyograms, all of which are summarised below.
Common Neurophysiological Investigations
The types of conditions in which EEGs are particularly needed are:
- known epilepsy: EEGs are used to diagnose the type of epilepsy and the level of risk of occurrence of further seizures
- “fits, faints and funny turns” – i.e., conditions in which the diagnosis is uncertain but in which epilepsy is possible. Patients in this category might complain of symptoms such as dizziness, light-headedness, blank spells, lapses in memory, “absences”, vertigo, unexplained falls, or unexplained problems with speech, sensation or motor-control. Sometimes, patients find it very hard to describe their symptoms, but simply register that “something is going wrong” with their brain
- Organic brain syndromes, e.g., when a doctor is trying to establish if symptoms such as confusion, hallucinations, memory impairment or poor concentration are likely to be due to electrical problems in the brain. Sometimes, the problem could be a specific pathology in the brain such as a brain tumour or other “mass lesion”, sometimes the problem could be a metabolic disorder or a drug side-effect, or intermittent epilepsy may be occurring. An EEG can be very helpful in making the distinction, particularly if the symptoms occur while the test is in progress-it is for this reason that the EEGs are sometimes performed, as this increases the likelihood of capturing and diagnosing one of the suspicious events
- Head injuries: up to 5% of patients with a head injury who have been admitted to hospital develop early-onset or late-onset post-traumatic epilepsy. Many other patients develop dizziness and impaired memory and concentration following a head-injury. An EEG is helpful in assessing the risk of epilepsy and in distinguishing between organic causes (including epilepsy) and other causes of symptoms where no organic brain pathology is demonstrable with imaging studies
- Blackouts: blackouts can be caused by cardiac disorders (heart problems), impairment of flow of blood to the brain (such as in vertebro-basilar insufficiency), and also by epilepsy and many other conditions. An EEG is a basic tool which is used in making the distinction.
Generally, EEGs are used to help establish whether or not a patient’s symptoms are due to organic brain-dysfunction and if so, whether the dysfunction is due to some primary, localised electrical disorder or to a diffuse, abnormal brain process (either local or systemic).
EEGs measure electrical signals generated by the brain. They are painless, non-invasive tests where the patient sits in a comfortable chair with a cap on the head. Electrodes are then attached to the scalp and transmit electrical signals to a computer. We use some of the most modern equipment available, and typically record 28 simultaneous channels for recording and graphic display brain data at all times. Older EEG systems cannot do this.
Professor Corbett’s facility provides for full digital EEG and video-telemetry when needed. This means that a video-recording can be made while the EEG is running so that if the brain waves are abnormal at any time, it is possible to play back the video and see if the patient was having a fit or “turn” at the time.
If a patient has had a fit or a possible fit, there is a better chance of diagnosing the type of epilepsy if the tracing can be recorded soon afterwards. In such circumstances, we therefore attempt to provide an early appointment.
Overnight Fasting and Sleep-Deprivation EEGs
When a patient is suspected of having epilepsy but the results of a standard EEG are normal, it is often possible to detect abnormalities if the tracing is repeated after overnight fasting and sleep deprivation. These are called “provocation tests”, as they often bring out hidden abnormalities. Patients having this type of test need to have their normal evening meal on the previous night but avoid breakfast on the morning of the study. These patients need also to stay awake all night (or for as much of the night as possible). Professor Corbett likes the patient to arrive for this particular EEG study feeling “tired and hungry and wrecked”.
Prolonged EEGs (PEEGs)
Because the nature of epilepsy is such that it occurs only intermittently, it is sometimes difficult to establish this diagnosis with certainty with standard-duration EEGs (which are typically recorded over a period of about 15-20 minutes). This issue is particularly important in assessing whether a patient can drive safely, or if they are suspected of having epilepsy but the diagnosis has not been confirmed. In this situation, and also when patients are being assessed for their fitness to return to driving when they are known to have suffered from epilepsy in the past, Professor Corbett sometimes performs prolonged EEGs (PEEGs). These PEEGs are recorded over a period of in excess of three hours, often after overnight fasting and sleep-deprivation. (Professor Corbett’s new digital EEG video-telemetry system is especially suited to this type of study, as it allows 24-48 channels to be recorded for 24, 48 or 72 hours if necessary). These PEEGs enable a much more comprehensive assessment of epileptic risk than is achievable with routine EEGs.
Nerve Conduction Study (NCS)
A nerve conduction study is used for the diagnosis of a variety of peripheral-nerve problems. This study can give an overall impression of nerve function, but it can also be used for diagnosing localised nerve-entrapments. Nerve Conduction Studies are typically performed by using an externally-generated electrical pulse to cause a nearby nerve to discharge, resulting in propagation of a signal proximally or distally along the nerve in question. These signals are then recorded by skin or muscle electrodes which are placed further along the nerve being tested. Nerve conduction studies are typically performed for symptoms such as tingling and numbness, weakness and muscle wasting. This type of testing is often performed in conjunction with EMG studies.
Electromyography (EMG) Studies
EMG studies are used to test the nerve supply to a muscle or group of muscles. Fine needles are inserted into the muscle being tested and the patient is asked to contract and relax that muscle. This type of testing is used in the diagnosis of nerve-root compressions, myopathies, and acute and chronic denervation following injury or disease. Typical symptoms requiring EMG studies are muscle-weakness and wasting, abnormal muscle twitches, and abnormal movements. This type of testing is often performed in conjunction with nerve conduction studies.