Electrodiagnostics

Electrodiagnostic examinations

Electrodiagnostic examinations are useful in the diagnosis of many neurological diseases of the brain and the peripheral nervous system (diseases of the nerves and muscles). Electrodiagnostic tests include: electroencephalography (EEG), electromyography (EMG), nerve conduction studies (NCS) and repetitive stimulation (RS).

Electroencephalography

Electroencephalography is the measurement of the spontaneous electrical activity of brain, mainly forebrain (cortex). It is non-invasive examination during which tiny small needles are placed under skin, while the animal is in sedation (sleeping) for about 20 minutes. EEG is usually performed in patients where the brain disease is suspected. EEG has been shown to be valuable in diagnosis of brain tumours, anomalies (mainly hydrocephalus), vascular lesions and brain trauma. Nevertheless, EEG is most frequently used in cats and dogs with epilepsy or where epilepsy is suspected. EEG currently is the only examination which can positively confirm presence of epileptic activity in the brain of the animal and therefore is the most useful in differentiation of the epileptic and non-epileptic events. For example if the dog or cat is having episodes during which the behaviour is changing suddenly and it is not clear if it is a seizure or behaviour problem. In these cases EEG can help to distinguish between them and help to choose the appropriate medication.

The two images below are showing examples of focal and generalised epileptic activity in a dog. 

 Another important role of EEG is in treatment of status epilepticus (continuous seizure) patients. In these cases EEG is the only tool which is able to say objectively if the patient has been treated sufficiently or not as the EEG is showing if the brain has calmed down already or not.

Electromyography

EMG is the recording of activity of the muscle. During the test the small needle is inserted in to the examined muscle and the activity recorded. EMG is performed on a patient under deep sedation or general anaesthesia therefore no pain is felt during examination.

The test idea is based on the fact that the normal resting muscle is generally electrically silent. In contrary the sick muscle has so called spontaneous activity which can be measured during electromyographic examination and based on the characteristics of the measured activity the decision can be made what kind of process is present in the muscle. EMG helps in confirmation of the existence of the disease of the peripheral nervous system and helps to distinguish between primary nerve and muscle disease. It says precisely if the muscle atrophy is because of disease of the muscle or nerve or because of because of too little activity. In case the nerve(s) of the patient have been damaged during trauma episode, the so called denervation potentials will be detectable on EMG 5 days after trauma. The measurement of the denervation potentials in different muscles will tell the examiner which nerves have been traumatised. If the nerve will recover from trauma, so called reinervation potentials will be detectable on EMG recording and are considered a good prognostic sign.

Nerve conduction studies

Nerve conduction studies evaluate the function of the of peripheral nerves. In the clinical practice, the function of the motor nerves is the most commonly evaluated. The most commonly tested nerves in dogs and cats are the peroneal and the tibial nerves in the hind and the ulnar and the radial nerves in the front limb.

The measurement is performed on the animal in general anesthesia. Two small needles are inserted near the nerve and the nerve is stimulated with low electrical current. The stimulation of the nerve initiates muscle contraction. Distance from the stimulation to the recording site is measured as well as time needed for impulse to arrive to the muscle. If distance and time is known velocity can be calculated. The normal nerve conduction velocity is around 55-75 m/s in most of the peripheral nerves of the dog or cat. The velocity is higher in young animals as in old patients and is also decreasing with the body temperature. Changes in the nerve conduction velocity indicate damage to the axon of the nerve, to its myelin sheath or both. Nerve conduction velocity helps to confirm the presence of the peripheral nerve disease and to choose the right nerve for the further investigation such as nerve biopsy.

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