by Carlos Ballario and Ma. Eugenia Ferri


The Electroencephalogram (EEG) is the graphic recording of brain electrical activity on the cranial surface. As early as 1875 it was known that excitable tissues could generate electrical stimuli, which is why, based on these principles, Hans Berger in 1929 made the first record, using electrodes placed on the scalp. From there and thanks to the important technical advances, the method has improved ostensibly, reaching the current digital records.

The EEG represents an average of the postsynaptic potentials that occur in a synchronized manner in broad cortical brain areas, with a radial spatial orientation.

The technique is very simple and risk-free. The potentials are registered by means of 21 electrodes attached to the scalp, arranged according to a preset and internationally accepted system, known as the 10-20 system or international system. The placement of the electrodes must be done properly to avoid electrical artifacts due to bad contact. The electrodes are connected in pairs in sequences called mounts. The study should have a minimum duration of 20 minutes, with the patient sitting or lying down, relaxed, in a quiet environment and with the eyes closed. Some activation techniques such as hyperventilation, eyelid opening and closing or light stimulation can be performed in order to facilitate the appearance of abnormal discharges.

In the normal adult, the EEG presents a rhythm of 8 to 12 Hz and 15 to 150 microvolts, known as the Alpha rhythm, which is observed symmetrically in both parieto-occipital regions when the eyes are closed and the patient is relaxed. In the frontal regions, irregular, faster, and low-voltage activity or Beta rhythm is recorded. When the eyes are opened, the alpha rhythm is quickly replaced by beta activity which is known as Activation. In the transition to sleep, the alpha rhythm disappears and irregular slow waves, Theta and Delta, are evident, interrupted by brief bursts of activity of 12 to 14 cycles per second called sleep spindles.

The most frequent clinical applications of EEG are the following:

Epilepsies and convulsive states : the most common patterns are spikes (tips) and sharp waves either in isolation or in complexes. They usually stand out clearly from the base rhythm and are called paroxysm. The EEG is altered during seizures and can return completely to normal between seizures, so a normal intercritical study does not rule out epilepsy or seizures.

Encephalopathies : such is the case of hyponatremia, hypoglycemia, liver, kidney or respiratory failure cause a global slowdown in brain activity. It can be seen from a slight excess of theta frequencies to registers where all that is observed is very slow, low voltage delta activity.

Focal structural lesions : any structural process, such as a tumor, infarction or abscess, can generate foci of slow activity, delta or theta, irregular and polymorphic.

Although this method may be indicative, it is not the first choice when a focal lesion is suspected and neuroimaging is clearly required.

Encephalitis : in the case of suspected herpetic encephalitis, this method acquires a lot of value since it can reveal the appearance of outbreaks of slow activity on a periodic basis or frank epileptiform discharges located in one of the temporal regions.

Suspicion of Spongiform Encephalopathy (Jacob-Creutzfeldt disease): in this subacute form of dementia, a characteristic pattern of periodic discharges is evident.

Diagnosis of brain death : the EEG is included within the brain death tests, which must be flat since no waves are recorded.

Videoencephalographic Record

By this method, the definitive diagnosis of the presence of epileptic seizures and the location of the epileptogenic zone is achieved. The patient is placed on a bed with CCTV. The video is synchronized with the continuous EEG signal to allow a perfect temporal correlation between the clinical and EEG manifestations of seizures. The indication occurs in the following situations:

Pre-surgical investigation of the patient with drug-resistant epilepsy

Faced with diagnostic difficulty, it is important to define if the patient really has seizures or is another type of non-epileptic paroxysmal disorders

To differentiate different epileptic syndromes

Electromyography and Study of Nerve Conduction Velocity

The Electromyogram (EMG) is the study of muscular electrical activity.

The different electrical changes that can be detected with this technique allow us to draw certain conclusions about the physiology of the peripheral nervous system, resulting in a useful tool in the diagnosis of different neuromuscular diseases.

Thanks to the technical advances of the last three decades that allowed the development of the lateral or single fiber micro-pickup electrode and the introduction of computer programs, a great advance in the electromyographic study was produced. Despite these advances, a complete study must include a needle electrode that must be inserted deep into the previously selected muscles with a thorough neurological examination.

These electrodes are best suited for recording the activity of a few motor units. In contrast, the use of surface electrodes, such as those used in electrocardiography, only allow the random sum of many motor units to be collected, providing an "insensitive average" of the desired information.

The first phase of the procedure consists in the introduction of the needle with the patient absolutely relaxed, in one of the muscles to be studied.

In a normal patient there is electrical silence.

Then the voluntary activity that causes muscle contraction gives rise to motor unit potentials (PUM). Amplitude, duration, morphology, and number of phases are the most characteristic electrophysiological elements that determine the normality or abnormality of PUM.

On the other hand, the study of the conduction velocities of the motor and sensory fibers offers objective information on the ability to conduct an electrical impulse that different nerve trunks of the four limbs have.

Using surface electrodes (needles are not used for this technique), a nerve accessible through the skin is stimulated and the resulting action potential can be recorded in different routes of the nerve or in the muscle innervated by it.

With a very simple formula, the conduction velocity of the scanned nerve is obtained, as well as other electrical hierarchical data (distal latency, amplitude and morphology of the potential, etc.).

The electromyographic study and the nerve conduction velocity is of great value in those muscular affections, of the roots and trunks of the peripheral nerves, and of the medullary anterior horn.

In addition, there is a specific technique to explore the functioning of neuromuscular transmission called "repetitive stimulation", consisting of stimulating a nerve trunk with a "shock train" at different frequencies and with certain sensitization techniques of the method.

Repetitive stimulation is very useful in the diagnosis of Myasthenia Gravis, Lambert-Eaton Syndrome, and other conditions of the neuromuscular junction.

Finally, it is necessary to remember that both EEG and EMG and the other techniques described, lose diagnostic value if they are not considered in the context of a detailed clinical and neurological examination.