Echoencephaloscopy

Echoencephaloscopy (EchoES, synonym - M-method) is a method for detecting intracranial pathology based on echolocation of the so-called sagittal structures of the brain, which normally occupy a median position relative to the temporal bones of the skull. When graphic registration of reflected signals is performed, the study is called echoencephalography.

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Indications for echoencephaloscopy

The main goal of echoencephaloscopy is express diagnostics of volumetric hemispheric processes. The method allows to obtain indirect diagnostic signs of the presence/absence of a unilateral volumetric supratentorial hemispheric process, to estimate the approximate size and localization of the volumetric formation within the affected hemisphere, as well as the state of the ventricular system and cerebrospinal fluid circulation.

The accuracy of the listed diagnostic criteria is 90-96%. In some observations, in addition to indirect criteria, it is possible to obtain direct signs of hemispheric pathological processes, i.e. signals directly reflected from a tumor, intracerebral hemorrhage, traumatic meningeal hematoma, small aneurysm or cyst. The probability of their detection is very insignificant - 6-10%. Echoencephaloscopy is most informative in case of lateralized volumetric supratentorial lesions (primary or metastatic tumors, intracerebral hemorrhage, meningeal traumatic hematoma, abscess, tuberculoma). The resulting shift of the M-echo allows us to determine the presence, side, approximate localization and volume, and in some cases the most probable nature of the pathological formation.

Echoencephaloscopy is absolutely safe for both the patient and the operator. The permissible power of ultrasonic vibrations, which is on the verge of damaging effects on biological tissues, is 13.25 W/cm2 ^, and the intensity of ultrasonic radiation during echoencephaloscopy does not exceed hundredths of a watt per 1 cm2 ^. There are virtually no contraindications to echoencephaloscopy; a successful study has been described directly at the scene of an accident even with an open craniocerebral injury, when the position of the M-echo could be determined from the side of the "unaffected" hemisphere through the intact bones of the skull.

Physical principles of echoencephaloscopy

The echoencephaloscopy method was introduced into clinical practice in 1956 thanks to the pioneering research of the Swedish neurosurgeon L. Leksell, who used a modified device for industrial flaw detection, known in technology as the "non-destructive testing" method and based on the ability of ultrasound to reflect from the boundaries of media with different acoustic resistance. From the ultrasound sensor in pulse mode, the echo signal penetrates the bone into the brain. In this case, three most typical and repeating reflected signals are recorded. The first signal is from the bone plate of the skull on which the ultrasound sensor is installed, the so-called initial complex (IC). The second signal is formed due to the reflection of the ultrasound beam from the median structures of the brain. These include the interhemispheric fissure, the transparent septum, the third ventricle and the pineal gland. It is generally accepted to designate all of the listed formations as a middle echo (M-echo). The third registered signal is caused by the reflection of ultrasound from the inner surface of the temporal bone opposite to the location of the emitter - the final complex (FC). In addition to these most powerful, constant and typical for a healthy brain signals, in most cases it is possible to register small amplitude signals located on both sides of the M-echo. They are caused by the reflection of ultrasound from the temporal horns of the lateral ventricles of the brain and are called lateral signals. Normally, lateral signals have less power compared to the M-echo and are located symmetrically with respect to the median structures.

I.A. Skorunsky (1969), who carefully studied echoencephalotopography under experimental and clinical conditions, proposed a conditional division of signals from the midline structures into anterior (from the septum pellucidum) and mid-posterior (III ventricle and pineal gland) sections of the M-echo. Currently, the following symbolism is generally accepted for describing echograms: NC - initial complex; M - M-echo; Sp D - position of the septum pellucidum on the right; Sp S - position of the septum pellucidum on the left; MD - distance to the M-echo on the right; MS - distance to the M-echo on the left; CC - final complex; Dbt (tr) - intertemporal diameter in transmission mode; P - amplitude of M-echo pulsation in percent. The main parameters of echoencephaloscopes (echoencephalographs) are as follows.

• Probing depth is the greatest distance in tissues at which it is still possible to obtain information. This indicator is determined by the amount of absorption of ultrasonic vibrations in the tissues being examined, their frequency, the size of the emitter, and the gain level of the receiving part of the device. Domestic devices use sensors with a diameter of 20 mm with a radiation frequency of 0.88 MHz. The specified parameters allow obtaining a probing depth of up to 220 mm. Since the average intertemporal size of the skull of an adult, as a rule, does not exceed 15-16 cm, a probing depth of up to 220 mm seems to be absolutely sufficient.
• The resolution of the device is the minimum distance between two objects at which the signals reflected from them can still be perceived as two separate pulses. The optimal pulse repetition rate (at an ultrasound frequency of 0.5-5 MHz) is established empirically and is 200-250 per second. Under these location conditions, good signal recording quality and high resolution are achieved.

Methodology for conducting and interpreting the results of echoencephaloscopy

Echoencephaloscopy can be performed in almost any setting: in a hospital, outpatient clinic, in an ambulance, at the patient's bedside, or in the field (if an autonomous power supply is available). No special preparation of the patient is required. An important methodological aspect, especially for novice researchers, is the optimal position of the patient and the doctor. In the vast majority of cases, the study is more conveniently performed with the patient lying on his back, preferably without a pillow; the doctor is on a movable chair to the left and slightly behind the patient's head, with the screen and panel of the device located directly in front of him. The doctor freely and at the same time with some support on the patient's parietal-temporal region performs echolocation with his right hand, turning the patient's head to the left or right if necessary, while using his free left hand to make the necessary movements of the echo distance meter.

After lubricating the frontotemporal sections of the head with contact gel, echolocation is performed in pulse mode (a series of waves with a duration of 5x10 ⁶ s, 5-20 waves in each pulse). A standard sensor with a diameter of 20 mm and a frequency of 0.88 MHz is initially installed in the lateral part of the brow or on the frontal tubercle, orienting it towards the mastoid process of the opposite temporal bone. With a certain amount of operator experience, a signal reflected from the transparent septum can be recorded near the NC in approximately 50-60% of observations. An auxiliary reference point in this case is a significantly more powerful and constant signal from the temporal horn of the lateral ventricle, usually determined 3-5 mm further than the signal from the transparent septum. After determining the signal from the transparent septum, the sensor is gradually moved from the border of the hairy part towards the "ear vertical". In this case, the mid-posterior sections of the M-echo reflected by the third ventricle and the pineal gland are located. This part of the study is much simpler. It is easiest to detect the M-echo when the sensor is positioned 3-4 cm above and 1-2 cm in front of the external auditory canal - in the projection zone of the third ventricle and the pineal gland on the temporal bones. Location in this area allows you to register the most powerful median echo, which also has the highest pulsation amplitude.

Thus, the main signs of M-echo include dominance, significant linear extension and more pronounced pulsation compared to lateral signals. Another sign of M-echo is an increase in the M-echo distance from front to back by 2-4 mm (detected in approximately 88% of patients). This is due to the fact that the overwhelming majority of people have an ovoid skull, that is, the diameter of the polar lobes (forehead and back of the head) is smaller than the central ones (parietal and temporal zones). Consequently, in a healthy person with an intertemporal size (or, in other words, a terminal complex) of 14 cm, the transparent septum on the left and right is at a distance of 6.6 cm, and the third ventricle and the pineal gland are at a distance of 7 cm.

The main objective of EchoES is to determine the M-echo distance as accurately as possible. Identification of M-echo and measurement of the distance to the median structures should be performed repeatedly and very carefully, especially in difficult and questionable cases. On the other hand, in typical situations, in the absence of pathology, the M-echo pattern is so simple and stereotypical that its interpretation is not difficult. To accurately measure distances, it is necessary to clearly align the base of the leading edge of the M-echo with the reference mark with alternate location on the right and left. It should be remembered that normally there are several echogram options.

After the M-echo is detected, its width is measured, for which the marker is first brought to the anterior and then to the posterior front. It should be noted that the data on the relationship between the intertemporal diameter and the width of the third ventricle, obtained by H. Pia in 1968 by comparing echoencephaloscopy with the results of pneumoencephalography and pathomorphological studies, correlate well with the CT data.

The relationship between the width of the third ventricle and the intertemporal dimension

Width of the third ventricle, mm	Intertemporal size, cm
3.0	12.3
4.0	13.0-13.9
4.6	14.0-14.9
5.3	15.0-15.9
6.0	16.0-16.4

Then the presence, quantity, symmetry and amplitude of lateral signals are noted. The amplitude of the echo signal pulsation is calculated as follows. Having received an image of the signal of interest on the screen, for example, the third ventricle, by changing the pressing force and the angle of inclination, we find such a location of the sensor on the scalp at which the amplitude of this signal will be maximum. Then the pulsating complex is mentally divided into percentages so that the peak of the pulse corresponds to 0%, and the base - 100%. The position of the peak of the pulse at its minimum amplitude value will show the magnitude of the signal pulsation amplitude, expressed as a percentage. The norm is considered to be a pulsation amplitude of 10-30%. Some domestic echoencephalographs have a function that graphically records the pulsation amplitude of reflected signals. For this, when locating the third ventricle, the counting mark is precisely brought under the leading edge of the M-echo, thus highlighting the so-called probing pulse, after which the device is switched to the pulsating complex recording mode.

It should be noted that recording of brain echopulsation is a unique, but clearly underestimated opportunity of echoencephaloscopy. It is known that in the non-stretchable cranial cavity during systole and diastole there occur successive volumetric oscillations of the media associated with rhythmic oscillation of the blood located intracranially. This leads to a change in the boundaries of the ventricular system of the brain in relation to the fixed beam of the transducer, which is recorded in the form of echopulsation. A number of researchers have noted the influence of the venous component of cerebral hemodynamics on echopulsation. In particular, it was indicated that the villous plexus acts as a pump, sucking cerebrospinal fluid from the ventricles in the direction of the spinal canal and creating a pressure gradient at the level of the intracranial system-spinal canal. In 1981, an experimental study was conducted on dogs with modeling of increasing cerebral edema with continuous measurement of arterial, venous, cerebrospinal fluid pressure, monitoring of echopulsation and ultrasound Dopplerography (USDG) of the main vessels of the head. The results of the experiment convincingly demonstrated the interdependence between the value of intracranial pressure, the nature and amplitude of the M-echo pulsation, as well as the indices of extra- and intracerebral arterial and venous circulation. With a moderate increase in cerebrospinal fluid pressure, the third ventricle, normally a small slit-like cavity with practically parallel walls, becomes moderately stretched. The possibility of obtaining reflected signals with a moderate increase in amplitude becomes very likely, which is reflected in the echopulsogram as an increase in pulsation up to 50-70%. With an even more significant increase in intracranial pressure, a completely unusual character of echopulsation is often recorded, not synchronous with the rhythm of heart contractions (as in the norm), but "fluttering" (undulating). With a pronounced increase in intracranial pressure, the venous plexuses collapse. Thus, with a significantly obstructed outflow of cerebrospinal fluid, the ventricles of the brain expand excessively and take a rounded shape. Moreover, in cases of asymmetric hydrocephalus, which is often observed with unilateral volumetric processes in the hemispheres, compression of the homolateral interventricular foramen of Monroe by the dislocated lateral ventricle leads to a sharp increase in the impact of the cerebrospinal fluid stream on the opposite wall of the third ventricle, causing it to tremble. Thus, the phenomenon of fluttering pulsation of the M-echo, recorded by a simple and accessible method against the background of a sharp expansion of the third and lateral ventricles in combination with intracranial venous dyscirculation according to the data of ultrasound Doppler imaging and transcranial Doppler ultrasonography (TCDG), is an extremely characteristic symptom of occlusive hydrocephalus.

After finishing the pulse mode, the sensors are switched to transmission research, in which one sensor emits and the other receives the emitted signal after it passes through the sagittal structures. This is a kind of check of the "theoretical" midline of the skull, in which the absence of displacement of the midline structures, the signal from the "middle" of the skull will exactly coincide with the distance measurement mark left during the last sounding of the leading edge of the M-echo.

When the M-echo is displaced, its value is determined as follows: the smaller distance (b) is subtracted from the larger distance to the M-echo (a) and the resulting difference is divided in half. The division by 2 is done because when measuring the distance to the midline structures, the same displacement is taken into account twice: once by adding it to the distance to the theoretical sagittal plane (from the side of the larger distance) and the other time by subtracting it from it (from the side of the smaller distance).

CM=(a-b)/2

For the correct interpretation of echoencephaloscopy data, the question of physiologically acceptable limits of M-echo dislocation is of fundamental importance. Much credit for solving this problem goes to L.R. Zenkov (1969), who convincingly demonstrated that an M-echo deviation of no more than 0.57 mm should be considered acceptable. In his opinion, if the displacement exceeds 0.6 mm, the probability of a volumetric process is 4%; a 1 mm shift of the M-echo increases this figure to 73%, and a 2 mm shift - to 99%. Although some authors consider such correlations to be somewhat exaggerated, nevertheless, from this study, carefully verified by angiography and surgical interventions, it is obvious to what extent researchers risk making a mistake who consider a displacement of 2-3 mm to be physiologically acceptable. These authors significantly narrow the diagnostic capabilities of echoencephaloscopy, artificially excluding small shifts that should be detected when damage to the cerebral hemispheres begins.

Echoencephaloscopy for tumors of the cerebral hemispheres

The size of the displacement when determining the M-echo in the area above the external auditory canal depends on the localization of the tumor along the long axis of the hemisphere. The greatest displacement is recorded in temporal (on average 11 mm) and parietal (7 mm) tumors. Naturally, smaller dislocations are recorded in tumors of the polar lobes - occipital (5 mm) and frontal (4 mm). In tumors of median localization, there may be no displacement or it does not exceed 2 mm. There is no clear relationship between the magnitude of the displacement and the nature of the tumor, but in general, with benign tumors, the displacement is on average less (7 mm) than with malignant ones (11 mm).

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Echoencephaloscopy in hemispheric stroke

The goals of echoencephaloscopy in hemispheric strokes are as follows.

• To roughly determine the nature of acute cerebrovascular accident.
• To assess how effectively cerebral edema has been eliminated.
• Predict the course of stroke (especially hemorrhage).
• Determine indications for neurosurgical intervention.
• To evaluate the effectiveness of surgical treatment.

Initially, there was an opinion that hemispheric hemorrhage is accompanied by M-echo displacement in 93% of cases, while in ischemic stroke the frequency of dislocation does not exceed 6%. Subsequently, carefully verified observations showed that this approach is inaccurate, since hemispheric cerebral infarction causes displacement of midline structures much more often - up to 20% of cases. The reason for such significant discrepancies in the assessment of the capabilities of echoencephaloscopy was the methodological errors made by a number of researchers. Firstly, this is an underestimation of the relationship between the rate of occurrence, the nature of the clinical picture and the time of echoencephaloscopy. The authors who performed echoencephaloscopy in the first hours of acute cerebrovascular accident, but did not carry out dynamic observation, really noted displacement of midline structures in most patients with hemispheric hemorrhages and the absence of such in cerebral infarction. However, daily monitoring has shown that if intracerebral hemorrhage is characterized by the occurrence of dislocation (on average by 5 mm) immediately after the development of a stroke, then in case of cerebral infarction, the displacement of the M-echo (on average by 1.5-2.5 mm) occurs in 20% of patients after 24-42 hours. In addition, some authors considered a displacement of more than 3 mm to be diagnostically significant. It is clear that in this case the diagnostic capabilities of echoencephaloscopy were artificially underestimated, since it is precisely in ischemic strokes that the dislocation often does not exceed 2-3 mm. Thus, in the diagnosis of hemispheric stroke, the criterion of the presence or absence of M-echo displacement cannot be considered absolutely reliable, however, in general it can be considered that hemispheric hemorrhages usually cause M-echo displacement (on average by 5 mm), while cerebral infarction is either not accompanied by dislocation, or it does not exceed 2.5 mm. It was established that the most pronounced dislocations of midline structures in cerebral infarction are observed in the case of prolonged thrombosis of the internal carotid artery with disconnection of the circle of Willis.

As for the prognosis of the course of intracerebral hematomas, we have found a pronounced correlation between the localization, size, rate of development of hemorrhage and the size and dynamics of the M-echo displacement. Thus, with the dislocation of the M-echo less than 4 mm, in the absence of complications, the disease most often ends well in terms of both life and restoration of lost functions. On the contrary, with the displacement of the midline structures by 5-6 mm, the mortality increased by 45-50% or gross focal symptoms remained. The prognosis became almost absolutely unfavorable with the shift of the M-echo more than 7 mm (mortality 98%). It is important to note that modern comparisons of CT and echoencephaloscopy data regarding the prognosis of hemorrhage have confirmed these long-obtained data. Thus, repeated echoencephaloscopy in a patient with acute cerebrovascular accident, especially in combination with ultrasound dopplerography/TCDG, is of great importance for noninvasive assessment of the dynamics of hemo- and cerebrospinal fluid circulation disorders. In particular, some studies on clinical and instrumental monitoring of stroke have shown that both patients with severe craniocerebral trauma and patients with progressive course of acute cerebrovascular accident are characterized by so-called ictuses - sudden repeated ischemic-cerebrospinal fluid dynamic crises. They occur especially often in the pre-dawn hours, and in a number of observations, an increase in edema (M-echo shift) along with the appearance of "fluttering" echo pulsations of the third ventricle preceded the clinical picture of blood breakthrough into the ventricular system of the brain with phenomena of sharp venous discirculation, and sometimes elements of reverberation in intracranial vessels. Therefore, this easy and accessible comprehensive ultrasound monitoring of the patient's condition can be a strong basis for repeat CT/MRI and consultation with an vascular surgeon to determine the appropriateness of decompressive craniotomy.

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Echoencephaloscopy in traumatic brain injury

Road traffic accidents are currently identified as one of the main sources of death (primarily from traumatic brain injury). The experience of examining more than 1,500 patients with severe traumatic brain injuries using echoencephaloscopy and ultrasound Doppler (the results of which were compared with CT/MRI data, surgical intervention and/or autopsy) indicates the high information content of these methods in recognizing complications of traumatic brain injury. A triad of ultrasound phenomena of traumatic subdural hematoma was described:

• M-echo displacement by 3-11 mm contralateral to the hematoma;
• the presence of a signal before the final complex, directly reflected from the meningeal hematoma when viewed from the side of the unaffected hemisphere;
• registration by ultrasound dopplerography of a powerful retrograde flow from the ophthalmic vein on the affected side.

Registration of the above ultrasound phenomena allows to establish the presence, side and approximate size of the subthecal blood accumulation in 96% of cases. Therefore, some authors consider it mandatory to conduct echoencephaloscopy in all patients who have suffered even a mild TBI, since there can never be complete certainty in the absence of a subclinical traumatic meningeal hematoma. In the overwhelming majority of cases of uncomplicated TBI, this simple procedure reveals either an absolutely normal picture or minor indirect signs of increased intracranial pressure (increased amplitude of M-echo pulsation in the absence of its displacement). At the same time, an important question about the advisability of expensive CT/MRI is resolved. Thus, it is in the diagnosis of complicated TBI, when increasing signs of brain compression sometimes do not leave time or opportunity to conduct CT, and trephination decompression can save the patient, that echoencephaloscopy is essentially the method of choice. It was this application of one-dimensional ultrasound examination of the brain that brought such fame to L. Leksell, whose research was called by his contemporaries "a revolution in the diagnosis of intracranial lesions." Our personal experience of using echoencephaloscopy in the conditions of the neurosurgical department of the emergency hospital (before the introduction of CT into clinical practice) confirmed the high information content of ultrasound localization in this pathology. The accuracy of echoencephaloscopy (when compared with the clinical picture and routine radiography data) in recognizing meningeal hematomas exceeded 92%. Moreover, in some observations, there were discrepancies in the results of clinical and instrumental determination of the localization of traumatic meningeal hematoma. In the presence of a clear dislocation of the M-echo towards the unaffected hemisphere, focal neurological symptoms were determined not contra-, but homolaterally to the identified hematoma. This was so contrary to the classical canons of topical diagnostics that an echoencephaloscopy specialist sometimes had to make a lot of effort to prevent the planned craniotomy on the side opposite to the pyramidal hemiparesis. Thus, in addition to identifying the hematoma, echoencephaloscopy allows one to clearly determine the side of the lesion and thus avoid a serious error in surgical treatment. The presence of pyramidal symptoms on the side homolateral to the hematoma is probably due to the fact that with sharply expressed lateral displacements of the brain, there is a dislocation of the cerebral peduncle, which is pressed against the sharp edge of the tentorial notch.

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Echoencephaloscopy for hydrocephalus

Hydrocephalus syndrome may accompany intracranial processes of any etiology. The algorithm for detecting hydrocephalus using echoencephaloscopy is based on assessing the relative position of the M-echo signal measured by the transmission method with reflections from lateral signals (midsellar index). The value of this index is inversely proportional to the degree of expansion of the lateral ventricles and is calculated using the following formula.

SI=2DT/DV ₂ -DV ₁

Where: SI is the midsellar index; DT is the distance to the theoretical midline of the head using the transmission method of examination; DV ₁ and DV ₂ are the distances to the lateral ventricles.

Based on a comparison of echoencephaloscopy data with the results of pneumoencephalography, E. Kazner (1978) showed that SI in adults is normally >4, values from 4.1 to 3.9 should be considered borderline with the norm; pathological - less than 3.8. In recent years, a high correlation of such indicators with CT results has been shown.

Typical ultrasound signs of hypertensive-hydrocephalic syndrome:

• expansion and splitting to the base of the signal from the third ventricle;
• increase in the amplitude and extent of lateral signals;
• amplification and/or undulating nature of the M-echo pulsation;
• increase in the circulatory resistance index according to ultrasound dopplerography and transcranial pressure dopplerography;
• registration of venous discirculation in extra- and intracranial vessels (especially in the orbital and jugular veins).

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Potential sources of error in echoencephaloscopy

According to the majority of authors with significant experience in using echoencephaloscopy in routine and emergency neurology, the accuracy of the study in determining the presence and side of volumetric supratentorial lesions is 92-97%. It should be noted that even among the most experienced researchers, the frequency of false-positive or false-negative results is highest when examining patients with acute brain damage (acute cerebrovascular accident, TBI). Significant, especially asymmetric, cerebral edema leads to the greatest difficulties in interpreting the echogram: due to the presence of multiple additional reflected signals with especially sharp hypertrophy of the temporal horns, it is difficult to clearly determine the anterior front of the M-echo.

In rare cases of bilateral hemispheric foci (most often tumor metastases), the absence of M-echo displacement (due to the “balance” of formations in both hemispheres) leads to a false-negative conclusion about the absence of a volumetric process.

In subtentorial tumors with occlusive symmetric hydrocephalus, a situation may arise when one of the walls of the third ventricle occupies an optimal position for reflecting ultrasound, which creates the illusion of displacement of the midline structures. Registration of the undulating pulsation of the M-echo can help to correctly identify the brainstem lesion.