Thymus Ultrasonography Biometry in Healthy Cuban Children Between 10 To 14 Years Old

Introduction: Thymic gland is the first of the lymphoid organs to form and grow considerably in childhood.  Experience dynamic changes mainly in the pediatric population.  A behavior has been observed that relates the size of the thymus to augmented susceptibility of infants to suffer frequent infections.  So, the dimensions of the thymic area can be used as a tool for immune clinical diagnosis.

Objective: Explore the biometry of the timic gland by ultrasonography in healthy Cuban children between 10 and 14 years of age. 

Methodology: A prospective descriptive study was carried out with 68 seemingly healthy subjects included in the ages of 10 to 14 years. 

Results: When performing the means comparison test for the area of the thymic silhouette according to the two age groups studied, it turns out to be non-statistically significant.  In turn, when comparing the means obtained for the area of the thymic silhouette according to sexes, these were to present statistically significant differences. 

Conclusions: Mensurations of thymic lobes in healthy children from 10 to 14 years of age according to sex were described, observing a marked difference towards larger dimensions in male individuals.  The value of the area of thymic silhouette turned out to be considerably greater in subjects of the male sex, which allowed us to state that the normal value for said subjects between 10 and 14 years is: 988.46 ± 233.87 mm2 and for female subjects from 10 to 14 years is: 756.76 ± 16.99 mm2.

Birth is a crucial moment for the maturation of the immune system, a transition occurs from the intrauterine sterile environment to the outer hostile environment, where the neonate will be exposed to a large microbial load of the environment.  The passage through the childbirth channel is the first stimulus that the immune system perceives to begin its progressive maturation.  The maternal vagina is impregnated with bacteria with which we usually live together, which in turn will be impregnated in the body of the newborn [1]. Thymus is located in the thorax, thyroid flow, sternum and ventral dorsal to the heart and large vessels.  It is the first of the lymphoid organs to form and grow considerably in childhood. After puberty it begins to involve due to the large influx of sex hormones.  The thymus plays a fundamental role in the development and maturation of the immune system during childhood, specifically in the maturation of T cells [2]. The elimination of this organ months before birth can avoid the development of all cellular immunity, which causes a serious deterioration of immunological defenses that, as it is well known, depend on thymic - derived T lymphocytes [3]. Thymic gland experiences dynamic changes more frequently in the pediatric population, that is why familiarity with embryology, anatomy and pathology of the thymic gland is essential for radiologists to make a precise diagnosis and avoid unnecessary interventions [4]. 

A behavior has been observed that relates the size of the thymus to an increased susceptibility of infants to suffer frequent infections even in a higher number than reported by pediatric texts, which declare for example, children with a normal immune system have an annual average of 6 to 8 infections of respiratory tract during the first 10 years of life, according to the group of infectious pathology of the Spanish Association Primary Care Pediatrics. 1 Rytter, et al. confirmed that the size of the thymus is reduced by nutritional aggressions and infections, relating it to high levels of cortisol and low levels of leptin and hemoglobin that according to animals’ studies, in animals produce atrophy of the organ [5]. Numerous studies have been focused on evaluating changes in its dimensions either natural or secondary to infections, use of immunomodulators, among others, using ultrasound as a tool.  Such is the example of the preliminary study published in 2004 in the Cuban Magazine of Pediatrics, by Rabasa Pérez and collaborators who studied 122 allegedly healthy children in the ages of 8 months to 6 years, defining for the first time the normal values of the area of the thymic silhouette and the thimic index measured by ultrasonography [6].  Consequently, in Cuba the normality range of the thymic silhouette area in healthy children from 8 months to 6 years of age ranges from 1010.6 - 1425.4 mm2, that is, 1 218 ± 207.4 mm2 and the timic index in the range of 82.97 - 117.03 %.  Below and above these values, hypoplasia and hyperplasia would be discussed, respectively. Severe hypoplasia is considered when the thimic area is less than 500 mm2; moderate hypoplasia when the area is between 500 and 799 mm2 and mild hypoplasia, between 800 and 999 mm2 [6].

There have been several methods used to measure the fluctuations of the size of this organ, but thymic ultrasound has allowed the measurement of the size of the thymus quickly, safe and non -traumatic, and even evaluate the changes in the dimensions of this organ.  Being a low cost, laptop and easily available mode without ionizing radiation, ultrasound is useful as frontline research for the evaluation of the suspicion of thymic pathology [7- 9].

There is no biometric standard of the normal parameters of the thymic gland in children between 10 and 14 years of age that serves as a pattern for the diagnosis of conditions associated with the dimensions of this gland, there is only a biometric standard of 8 months to 6 years of age. The research that was proposed aimed to explore and normalize the biometry of the timic gland by ultrasonography in healthy Cuban children from 10 to 14 years of age in the municipality of Arroyo Naranjo, for use in the immune clinical diagnosis, and thus give continuity to the previous studies carried out in the Maternal Child Hospital Angel Arturo Aballi Arellano by Rabasa J and collaborators [6].

A prospective descriptive study, series of cases, was carried out in the Arroyo Naranjo municipality in Havana, Cuba; in the age groups of 10 years old to 14 years old, 11 months and 29 days.  The sample was constituted by children from 10 to 14 years of the Arroyo Naranjo municipality, who complied with the study criteria and whose parents or guardians provided their informed consent to participate in it.

Inclusion criteria: Healthy, eutrophic children, included in the age group from 10 to 14 years, 11 months and 29 days, of both sexes, whose parents or guardians offered their informed consent to participate in the research proposed, that they presented the fingerprint of vaccination with BCG. 

Exclusion criteria: Having used immunostimulant or immunosuppressive medications three months before the realization of measurements, suffering from chronic disease, suffering from some benign or malignant hemopathy, suffering from some immunodeficiency disease either primary (innate errors of immunity) or secondary, that the normal ossification process of the thoracic box prevents the realization of the planned measurements.

We study a total of 68 individuals included in the age of 10 years of age at 14 years, 11 months and 29 days, belonging to the municipality of Arroyo Naranjo, Havana.  There was representation of both biological sexes: 35 male subjects and 33 female subjects.  For a better understanding and analysis of the results we divide the sample into 2 age groups, as shown in Table 1.

Table 1: Ages groups.

To carry out the recruitment of the research subjects, visits to different primary and secondary schools were made, calling for parents’ meetings, with prior coordination with the managers of the educational centers; and visits to the homes of the candidate subjects, providing parents or guardians with extensive information about the study, motivating them through educational talks about the importance and transcendence of the project.  An exhaustive physical examination was performed on the participants, always in the presence of one of the tutors or other specialists who also participated in the investigation and the presence or not of vaccination BCG-fingerprint was verified.  The informed consent of all parents or guardians of the participating subjects were obtained and all their data were collected in the primary data collection model.

During the study, the information could be extended, rectified and corrected by telephone, in case of omissions or incomplete information at the time of filling the primary data collection model when evaluating the research subjects. This implied the collection of the father or tutor telephone number by voluntary decision, thus providing the possibility of obtaining the information for any necessary data rescue or correction. The thymic area was measured by mediastinal ultrasound, using a real -time mobile Toshiba Model SSA 320A model with 3.75 MHz pediatric transducer and 8 MHZ linear and sectorial.  To determine the thimic mass index, the area of the longitudinal ultrasound session of both thymic lobes between the upper edge of the second rib and the lower edge of the fourth rib, at the sternum level, by means of left parasternal cuts, according to the technique introduced by Christian López L and collaborators, and with modification of the pediatric transducer used. 

The areas and volumes of the thymic lobes; and the total area and volume of the thymus were calculated through the following formulas:

Right lobe area (mm2) = length RL x width RL

Left lobe area (mm2) = length LL x width LL

Total thymic area (mm2) = (length RL x width RL) + (length LL x width LL) 

Right lobe volume (mm3) = length RL x width RL x depth RL 

Left lobe volume (mm3) = length LL x width LL x depth LL

Thymic total volume (mm3) = (lenght RL x width RL x depht RL) + (lenght LL x width LL x depth LL) 

Where RL is right lobe and LL is left lobe. 

To process the information, the Graphpad statistical package was used.  For each calculated value, means and standard deviation were determined. Parents and/or legal guardians of the subjects under study were informed about the purpose of the investigation.  The approval to execute the study was received through the signing of the informed consent by the parents and/or legal guardians, a document that was previously prepared and approved by all the corresponding instances and organizations, including the Ethic Committee of the Investigation of the Medical Sciences Faculty Julio Trigo López and the Scientific Council of said institution. It was read and explained by the main researcher of the project, highlighting the degree of privacy and confidentiality of the information obtained and the commitment that, when publishing the data, this would be carried out in an aggregate manner.  The doubts and concerns that arose, making clear the full freedom to participate or not as a subject of the investigation and its right to abandon it at will, were responded without interfering with the continuity or quality of medical care.  The investigation was conducted according to Helsinki's criteria for research in human subjects.  At no time in research have been used technologies assisted by artificial intelligence.

It is important to explore the dimensions both in terms of the area of thimic silhouette, and in terms of thimic volume in children between 10 to 14 years of both sexes. According to the ultrasonographic measurements carried out in this research in children of both sexes, between 10 and 14 years of age, there are no statistically significant differences between the dimensions (long, width and depth) of both timic lobes (right lobe and left lobe).  However, despite not representing a statistically significant difference, there is a slight trend in the long dimensions and depth to major figures for the left lobe.  The above can be observed in Table 2. 

Table 2: Dimensions of both thymic lobes (mm), n=68.

Source: Primary data collection model, data record

We divide the study sample into two age groups (10 to 11 years, 11 months and 29 days; and 12 years to 14 years, 11 months and 29 days); in order to explore if there were differences within the age group that we study in terms of dimensions (long, wide, depth, area of thymic silhouette). There is a discreet decrease in the meant parameters in the group of 12 to 14 years; with respect to the group of 10 to 11 years.  The above can be seen in Table 3. 

Table 3: Dimensions of both thymic lobes according to age (mm), n=68.

Source: Primary data collection model, data record

We set out to explore the possible differences in the measurements made to the thymic lobes (right lobe and left lobe) according to sexes (female and masculine).  A marked difference is observed towards the major dimensions in the subjects of the male sex.   The above can be observed in Table 4.

Table 4: Dimensions of both thymic lobes according to sex (mm), n=68.

Source: Primary data collection model, data record.

When calculating the area of the thymic silhouette (length x width of both lobes); and compare said parameter between groups from 10 to 11 years, 11 months and 29 days; and 12 years to 14 years, 11 months and 29 days; A tendency to a lower area of the thymic silhouette in the group of 12 to 14 years is observed.  The above can be observed in Table 5.

Table 5: Thymic silhouette area according to age (mm2), n=68.

Source: Primary data collection model, data record.

When carrying out the means comparison test (T-test) for the area of thymic silhouette according to the two age groups studied, it turns out to be non-statistically significant, which allows us to affirm that there are no statistically significant differences in the values of the area of the thymic silhouette according to age subgroups within the group between 10 and 14 years.  The above can be observed in Table 6.

Table 6: Total area of the thymic silhouette according to age (mm2), n=68.

Source: Primary data collection model, data record 

When comparing the means obtained for the area of the thymic silhouette according to sexes (male and female); these are statistically significant differences. This finding allows us to make the decision that we will state two average values ± 1 of as a normal reference within the group of 10 to 14 years (one for female subjects and another for male subjects) since according to conventional criteria, this difference is considered extremely significant from the statistical point of view.  The above can be observed in Table 7.

Table 7: Thymic silhouette area by sex (mm2), n=68.

Source: Primary data collection model, data record

As for the thymic volume, there is a discreet difference towards the decrease in the age group of 12 to 14 years, with respect to the group of 10 to 11 years. The above can be observed in Table 8.

Table 8: Thymic volume according to age (mm3), n=68.

Source: Primary data collection model, data record

When evaluating the thymic volume according to the sexes, there is an important tendency to a greater thymic volume in the subjects of the male sex.  According to conventional criteria, this difference is considered extremely significant from the statistical point of view.  The aforementioned can be seen in Table 9.

Table 9: Thymic volume by sex (mm3), n=68.

Source: Primary data collection model, data record 

As for the thymic volume according to the age groups (10 to 11 years, 11 months and 29 days; and 12 years to 14 years, 11 months and 29 days), the described differences are considered non -statistically significant.  The aforementioned can be seen in Table 10.

Table 10: Thymic volume according to age groups (mm3), n=68.

Source: Primary data collection model, data record

Thymus, formerly immersed in the darkness of the mediastinum, has resurfaced to play a role of transcendental importance in cellular immunity.  It is a gland that is located in the anterior mediastinum and its immune function makes it a decisive organ in the development and maturation of T lymphocytes in the fetal and childish stage [10].

Thymic gland has been studied for several years due to its value as a diagnostic marker and prognosis in medical practice [10]. We can cite several studies carried out in children, both healthy and sick, whose purpose was to establish a thymic gland-health relationship that would allow using the biometry of the thymic gland as a clinical-immunological diagnostic tool. Among these investigations is the pioneer study carried out in 1999, by Christian López L and Rabasa Pérez J and collaborators; this research team studied 283 children, included in the ages of 8 months and 6 years of age, valued nutritionally as eutrophic, with recurrent infections, mainly of the digestive, respiratory and skin system. They reached the following conclusions: 73,4% of the children studied presented decreased thymic areas, said hypoplasia associated with primary cause or aggressions either of nutritional, infectious or emotional cause.  In all the age groups studied the most frequent clinical manifestations were high respiratory infections and chronic diarrheal disease. No significant differences were found between the means and standard deviations for any of the dimensions analyzed between the right and left timid lobes.  32,8% of cases presented lower values of serum immunoglobulins levels to those established for their age, and the IgA selective deficit prevailed, so a strong association between the levels of serum immunoglobulins and the size of the thymic silhouette [10] predominated. 

A similar study was published in the Cuban magazine of Hematology, Immunology and Hemotherapy, in 2021, by Guardia Peña O and collaborators, this research team attended in external consultation of pediatric immunology to 62 patients between one and five years of age, with a history of frequent acute respiratory infections, with two or more episodes in a month, to which they were diagnosed by ultrasonography a decrease in thymic area (hypoplasia).  Of these, 50 % also presented decrease in the concentration of IgA in serum [11]. In 2004, Rabasa Pérez J and collaborators, studied 122 allegedly healthy children at the ages of 8 months to 6 years of age, defining for the first time the normal values of the thymic silhouette and the silk index measured by ultrasonography.  The results of this study were already mentioned above in the present work [6]. In order to continue studying the thymic gland and give continuity to the research described above, we decided to expand the sample of study of the normal biometry of the thymic gland by ultrasonography, towards subjects between 10 and 14 years of age, since this group is still understood in the pediatric age and therefore this organ still has an important functional thymic mass in charge of the maturation processes of T cells. 

Thymus is an encapsulated bilobed soft organ.  It is located in the upper mediastinum and in the anterior mediastinum, near the pericardium. It is lodged before the great vessels of heart and deep to the sternum.  It extends from the height of the lower pole of the thyroid gland above, to the fourth costal cartilage.  It has two lobes, right and left, connected to each other on the midline by an isthmus [12]. In our study, no statistically significant differences were found between the dimensions (long, width and depth) of both thymic lobes (right lobe and left lobe).  However, despite not representing a statistically significant difference, there is a slight trend in the long dimensions and depth to major figures for the left lobe, finding consistent with the literature studied [12]. The development of the thymus begins towards the sixth week of gestation.  Throughout the embryonic and fetal period, it grows rapidly and reaches a great weight before birth, in relation to body weight. The involution suffered by the thymus in humans begins during puberty and is completed at the end of the sixth decade of life, so that in adulthood much of the thymic parenchyma has been replaced by fat [13]. In this process its size, weight and activity decrease as a result of the influence that high levels of circulating sex hormones exert over puberty, the low number of precursor cells derived from the bone marrow and the changes suffered by the thymic microenvironment [13]. 

The size of the thymus has shown great variability between different children and in the same infant, in different stages of life and in different situations.  Cause of this is the energy protein malnutrition, in the first place, although there are other causes that produce the same effect, such as the exogenous administration of steroids, emotional and infectious stress that increase the rate of free cortisol and accentuate a phenomenon of programmed cell death of immature sensitive steroids [14]. In our research we divide the study sample into two age groups (10 years to 11 years, 11 months and 29 days; and 12 years to 14 years, 11 months and 29 days); with the objective of seeking significant differences in the month -old of both thymic lobes in relation to age.  We do not find such differences, but a discreet decrease in the parameters analyzed in Group 2, with respect to Group 1. The result described above was expected due to the natural development of the organ.  For this reason, it is also explained that our thymic parameters were lower than those measured by Rabasa Pérez J and collaborators, in 2004, in children from 8 months to 6 years of age, in which the thymic dimensions were greater [14].

Puberty and its influence on the biometry of the thymic gland

Puberty is a very complex biological phenomenon through which the secondary sexual characters are developed, complete sexual maturation is obtained and the carving of the adult is reached.  In humans, puberty manifests not only in the form of hormonal and physical changes, but also with behavioral and psychological change [15, 16]. The puberty period is produced by a gradual increase in the secretion of gonadotrophs hormones by the pituitary, which begins approximately in the eighth year of life and usually culminates with the beginning of the menstruation between 11 and 16 years of age (on average, at age 13) [16]. The beginning and maintenance of puberty results from the interaction between important genetic determinants and a large number of regulatory factors such as nutrition, environmental toxicity, light/dark cycles and psychosocial situation.  These interactions begin in early stages of development, so puberty must be considered the final part of a complex maturative process regulated by interactions between genes and the environment [17-19]. Thanks to the historical studies carried out by Marshall and Tanner in the late 60s, the basis of normal pubertal development knowledge laid.  In this way, on the one hand a cohort was studied composed of 192 girls in which the loom (appearance of the breast button for the first time in women) was presented between 8.5 and 13 years in 95% of girls studied.  On the other, a cohort was analyzed formed by 228 children in which the start of pubertal development was objectified between 9.5 and 13.5 years in 95% of children studied.  Thus, today, it is considered a normal puberty that occurs between 8 and 13 years in girls and between 9 and 14 years in boys [18, 19]. 

The beginning of puberty in girls is marked by the appearance of progressive loom (approximately between 10.5 and 11 years on average) compared to the increase in testicular size (≥ 4ml) observed in boys (about 11.5-12 years on average).  Then, progressively, the rest of the secondary sexual characters develop.  Thus, the time elapsed between the start of the Tanner II pubertal stadium until the pubertal stadium V oscillates in most cases between three and four years. Generally, this interval is lower in girls.  The first menstruation usually occurs about two years after the appearance of loom coinciding with the Tanner IV pubertal stadium.  On the other hand, the change of voice in the child appears in the pubertal stadium Tanner IV.   Also, during the pubertal period an increase in growth speed is objective.  In sum, from the beginning of the Tanner II pubertal stadium to Tanner V, girls grow between 20 and 25 cm on average compared to 25-30 cm that boys usually grow [18, 19]. Normally the thymic gland decreases its size due to the action of the different circulating sex hormones (androgens and estrogens).  According to the bibliography studied, we can state that the hormonal influence present during puberty, manifests itself earlier in girls, suggesting a faster thymic involution with respect to boys [18]. This argument could be demonstrated during the study that is presented and we explained below. 

We compare the measures of the right and left thymical lobes according to sex, observing marked differences between their dimensions, being the timic lobes of the larger male subjects.  The same goes for the areas of the Thimic Silhouette, where they do not yield significant differences according to age but marked disparity between both sexes, the male individuals being again higher.  This finding allows us to propose two average values ± 1 for the total thymic area as a normal reference within the group of 10 to 14 years (one for female subjects and another for male subjects) since, according to conventional criteria, this difference is considered extremely significant from the statistical point of view. 

A normal value for male subjects between 10 and 14 years old was 988.46 ± 233.87 mm2 and for female subjects from 10 to 14 years old was 756.76 ± 166.99 mm2. 

Hence: Total thymic area in male individuals from 10 to 14 years must be included within the range: 754.59 - 1222.33 mm2. Total thymic area in female individuals aged 10 to 14 years must be included within the range: 589.77 - 923.75 mm2.

We decided to add to our analysis the study of the measurements of the thymic volume, because it is also a measure with a deep diagnostic value.  Rabasa Pérez J and collaborators explained it in their preliminary study in 2004, which demonstrated variations with respect to the volume given, possibly due to the differences for the different ages that were studied (from 8 months to 6 years), in the average depth values that increase according to age, suggesting that the successive increase of this organ with age occurs at the expense of depth [6]. When evaluating the timic volume, no distinctions were found between age groups 1 and 2, while an important tendency was observed to a greater volume in the subjects of the male sex.  Finding consistent with the aforementioned referring to the hormonal differences between both sexes that have an impact on the size of the thymic gland. The present study provides valuable and reliable data, for the correct performance in our medical work.  The results that are presented were also obtained by ultrasound, whose technique is fast, harmless and easily accessible.  In our country there are no studies to date on normalization of thymic biometry in children from 10 to 14 years of age.  Therefore, not only gives continuity to previous studies, but opens a gap for future research related to this body in other age ranges.

Mensuration’s of the thymic lobes in healthy children from 10 to 14 years of age were described, observing a slight tendency in the long dimensions and depth to major figures for the left lobe; as well as the month -to -lobes of the timic lobes in healthy children from 10 to 14 years of age, observing a discreet decrease in the parameters in the group of 12 to 14 years; with respect to the group of 10 to 11 years. Mensurations of the thymic lobes in healthy children from 10 to 14 years of age according to sex were described, observing a marked difference towards the major dimensions in the subjects of the male sex.  The area of the thymic silhouette was measured according to subgroups of age and sex, observing a tendency to a lower area of it in the group of 12 to 14 years, not being this statistically significant difference.  As for the sexes, if the difference in the values of the thymic silhouette area was statistically significant.  What allowed us to state that the normal value for male subjects between 10 and 14 years is: 988.46 ± 233.87 mm2 and for female subjects from 10 to 14 years is: 756.76 ± 166.99 mm2.  The volume of the thymic gland in healthy children from 10 to 14 years according to age and sex; finding differences in these values according to sex and not describing significant differences in age.