Abstract
In this condition, which affects one in every 8,500 people the chest and/or abdominal organs have their position inverted on the saggital plane, constituting a perfect mirror image. The relations between the organs are the same, so normally it won’t bring any harm to the patient with such condition. The onset of the situs inversus begins in the embryonic stage, when the tubular structure that will be the heart forms its loop.
However, there are situations of heart defects associated with this condition (or of other organs) and such represent much more severe risks for the patient. If any defect or abnormality is absent, then the person is expected to live normally and without any decrase on the life expectancy.
Keywords: situs inversus, organs, saggital, plane, right, left, relation, symptoms, problems, life, heart, defects, consequences.
Situs Inversus
Situs inversus is defined as being a not very usual condition, in which the heart and other organs of the human body have their position inverted saggitally, lying on the opposite side of the human.
When the development of the human body is normal, there is an assymetrical arrangement of the internal chest and/or abdominal organs, being that the heart is placed on the left side of the chest cavity (levocardia), the liver and spleen are on the right, the left lug had two lobes and the right lung has three – the so called situs solitus.
However, 1 in every 8,500 people has their chest and/or abdomen organs with their places inverted in the saggital plane. On this case, one finds the heart on the right side – dextrocardia – together with the lung that has two lobes, and the liver, spleen and three-lobed lung are on the left side. It is a mirror image of the organs location solely, which means that the relations between them are the same, not implying, in general, any harm to the person’s health.
This condition is related to the human development in the embryonic stage. Humans’ (or other vertebrates’) body morphology and implied asymmetry develops on the embryogenesis stage, where the left-right axis is determined by the dorsal-ventral and cephalocaudal axes that are formed. Asymmetry begins with the formation of the cardiac tube’s curve to the right (cellular information, however, has already determined which is the left-right gradient). So, on the thorax and abdomen, the structures are place on one of the sides of this median line: heart with aortic arch, stomach and spleen are located on the left; on the right, one can find the liver and gall bladder. This is the situs solitus.
Moving on to a situation in which a complete situs inversus is present, the lateralized (to the left or right) organs switch their positions to a complete mirror inversion. But this is not the only form that a human body can present of an abnormal, or unusual, internal organ organization and structure.
It is also possible to have an absent lateralization of an organ – situs ambiguous (also called heterotaxia . It can be either absent or present on both sides – asplenia, or polysplenia syndrome, respectively. Both are very serious conditions that cause the heart’s apex to be located in either of the hemitorax, associated with common heart abnormalities. The asplenia cases have worse prognosis. This type of heart condition that is called right isomerism, or bilateral right-sidedness, when present, presents the atria both having right morphology, the liver is placed medially, having identical left and right lobes, the spleen is absent and both lungs are tri-lobed. In polysplenia, where there is left isomerism or bilateraç left-sidedness, there is an association with a left and bilateral atrial morphology, the presence of multiple spleens of small dimensions, no inferior vena cava’s intrahepatic segment, and bilateral bi-lobed lungs.
About the abnormalities in position that concern the heart, this can be located, as said before, on the right side instead of the left – dextrocardia – with the apex pointing to the right instead of left, in the middle (or on the median line, as it can also be said) – mesocardia – with the apex being probable to point either to the left or right, on the right side (dextrocardia) with situs solitus – dextroversion –, and on the left side with situs inversus - levoversion. Where the atria are located is the factor that determines the heart’s situs. Both in situs solitus and inversus, the ventricles can present a right-sidedness or D-loop – the right ventricle being anterior and positioned on the right of the left ventricle, which is normal – or have an L-loop, or left-sidedness – right ventricle posterior and to the left of the left ventricle, case in which they present an inversion.
Going back to what complete situs inversus comprehends; this is a rare, autosomal recessive genetic disorder, affecting 1 in every 10,000 births, distributed equally between the genders and races. For this situation to be present, the cardiac tube changes its rotation, being inverted, when in the embryonic stage. There is not much information about what molecular events determine the antero-posterior and dorsal-ventral axes, or left-right asymmetry, but recent genetic studies have found genes that seem to be involved with the left-right position and their mutations associated with situs inversus cases.
For example, lefty 1, 2 and nodal genes are expressed mainly on the left side of the ventral neural tube and left side of the lateral plate mesoderm, respectively, which is suggesting a relation with TGF-® (transforming growth factor); iv gene, on chromosome 14, has the code for the heavy chain of axonemal dynein, an ATPase that is related to ciliary motion and, thus, associated to organ positioning when on the embryonic stage; HAND genes (being dHAND and eHAND expressed on the right and left ventricles, respectively) are related to the development of the cardiac tube’s segments and not the rotation to the right that is suffers, being its expression inverted on situs inversus cases; ZIC3 gene was found, in clone studies, on chromosome Xq26.2, at the locus HTX1, and as possibly influencing the left-right asymmetry; the Shh gene, which in normal cases is expressed on the left side, determines the heart’s position during the embryonic stage and, when bilaterally expressed, is also thought of causing this type of condition; mutations on the ACVR2B gene have, in rare cases, also been associated with left-right axes malformations, being that this gene codes for type IIB activin receptor; the Pitx2 gene is also thought to be involved in such cases, being its left lateral plate mesoderm’s expression induced by inhibition of Shh and nodal genes, and also blocked by the inhibition of activin.
So, in resume, when in the embryonic stage, the structure with a tubular form that will later become the heart, when developing its left loop, will determine the left-right axis and this one, on its turn, the position of the organs that will be formed. Why this left loop occurs is not fully understood yet, but as it was described, there are genes already identified as being part of the cause, together with other possible factors.
In general, people that have situs inversus do not present symptoms or any complications that may result from it, but 3-5% do have a functional heart disorder, which was already said above and it is a higher rate than the one of population in general suffering from such heart abnormal conditions (1%). So, situs inversus patients do have a higher probability for heart disorders and, in some cases, these can also be more serious and hard to resolve.
There are also other health conditions that can be related or coexist with situs inversus: an estimate of 25% of the population with situs inversus may also have primary ciliary dyskinesia (PCD), a disorder found on the cilia, which impairs the normal movements of these structures; together, they form a disorder that is called the Kartagener’s Syndrome. This health condition is a serious one, which, along with situs inversus, is associated with chronic infections of the sinus, an increase of lungs’ mucous secretions and also an increase of respiratory infections’ susceptibility.
A situs inversus situation can be diagnosed through a simple physical examination, like hearing the heart sounds with a stethoscope, which on this case will be heared on the right side, instead of being heard on the left. To further confirm the situation, Magnetic Resonance Imaging, Computerized Tomography or Ultrasound exams can be performed. Also, having some relation to heart conditions, it may be advised to consult a cardiologist or internist. With those cited exams, a full understanding of how the organs are arranged can be present; if it is random – heterotaxia – this will represent an increase on the risk for having other complications that can also be associated in a severe health disorder.
As it can easily be perceived, such condition does not have a treatment. If a heart disorder is present, than this one may have the according treatment, performed by a cardiologist. Of course, the patients must know that they should always inform every physician they consult when treating their health, so that the knowledge of this condition, a priori, may reduce diagnosis complications, time to establish the diagnosis and to be more aware of the possible associated complications.
When a heart disorder or defect is absent (or of any other kind) the patient with situs inversus is expected to have a pretty normal life, not being expected any complications or decrease in life expectancy.
Conclusion
Situs inversus, in conclusion, is a rare condition, only observed in one in every 8,500 people of the general population, and present in one in every 10,000 births, characterized by opposite placement of the chest and/or abdominal organs in the saggital plane. Its onset is related to the loop formation on the tubular structure that later on will form the heart, during the embryonic stage. There are also genetic factors, but these need further investigation.
It has no distinction between races or gender and is not necessarily associated with life-threatening situations, but in a small percentage, there may be heart defects or of other organs, which will need correction by a specialized surgeon.
Without any defect, a person with situs inversus is expected to have a norma life. However, it should be informed to every consulted physician the presence of such condition, in order for not complicate or delay needed diagnoses.
References
Definition of Situs Inversus Totalis. MedicineNet, n.d. Retrieved from http://www.medterms.com/script/main/art.asp?articlekey=5499
Marta, M. João, Falcão, L. Menezes, Saavedra, J. A., Ravara, Luciano (2003). A Case of Complete Situs Inversus. Retrieved from http://www.spc.pt/dl/rpc/artigos/511.pdf
Situs Inversus.Genetic and Rare Diseases Information Center (GARD), n.d. Retrieved from http://rarediseases.info.nih.gov/GARD/Condition/4883/Situs_inversus.aspx
Situs Inversus. The Free Dictionary by Farlex, n.d. Retrieved from http://medical-dictionary.thefreedictionary.com/situs+inversus
Sorrentino, Sajoscha, Gaillard, Frank. Situs Inversus. Radiopaedia, n.d. Retrieved from http://radiopaedia.org/articles/situs_inversus
