University of Messina, Italy.

Synonyms:  None
Definition: Mesocardia means "heart in the middle of the thorax". In mesocardia, the heart is centrally located in the chest.
The apex is central or rightwardly displaced in the chest.

Cardiac Axis
The cardiac axis describes the orientation of the apex of the heart within the thorax.
Terms used to describe cardiac axis orientation:

Levocardia: The cardiac apex points to the left (normal). Greater than 57 degrees of leftward axis deviation may be considered abnormal . 
Mesocardia: The cardiac apex points to the midline 
Dextrocardia: The cardiac apex points to the right

Cardiac Positon
Cardiac position describes the location of the heart within the thorax.
Terms used to describe cardiac podition:

Levoposition: The heart is in the left thorax. 
Dextroposition: The heart is in the right thorax.
Mesoposition: The heart is in the middle thorax

Mesoversion Type I: heart vertical and central with both mixed mesocardia with ventricular inversion and mixed mesocardia with atrial inversion
Mesoversion Type II: inversion of both right and left chambers

Etymology: Greek:  Mesos, middle; Kardia, heart. 

Prevalence: Rare: 0.2:10.000 deliveries; 0,2 % of congenital heart disease, Strong male preponderance: male/female 11/5 ^{(1, 3)}

Etiology:  Cardiac location is affected by many factors including underlying cardiac malformation, abnormalities of mediastinal and thoracic structures (eg., Bronchogenic cysts, lung mass, tumors, esophageal atresia, diaphragmatic hernia, kyphoscoliosis, abnormalities of the diaphragm ecc.), Otherwise  mesocardia reflects that embryonic malrotation before the formed heart points to the right or left and acquires an apex, situs viscerum inversus.

Pathogenesis: Bulboventricular loop A portion of the embryonic cardiac tube  that grows so that the tube assumes an S-shape. The ventricles evolve from the loop. The type of bulboventricular loop describes the relationship of the ventricles to each other. In a D-loop (dextro loop) the morphological right ventricle is located to the right of the morphological left ventricle. In L-loop (levo loop) the morphological right ventricle is located to the left of the morphological left ventricle

Sonographic findings: Demonstration a midline cardiac apex and frequently abnormal cardiac contour and pulmonary vascularity

Implications for targeted examinations:  Special attention should be directed to the detection of inversion of symmetrical anatomical landmarks (situs inversus, situs ambiguous).  Complex anomalies such as transposition of great artery (TGA), single ventricle (SV), common trunk (CMT), ventricle septal defect (VSD) and pulmonary stenosis.
Assess the situs by the position of the: Heart, Apex, Stomach and Great vessels.

Differential diagnosis: dextrocardia, dextroversion.

Associated anomalies

Thoracic defect

• Situs inversus
• Bilobed lung, bilateral
• Polylobulation of both upper lobes of lung
• Abnormal lobation of lungs
• Incomplete lung trasverse fissure
• Kyphoscoliosis
• Spondylocostal dysplasia
• Lung mass
• Bronchogenic cyst
• Thymic mass

Abdominal situs

• Solitus
• Inversus
• Ambiguous

Spleen

• Asplenia
• Polysplenia
• Accessory spleen

Heart

• Coronary artery distibution
  • Dominant left
  • Ostia exchange vessels
  • Ostia rotated
• Ventricular septal defect
• Atrial septal defect
• Tricuspid stenosis
• Pulmonary stenosis
• Complete transposition
• Partial inverted transposition
• Partial transposition with common A-V orifice
• Common A-V
• Persistent ostium primum complex
• Single ventricle
• Ebstein's anomaly
• Bicuspid pulmonic valve
• Bicuspid aortic valve
• Right subclavian from descending aorta
• Absent coronary sinus
• Complete A-V block
• Cleft aortic leaflet of mitral

Chromosomal anomaly  

• Trisomy 18
• Trisomy 13

Other

• Esophageal atresia
• Diaphagmatic hernia
• Hydrops Fetalis
• Renal agenesis

Prognosis:   Good for isolated form, without associated anomalies.

Recurrence risk:  Unknown. Depend upon the underlying etiology in the secondary form.

 
Reference
1) Moss" Heart Disease in Infants, Children and Adolescents
Forrest Adams (Editor), Thomas A. Riemenschneider, George C. Emmanouilides, Forrest H. Adams
Hardcover, March 1989

2) The Clinical Recognition of Congenital Heart Disease
Joseph K. Perloff
Hardcover, March 1994

3) Syndromes of asplenia and polysplenia. A review of cardiac and non-cardiac malformations in 60 cases with special reference to diagnosis and prognosis.
Rose V, Izukawa T, Moes CA. Br Heart J 1975 Aug;37(8):84052

4) Asplenia-polysplenia syndromes
Sandra R Silva, MD & Philippe Jeanty, MD, PhD
1999-05-17-03 Asplenia-polysplenia syndromes © Silva /www.thefetus.net/

5) Adult congenital heart disease.
Marelli AJ, Moodie DS:
In Topol EJ (ed): Textbook of Cardiovascular Medicine. Philadelphia, Lippincott-Raven, 1998

6) Sporadic occurrence of spondylocostal dysplasia and mesocardia in a Japanese girl.
Am J Med Genet 1990 Nov;37(3):427-8
Ohzeki T, Shiraishi M, Matsumoto Y, Takagi J, Motozumi H, Hanaki K, Ishitani N, Matsuda-Ohtahara H, Okuda H, Hoshika T, et al.

7) Developmental defects of the lungs.
DaCosta H, Pathak A, Noronha O, Dalal S, Shah K, Merchant S.
 Eur J Nucl Med 1981;6(6):265-7

8) Polyasplenia complex with mesocardia and renal agenesis in an infant of a diabetic mother
Gonzalez A, Krassikoff N, Gilbert-Barness EF.
Department of Pathology, University of Wisconsin, Madison
Am J Med Genet 1989 Apr;32(4):457-60