Dextrocardia is a type of anatomical heterotaxy. Yeah, I know. Big words. Let me break them down for you:
Dextrocardia: from the Latin 'dexter', right; and from the Greek 'kardia', heart: so, right-sided heart. And yes, medicine has a tendency to mix its ancient languages up a little...
Heterotaxy: from the Greek 'heteros', other; and 'taxis', order: so, an abnormal arrangement of parts.
The first thing I've learnt in my medical career is that being able to break words down to their origins is a good skill to have. I mean, if we were to say 'high blood pressure' instead of 'hypertension', the patients might know what we were talking about. That'd ruin the medical profession's image of being considerably cleverer than you. And we couldn't have that now, could we? We have to justify our private consultation fees somehow.
Anyway. Dextrocardia. A right-sided heart. This is a relatively rare condition, with an incidence of 2 per 10,000 live births; the normal heart position for the population is a laevocardia (from the Latin 'laevus', left), which means the heart points down and to the left. With dextrocardia, the heart is anatomically positioned to point down to the right. There are different types of dextrocardia:
Idiot doctor dextrocardia: I've actually just completely made that name up, but it's the best way of describing a situation where a medical professional puts a chest x-ray on a light box the wrong way around; the most classic example of this being the end of the opening credits of Scrubs. Every time I watch that show, those few seconds of screen time make my eyes hurt. Another version of idiot doctor dextrocardia occurs when people attach ECG chest leads incorrectly. ECG tracings are hard enough to read at the best of times; when some idiot mixes up the arm and the leg, it's actually impossible.
Dextroposition: This is where the anatomy of the heart is normal, but there is another anatomical abnormality that is pushing the heart away from its usual position in the chest, such as a scoliosis.
Developmental dextrocardia:
- with situs inversus, where both the contents of the chest and the contents of the abdomen are the wrong way round (reversed symmetry), so as well as the heart being on the right, the liver is on the left, and the spleen is on the right. (occurs in 39% of developmental dextrocardia cases (Garg et al, 2003))
- with situs solitus, where the chest contents are reversed, but the abdominal contents are anatomically normal (normal symmetry). (34%)
- with situs ambiguus, where the anatomical positioning of organs is indeterminate. This is anatomically and developmentally fascinating, but maybe I should leave the details for another writeup. (27%)
The embryology of dextrocardia
The stage where an embryo organises itself into 'left' and 'right' occurs relatively early in its development. There are two sets of contractile pre-cardiac cells that are brought together when the embryological disc structure folds in half; these then fuse together to form the heart tube. Up until this point (around day 21) the embryo was symmetrical; now the heart tube starts to deviate, twist, loop and fold. Around day 22-23, it rotates so that the heart's apex points to the left. By day 28, the embryological heart has formed the four chambers, and the rudiments of the great vessels. And it does all this without missing a beat.
So, what goes wrong in dextrocardia? Well, there's no definitive reason; research suggests that the cause could be gene-linked, but that other risk factors include maternal diabetes; family history of malformations; cocaine use during the 2 months before conception and through the first trimester; or being a conjoined twin all increase the risk of heterotaxy (Olbrich et al, 2002). Whatever the precipitating cause may be, the error itself is thought to occur on day 23/24 when the bulboventricular loop of the embryological heart folds to the left rather than the right.
Prognosis
The prognosis of developmental dextrocardia depends on the type of abnormality. Those with situs inversus are usually completely normal and have the same life expectancy rates and coronary disease rates as those who are laevocardia. The only problem is with regard to medical care; a situs inversus dextrocardiac who presents to A&E acutely unwell will have a delay in the correct treatment being given while the medical staff try to work out which way around everything goes. The exception to this is for those whose situs inversus is associated with Kartagener's syndrome. This syndrome is composed of a triad of abnormalities: situs inversus; abnormal development of the frontal sinuses that causes a constant sinusitis and bronchiectasis; and immobility of the various cilia found throughout the body. Kartagener's syndrome is seen in 20% of those with situs inversus.
Those with situs solitus tend to have a lot more problems; there are commonly other developmental abnormalities seen, such as a transposition of the great vessels. The worst prognosis is for those with situs ambiguus, which is almost always associated with severe cardiac developmental abnormalities and cardiovascular impairment.
Dextrocardia and medical education
Patients with dextrocardia are usually identified as a child or a teenager, even if it's an asymptomatic situs inversus. They then find themselves being utilised as teaching tools every time they go to the doctors for the next sixty years. I've encountered two dextrocardiacs so far in my medical education.
The second thing that I've learnt in my medical career is that if, after doing an examination of (for instance) the abdomen, the consultant then asks you to take a quick listen to the heart, 99/100 times it means there's an interesting and obvious abnormality there that they'd like you to pick up. The other 1/100 times, they're just messing with your head. The bastards. The abnormality in this situation is either going to be a heart murmur, or dextrocardia. Guaranteed. My consultant tried to pull this trick on me today. After listening to the centre of the chest to confirm there was no audible murmur, I immediately knew that the patient had a dextrocardia, and so moved my stethoscope to the right rather than the left to listen to the heart's apex. Guess what; I was right.
In a later teaching session with the same consultant, I was hounded relentlessly for an entire hour about the physiology and treatment of heart failure and acute pulmonary oedema. The third thing that I've learnt in my medical career is that no one likes a smart-arse.
References
- Garg N, Agarwal BL, Modi N, et al, 2003, "Dextrocardia: an analysis of cardiac structures in 125 patients", Int J Cardiol; 88:143-155
- Marieb E N, 2001, "Human Anatomy and Physiology", 5th edition, Addison Wesley Longman
- Olbrich H, Haffner K, Kispert A, et al, 2002, "Mutations in DNAH5 cause primary ciliary dyskinesia and randomization of left-right asymmetry" Nat Genet.; 30:143-144