Discussion

Vitamin D is a fat-soluble vitamin essential for bone metabolism and calcium homeostasis. It can be synthesised after sunlight exposure or absorbed from the diet or supplements. Maternal vitamin D deficiency during pregnancy, exclusive breastfeeding without supplementation, skin pigmentation, decreased sun exposure, malabsorption, and certain medications can all lead to vitamin D deficiency. ^{Reference Weisberg, Scanlon, Li and Cogswell1} Hypocalcaemia secondary to calcipenic rickets is an uncommon but known aetiology of dilated cardiomyopathy. ^{Reference Price, Stanford, Braden, Ebeid and Smith2–Reference Fabi, Gesuete, Petrucci and Ragni8} Calcium release from the sarcoplasmic reticulum is essential for cardiac muscle contraction and inadequate levels may impair cardiac contractility and precipitate, or worsen, existing heart failure. ^{Reference Marks9}

In the United States, we are aware of only three similar reports. Two were individual case reports from 2003 and 2007. ^{Reference Price, Stanford, Braden, Ebeid and Smith2,Reference Cramm, Cattaneo and Schremmer3} The last was a single centre case series of four patients from 2009. ^{Reference Brown, Nunez, Russell and Spurney4} In each of these cases, the patients were African-American, breastfed, and between 4 and 10 months of age when diagnosed. This is consistent with a systematic review from 2013, which found a mean age of diagnosis of 5 months. ^{Reference Elidrissy, Munawarah and Alharbi5} While our patient was also African-American and exclusively breastfed, he was significantly older at 15 months of age. We could only find two other case reports of a toddler developing this condition in the past 50 years; one being a 14-month-old from Turkey in 2007 and the other a 15-month-old child from India in 2011. ^{Reference Kosecik and Ertas6,Reference Verma, Khadwal, Chopra, Rohit and Singhi7} In all three cases, there was prolonged breastfeeding with failure to provide vitamin D supplementation or introduce solid foods.

The clinical presentation of rickets-associated cardiomyopathy is variable, ranging from tachypnoea and cardiomegaly to cardiogenic shock requiring extracorporeal membrane oxygenation. ^{Reference Elidrissy, Munawarah and Alharbi5} In our case, the patient suffered a cardiac arrest after presenting in cardiogenic shock and required inotropic support while initiating treatment with calcium supplementation. Another unique feature that is inconsistently present in described cases is electrocardiogram changes, including prolonged PR interval, prolonged QTc, and T wave changes. ^{Reference Fabi, Gesuete, Petrucci and Ragni8} In our patient, the first electrocardiogram was notable for QTc prolongation and T wave abnormalities, which resolved by the time of discharge. However, it is worth noting that these changes can also occur in the setting of sick myocardium and typically resolve over time as the patient clinically improves. Although the prognosis for dilated cardiomyopathy is often poor, the outcome of calcipenic rickets-induced cardiomyopathy is typically quite favourable with an 8.5% incidence of mortality in a systematic review. ^{Reference Elidrissy, Munawarah and Alharbi5} The average time for systolic ventricular function to normalise is 12 months; however, it may take up to 28 months for complete recovery after diagnosis. ^{Reference Brown, Nunez, Russell and Spurney4} In our case, the patient demonstrated normal systolic function by 19 months post diagnosis.

Vitamin D deficiency is an underrecognized condition with an increased prevalence of nutritional rickets in exclusively breastfed infants over the last few years. ^{Reference Price, Stanford, Braden, Ebeid and Smith2} Current American Academy of Pediatrics guidelines recommend 400 IU/day of vitamin D for all exclusively breastfed infants. ^{Reference Casey, Slawson and Neal10} Because of the potential life-threatening implications, routine supplementation, early recognition, and prompt correction of hypocalcaemia are paramount.

Vitamin D deficiency should be considered as a possible aetiology of dilated cardiomyopathy in any exclusively breastfed infant as it is easily treatable with an excellent prognosis.