Situation D and M are conjoined twins born without an antenatal diagnosis and assessed as not suitable for separation. At the time of admission they were 21 months old with a combined weight of 17.1 Kg. D presented unwell with a raised heart rate and respiratory rate. A working diagnosis of sepsis (possibly urinary tract infection) was made. Advice was sought from pharmacy on the doses of ceftriaxone and paracetamol. Peripheral intravenous (IV) access was only available in twin M.

Background The twins are joined side by side from the upper chest to the pelvis. They have separate heads, three arms and 2 legs. They have 2 hearts with a fused aorta, a shared liver, 2 gallbladders, 2 stomachs, 3 kidneys and a single bladder. D has a complex congenital heart condition and a poor prognosis. On admission, D was receiving propranolol, but M was not. The dose was based on the combined weight of the twins divided by 2. Conjoined twins are a rare phenomenon, occurring 1 in 50,000 to 100,000 births.¹ Around 60% of these are stillborn or die shortly after birth. There are many different types of join with differences in shared organs and limbs. Consequently each twin pair is almost unique and consideration must be given as to how medication is dosed according to pharmacokinetic principles.

Outcome Opinion of the multidisciplinary team was that the twins have relatively separate circulations, although some cross-circulation would be expected. On admission, saturations in the right arm (twin D) were 75%. On the left side (twin M) this was 95%. Ceftriaxone is a highly protein bound, hydrophilic antibiotic,² The degree of cross circulation (how much blood volume is shared between the twins) would affect the volume of distribution and hypoalbuminaemia was likely to increase the apparent volume of distribution. Based on this, ceftriaxone dosing was advised on the combined weight of the twins and given at 50 mg/Kg to M only. Ceftriaxone is excreted mainly unchanged in the urine and bile with little renal clearance or hepatic metabolism so this was not a concern. After 2 days, Ds CRP had reduced and the twins were switched to oral amoxicillin. Dosing was based on the combined weight of the twins and each was given half the dose. As each twin has a separate stomach, it was assumed relatively individual enteral absorption occurs. Ds CRP continued to drop and the twins were discharged home on day 4 with a further 3 days of oral amoxicillin. Paracetamol dosing was advised at 15 mg/kg based on the combined weight and half given to each twin. As required use was agreed, as there was uncertainty over the amount of hepatic metabolism that would occur by the twins shared liver.

Lessons learnt Conjoined twins are a complex yet interesting challenge in terms of medication dosage and administration. There is a lack of evidence and dosing has been based on pharmacokinetic principles and adjusted according to clinical response.

References

1. Owolobi AT, Oseni SB, Sowande OA, et al. Dicephalus dibrachius dipus conjoined twins in a triplet pregnancy. Tropical Journal of Obstetrics and Gynaecology 2005;22:87–88.

2. Electronic Medicines Compendium: Rocephin 1g powder for solution for injection or infusion - Summary of Product Characteristics. http://www.emc.org.uk/(Accessed 12 Jun 2018)