Dicephalic parapagus - A rare variety of conjoined twins

A 29-year-old primigravida with a history of six months of amenorrhoea was referred to our department for the second-trimester anomaly scan. The conception was spontaneous after ten years of married life with a history of second-degree consanguineous marriage and history of treatment for infertility five years back.

The antenatal ultrasound (Fig 1) showed two fetuses corresponding to 19 weeks gestation with evidence of conjoined twinning. Ultrasound showed two heads, three upper limbs, a fusion of anterolateral aspects of the trunks with two thoracic cavities, two fused cardiae, two kidneys, one bladder, two lower limbs and two vertebral columns with a single pelvis. Fetal Magnetic Resonance Imaging (MRI) (Fig 2) confirmed the ultrasound findings. Conventional radiograph (Fig 3) and Computed Tomography (CT) (Fig 4) were performed after the demise of the abortus and demonstrated the presence of two separate vertebral columns, three upper limbs and two lower limbs. Two separate set of fingers were seen arising from the medial upper limb. Gross specimen (Fig 5) and autopsy findings suggest the diagnosis of a dicephalic parapagus variety of conjoint twins.

Conjoined twinning is a rare event occurring in only 1/50000-1/100000 pregnancies [1]. Of these, less than 0.5% of the cases are dicephalus parapagus twins, which is, therefore, a very rare occurrence [2].

‘Dicephalus’ is one among the various subsets of parapagus. Here the twins have two heads, sharing a common body from the neck or upper chest downwards, with a pair of lower limbs and a single set of reproductive organs [2].

Conjoined twinning is explained by two proposed theories. First is the fission theory which postulates that incomplete fission may be seen in a single embryonic disc, 13 to 15 days after the fertilisation of the ovum. The second theory is the fusion theory which postulates that two separate monovular embryonic discs could secondarily fuse leading to this developmental anomaly [3].

Parapagus twins are fused ventrolaterally and lie side by side. These twins are most frequently conjoined at the chest. However, respiratory, gastrointestinal and genito-urinary tracts are separate with fused liver and diaphragm. Two spinal cords, two vertebral columns, two legs and a variable number of upper limbs are present. Complex anomalies of the heart have been described in previous studies which comprise of the defects of lateralisation, ventricular cardiac union and a common pericardium [3].

Conjoined twins can be detected as early as 12 weeks of gestation on ultrasonography. Conjoined twinning should be kept in mind when monozygotic twins are encountered. Both twins in face-to-face position or in breech with polyhydramnios should raise the suspicion of conjoined twinning [3].

MR imaging demonstrates greater anatomic detail compared to ultrasonography. Thus fusion or separation of the visceral organs can be well assessed [4].

The prognosis is determined by the degree of fusion. Continuation of pregnancy is advised in cases without organ anomalies and when the surgical separation of the twins can be done [5]. Premature birth is common in conjoined twins, being stillborn in 40% of the cases and death within first 24 hours of life occurs in 35% of the cases [6].

Allowing the twins to remain conjoined is not possible because of a large degree of shared organs which prevents survival. If the surgery is not an emergency, it is recommended to delay it until late infancy to allow detailed preoperative assessment [6].