This is thought to be the most likely reason why conjoined twins occur, and could possibly play a role in the development of craniopagus parasiticus.One hypothesis is that craniopagus parasiticus starts with the development of two fetuses from a single zygote that fail to separate at the head region around the second week of gestation. Some believe, however, that it occurs later in development, around the fourth week of gestation, at which time the two embryos fuse together near the anterior open neuropore.Another hypothesis is that there is joining of the somatic and placental vascular system of the twins, as well as a degeneration of the umbilical cord of the parasitic twin. This suggests that craniopagus parasiticus develops due to the lack of blood supply to one of the twins. Diagnosis
Related conditions
In addition to craniopagus parasiticus, a few other conditions involve a childs, or animals, birth with two heads or faces. Dicephalic parapagus
Dicephalic parapagus is a condition where two heads are side by side on a single torso, with varying degrees of twinning of organs and structures within the torso. Diprosopus
Diprosopus occurs when a single body and neck are present, but there is a duplication of the structures of the face. This is different from craniopagus parasiticus in that there is only one head, although there is a duplication of the craniofacial features. Diprosopus can range from having two fully formed faces to just a duplication of the nose or eyes. Cats with the condition are referred to as Janus cats.
Higher plasma concentrations also lead to cardiovascular effects, though cardiovascular collapse may also occur with low concentrations. Adverse effects on the central nervous system may indicate impending cardiotoxicity and should be carefully monitored. Central nervous system: circumoral numbness, facial tingling, vertigo, tinnitus, restlessness, anxiety, dizziness, seizure, coma
Cardiovascular: hypotension, arrhythmia, bradycardia, heart block, cardiac arrestToxicity can also occur in the setting of subarachnoid injection during high spinal anesthesia. These effects include: paresthesia, paralysis, apnea, hypoventilation, fecal incontinence, and urinary incontinence. Additionally, bupivacaine can cause chondrolysis after continuous infusion into a joint space.Bupivacaine has caused several deaths when the epidural anaesthetic has been administered intravenously accidentally. Treatment of overdose
Animal evidence indicates intralipid, a commonly available intravenous lipid emulsion, can be effective in treating severe cardiotoxicity secondary to local anaesthetic overdose, and human case reports of successful use in this way. Plans to publicize this treatment more widely have been published. Pregnancy and lactation
Bupivacaine crosses the placenta and is a pregnancy category C drug. However, it is approved for use at term in obstetrical anesthesia. Bupivacaine is excreted in breast milk. Risks of stopping breast feeding versus stopping bupivacaine should be discussed with the patient. Postarthroscopic glenohumeral chondrolysis
Bupivacaine is toxic to cartilage and its intra-articular infusions may lead to postarthroscopic glenohumeral chondrolysis. Pharmacology
Pharmacodynamics
Bupivacaine binds to the intracellular portion of voltage-gated sodium channels and blocks sodium influx into nerve cells, which prevents depolarization. Without depolarization, no initiation or conduction of a pain signal can occur.