Midline cysts of the anterior brain are common in infants e.g. cavum septum pellucidum et vergae. We report twin brothers with midline intracranial cysts, mildly increased ventricular size, and macrocephaly. One brother had a cavum septum pellucidum and vergae and the other a cavum septum pellucidum. With the obliteration of each brother’s midline cyst the head circumference was noted to return to a normal head circumference curve and the ventricles decreased in size. We hypothesize that in infancy some cases of infantile macrocephaly are induced by a midline cyst and that as the cyst obliterates that physiologic cerebrospinal fluid pathways are established so that the head circumference returns within normal parameters. This knowledge should be kept in mind by the clinician who may wish to surgically intervene with a midline cyst prior to it demonstrating its natural history. Moreover, these data should especially be entertained in the premature infant. (J Pediatr Neurol 2004; 2(2): 107-110)
