BPC, a cystic malformation in the posterior fossa, is thought to be caused by the absence of spontaneous regression of Blake’s pouch. In normal subjects, the Blake’s pouch disappears with leaving a median opening, Blake’s metapore, considered as the precursor of the foramen of Magendie . Although approximately 20 cases have subsequently been reported in the literature, the incidence of this rare malformation is not well known, and neither its pathogenesis nor natural history is fully understood.
Diagnosis of this anomaly is mainly based on its radiological features. The main characteristic is the cyst communicating with the fourth ventricle with protrusion of the choroid plexus into the cyst. Although the vermis often has slight hypoplasia because of the mass effect, neither cerebellar rotation nor hypoplasia is observed. The confluence of sinuses is located in the normal position. These features not only reflect the embryological background of this malformation but are also useful for differential diagnosis from DWC, DWM, and MCM. Our case completely matched these features and was confirmed by intraoperative neuroendoscopic observation through the aqueduct and the fourth ventricle.
According to past case reports, hydrocephalus is the most common condition in BPC. However, there are some asymptomatic cases with BPC, which were discovered incidentally after routine examination by ultrasonography  or MR imaging . Furthermore, antecedent triggers such as meningitis existed in some , showing that the clinical presentation of BPC differs among patients.
In addition to the wide variety of BPC symptoms, the age at diagnosis is diverse. The majority of patients with BPC are infants [4, 7]. Recently, greater awareness of this entity has increased the rate of prenatal diagnosis of BPC by obsteric ultrasonography [5, 8], which is consistent with the proposed pathogenesis of BPC. By contrast, interestingly, there are some adult patients who are diagnosed with normal pressure hydrocephalus-like initial symptoms [3, 4]. One question raised, especially with those adult patients, was has BPC existed since the failure of Blake’s pouch’s regression during embryogenesis, with a long asymptomatic period?
The present case, which shows clear evidence that BPC developed a few years after birth, clearly suggests that some BPC might occur postnatally. Our case is unique in the sense that repeated MRI was performed before the diagnosis of BPC, verifying that BPC developed in the second or third year of her life, not during gestation. We speculate that in addition to the classic congenital BPC in which the remnant of Blake’s pouch remains persistent, there could be postnatal or secondary BPC, which develops after birth for some as yet unknown reason.
Another question raised by our case was how does postnatal or secondary BPC occur? Possible mechanisms include that the remnant of Blake’s pouch, which originally disappears, may re-expand after birth with some unknown triggers or a change in CSF dynamics or absorption (Fig. 4, modified from ). Even if the remnant of Blake’s pouch persists and the foramen of Magendie is not completely formed, CSF can circulate through the foramina of Luschka or the small Blake’s pore to reach the subarachnoid space; therefore, the patient does not develop hydrocephalus, which is similar to the situation in the first 2 years of our patient. Subsequently, deterioration of the CSF absorption system was induced by the multiple tumors in our case and CSF flow via foramina of Luschka, or the small pore could no longer compensate, which resulted in re-expansion of the pouch and the onset of hydrocephalus. Considering our case, the intraoperative observation confirmed that the foramina of Luschka was open without any membranous structure to block the CSF outlet from the foramina to the subarachnoid space. Therefore, the widely disseminated tumor in the ventricle’s wall might impair the transventricular absorption of CSF through the brain parenchyma, which resulted in stagnated CSF and finally induced re-expansion of the BPC. Some past cases, excluding the prenatally or neonatally diagnosed cases, which may be classified as classic congenital BPC, were exposed to a trigger altering the CSF environment, such as meningitis  or an association of syringomyelia . This hypothesis explains why BPC, originally thought as congenital, is sometimes found in adults or even the elderly. However, this is not certain and should be validated in a much larger series of cases.
Although the standard surgical treatment of hydrocephalus associated with BPC has not yet been established, it is more commonly managed by ETV, which shows a relatively good outcome with minimal side effects even in neonates and infants [4, 10]. We initially performed ETV, but the subsequent bacterial meningitis exacerbated the efficacy of ETV, and finally necessitated the shunt. Reviewing the pathogenesis of our patient’s hydrocephalus retrospectively, compared with the meningitis, the multiple tumors inducing communicating hydrocephalus were a major cause, and the hydrocephalus was finally managed by the shunt. Therefore, it is important to exclude the underlying mechanism of BPC-associated hydrocephalus, whether it is a simple problem of CSF obstruction at the BPC that can be managed by ETV or a concomitant factor of communicating hydrocephalus, which may need a shunt.
In summary, this report of a rare but important case provides evidence that BPC might develop after birth following a change in CSF dynamics, suggesting that BPC may be classified to two subgroups: a classic congenital subgroup and a postnatal secondary subgroup. We speculate that asymptomatic BPC, cases with a remnant Blake’s pouch or re-expanded BPC with no CSF alterations, may be more common than previously thought.