Occipital encephalocele: a morphological study

Natural history of posterior fetal cephaloceles and incidence of progressive cephalocele herniation

OBJECTIVE

In utero repair of fetal posterior cephaloceles (meningocele and encephalocele) is being performed based on the premise that fetal surgery prevents progressive herniation of neural tissue and brain damage during pregnancy. However, the extent to which progressive herniation occurs during pregnancy, specifically from prenatal diagnosis to after delivery, is not well known. The objective of this study was to describe the natural history of patients with fetal cephaloceles focusing on the incidence of progressive herniation.

METHODS

The authors conducted a retrospective cohort study of all patients referred to their center for posterior fetal cephalocele between 2006 and 2021. All patients underwent prenatal and postnatal MRI. Progressive herniation (primary outcome) was defined as an increase in the absolute volume of neural tissue within the cephalocele of > 5% or new herniation of a critical structure into the cephalocele. Total brain and cephalocele volumes were calculated to determine herniation progression from prenatal to postnatal MRI. Information on the presence of hydrocephalus, epilepsy, and developmental delay (secondary outcomes) was collected at 1 year of age.

RESULTS

Twenty patients met all study criteria. Ten patients (50%; 95% CI 0.27-0.73) demonstrated progressive herniation from prenatal to postnatal MRI. Three patients with progressive herniation were diagnosed with a meningocele prenatally and had an encephalocele postnatally. Two patients without progression had meningocele identified prenatally that regressed and became atretic by birth. Both prenatal hindbrain herniation (p = 0.03) and prenatal microcephaly (p = 0.05) were predictive of progressive herniation. The rates of hydrocephalus (44%), epilepsy (44%), and developmental delay (63%) were not associated with the occurrence of progressive herniation in this study.

CONCLUSIONS

In this study, progressive herniation was not a rare event (50%). Fetal hindbrain herniation and fetal microcephaly were associated with the occurrence of progressive herniation. These results support further investigations into why progressive herniation occurs in utero and if progressive cerebral herniation in utero plays a significant role in determining clinical outcome.

Prenatal imaging of occipital encephaloceles

INTRODUCTION

This retrospective study aims to describe systematically the fetal cerebral MR morphology in cases with occipital meningoencephaloceles using standard and advanced fetal MRI techniques.

MATERIAL AND METHODS

The 1.5-tesla MR examinations (T1- and T2-weighted imaging, echo planar imaging, EPI, diffusion-weighted imaging, DWI) of 14 fetuses with occipital/parietal meningoencephaloceles were retrospectively analyzed for the classification of anatomic characteristics. A diffusion tensor sequence was performed in 5 cases.

RESULTS

In 9/14 cases the occipital lobes were entirely or partially included in the encephalocele sac. Typical features of Chiari III malformation were seen in 6/14 cases. The displaced brain appeared grossly disorganized in 6/14. The brainstem displayed abnormal 'kinking'/rotation (3/14), a z-shape (1/14) and/or a molar tooth-like configuration of the midbrain (3/14). Tractography revealed the presence and position of sensorimotor tracts in 5/5 and the corpus callosum in 3/5. DWI was helpful in the identification of a displaced brain (in 8/9). EPI visualized the anatomy of draining cerebral veins in 7/9 cases. Clinical (9/14) and MRI (7/14) follow-up data are presented.

DISCUSSION

Encephaloceles show a wide range of morphological heterogeneity. Fetal MRI serves as an accurate tool in the visualization of brainstem, white matter pathway and cerebral venous involvement and facilitates the detection of specific underlying syndromes such as ciliopathies.

Development and malformations of the human pyramidal tract

The corticospinal tract develops over a rather long period of time, during which malformations involving this main central motor pathway may occur. In rodents, the spinal outgrowth of the corticospinal tract occurs entirely postnatally, but in primates largely prenatally. In mice, an increasing number of genes have been found to play a role during the development of the pyramidal tract. In experimentally studied mammals, initially a much larger part of the cerebral cortex sends axons to the spinal cord, and the site of termination of corticospinal fibers in the spinal grey matter is much more extensive than in adult animals. Selective elimination of the transient corticospinal projections yields the mature projections functionally appropriate for the pyramidal tract. Direct corticomotoneuronal projections arise as the latest components of the corticospinal system. The subsequent myelination of the pyramidal tract is a slow process, taking place over a considerable period of time. Available data suggest that in man the pyramidal tract develops in a similar way. Several variations in the funicular trajectory of the human pyramidal tract have been described in otherwise normally developed cases, the most obvious being those with uncrossed pyramidal tracts. A survey of the neuropathological and clinical literature, illustrated with autopsy cases, reveals that the pyramidal tract may be involved in a large number of developmental disorders. Most of these malformations form part of a broad spectrum, ranging from disorders of patterning, neurogenesis and neuronal migration of the cerebral cortex to hypoxic-ischemic injury of the white matter. In some cases, pyramidal tract malformations may be due to abnormal axon guidance mechanisms. The molecular nature of such disorders is only beginning to be revealed.

Sincipital encephalocele mismanaged as a facial hemangioma

A 3-month-old male infant was found to have a protruding mass with irregular contour at the root of his nose since birth. The mass protruded more when he cried. It was diagnosed and treated as a hemangioma by laser surgery in another hospital. However, the lesion recurred 1 week later. Reviewing the previous computed tomographic study of the brain, it revealed that the protruding mass communicated with the brain. Sincipital encephalocele was diagnosed, and craniotomy for excising the encephalocele and repair of the dura were undertaken successfully. We report this patient with sincipital encephalocele that was misdiagnosed and treated as a vascular malformation on the face. Fortunately, there was no complication. The authors stress the importance of accurate diagnosis of a lesion near the craniofacial junction to avoid unnecessary surgery and possible catastrophes.

Congenital cerebral and cerebrovascular anomalies: magnetic resonance imaging

Magnetic resonance is the imaging modality of choice for the evaluation of infants and children with known or suspected cerebral and cerebrovascular abnormalities. Entities described include cephaloceles, holoprosencephaly, dysgenesis of corpus callosum, and anomalies of neuronal migration. Congenital vascular lesions described include aneurysm, arteriovenous malformation, cavernous malformation, and Galenic malformations.

Falcine sinus and occipital encephalocele: a magnetic resonance venography study

OBJECT

Occipital encephaloceles are relatively frequently encountered. Many investigators have addressed the embryogenesis of these formations, but the dural system has never before been studied. In this retrospective analysis the authors sought to gain a better understanding of the origins of these defects.

METHODS

The charts and radiological examinations, especially the magnetic resonance venography studies, were reviewed in seven patients. In six patients the straight sinus was absent. Drainage of the galenic system took place through a sinus within the falx, also known as a falcine sinus. The tentorium was not seen in five patients.

CONCLUSIONS

The combination of an absent straight sinus and dysplastic tentorium is no coincidence: both develop within the same mesenchyme in the mesencephalic flexure. Distortion of the mesenchyme by a neural tube defect, causing an occipital encephalocele, will lead not only to disorders of the tentorium but also of the straight sinus.

Telencephalosynapsis (synencephaly) and rhombencephalosynapsis with posterior fossa ventriculocele ('Dandy-Walker cyst'): an unusual aberrant syngenetic complex

Agenesis of the cerebellar vermis (paleocerebellar agenesis) with fusion of the cerebellar hemispheres (rhombencephalosynapsis) is a rare malformation of the central nervous system (CNS). Its combination with synencephaly (telencephalosynapsis), telencephalic ventricular aplasia, aqueductal atresia and cystic fourth ventricle has not yet been described, as far as we know. Here, we report this combination in a 23-weeks' gestation male fetus who was aborted to a 24-year-old diabetic mother. In this fetus with cerebral and cerebellar hemispheric fusion, vermian agenesis was associated with a Dandy-Walker-like posterior fossa cyst, in spite of the fusion of the hypoplastic cerebellar hemispheres. The CNS malformations were further accompanied by dysmorphic facial stigmata such as unilateral atresia of the external ear, ocular hypertelorism and a broad nasal bridge. Preaxial polydactyly and contractures of the upper limbs were the only associated non-cranial abnormalities. Cytogenetic studies revealed a numerically and structurally normal male (46, XY). The malformation complex described in this fetus of a mother with antedating pregnancy diabetes appears to represent a previously undescribed aberrant syngenetic CNS phenotype, some basic teratogenetic aspects of which will be discussed in this paper.

Knobloch syndrome in a large Brazilian consanguineous family: confirmation of autosomal recessive inheritance

Knobloch syndrome is a rare genetic disorder characterized by high myopia, vitreoretinal degeneration with retinal detachment and occipital cephalocele. The inheritance has been described as autosomal recessive (AR) but in addition to the original report with 5 affected patients [Knobloch and Layer, 1971] only one other family with 2 affected sibs has been described [Czeizel et al., 1992]. We have studied a large consanguineous kindred in which there are 12 patients with severe ocular alterations associated with a congenital occipital encephalocele, compatible with the diagnosis of Knobloch syndrome. CT scan and MRI performed in one of the patients, allowed a better understanding of the cranial and ocular alterations in this syndrome. The pattern of occurrence in this highly inbred family clearly confirms autosomal recessive inheritance of Knobloch syndrome.

Ventricular volume reduction technique--a new surgical concept for the intracranial transposition of encephalocele

The authors propose a new operative technique for the intracranial transposition of an encephalocele by using the associated hydrocephalic state. In the first stage, the intracranial pressure buffering effect of the encephalocele sac is reduced by the closing of the dural defect. The repaired dural defect allows intraventricular pulse pressure to produce ventriculomegaly. In the second stage or in infants with hydrocephalus, the extracranially herniated brain is transposed into the intracranial cavity during ventricular shunting with appropriate drainage of cerebrospinal fluid.

Meningoceles, meningomyeloceles, and encephaloceles: a neuro-dermatopathologic study of 132 cases

Because there have been few comprehensive histopathologic studies of meningomyeloceles and related malformations, we undertook a systematic study of these lesions. One hundred and thirty two cases were obtained from our surgical pathology files; these included 38 meningoceles, 71 meningomyeloceles, and 23 encephaloceles. Tissue sections were stained with hematoxylin and eosin; special stains included trichrome, alcian blue, Fontana-Masson, Nissl, Holzer, and immunoperoxidase for glial fibrillary acidic protein. Epithelial changes included ulceration, atrophy, or nevoid hyperplasia of the epidermis, and loss of appendages. Mesodermal features included fibrous zones resembling dura, subarachnoid tissue or scar (99% of cases), increased numbers of blood vessels (83%), hypertrophy of arrector pili muscle (42%), lipoma formation (38%), and immature skeletal muscle fibers (5%) that rarely intermingled with neuropil-like matrix. The latter tissue was identified in 71% of cases and included neurons, astrocytes, oligodendroglia, and ependyma. Forty-eight percent of cases included peripheral nerve fibers or roots, and some fibers formed onion bulb or Pacinian corpuscle-like structures. Meningothelial cells were observed in 26% of cases and sometimes formed recognizable whorls. Choroid plexus was noted in 3 cases, one example showing an unusual dystrophic calcification that formed long parallel spicules. Pigmented dendritic cells were observed within zones of fibrous tissue in 10% of cases. These malformations involve complex arrangements of cutaneous, neuroectodermal, and mesodermal elements. Because they may be encountered by dermatopathologists, familiarity with the microscopic features of dysraphic lesions is essential.

Surgical implications of cerebral dysgenesis

Cerebral dysgenesis encompasses varied disorders of brain development. Based on the understanding of these conditions provided by histopathologists, embryologists, radiologists and developmental pediatricians, surgeons are able to appropriately assist in the care of these patients. The surgeon can offer assessment of the ventriculomegaly that commonly accompanies cerebral dysgenesis in addition to providing methods to control hydrocephalus, to reconstruct cranial and facial malformations and to remove dysfunctional tissue. For most patients, surgical intervention is only one of the many factors that determine developmental prognosis. Based on the foundation built by other specialists, this review discusses cerebral dysgenesis from the perspective of historical and current surgical interventions.

Intractable complex partial seizures associated with occult temporal lobe encephalocele and meningoangiomatosis: a case report

Occult congenital temporal lobe encephalocele has rarely been reported in association with medically intractable complex partial seizures. The four previously reported cases were unsuspected preoperatively. We present the case of an 18-year-old woman with intractable complex partial seizures since age 13. Seizure onset was electrically localized to the right temporal lobe. Preoperative neuroimaging studies revealed a middle fossa defect and inferior herniation of the right temporal lobe. Pathologic examination of the resected encephalocele revealed prominent features of meningoangiomatosis. We believe this to be the first case of temporal lobe encephalocele and epilepsy to be diagnosed preoperatively, and the first case also to be associated with meningoangiomatosis. The relevant literature on meningoangiomatosis and on temporal lobe encephalocele as a cause of epilepsy is reviewed.

Morphological evaluation for neuronal maturation in anencephaly and encephalocele in human neonates. A proposal of reclassification of cephalic dysraphism

The developmental pathogenesis of cranium bifidum and exencephaly, as well as other forms of the dysraphic state, remains unresolved. The process of neuronal maturation under such conditions is the major definitive factor for functional prognosis and may be the key to the embryopathogenesis. The neuronal maturation processes in cranium bifidum and anencephaly were analyzed in both human neonatal autopsy and surgical specimens, utilizing the technique of immunohistochemical morphological analysis. The results suggest that anencephaly is a form of neuroschisis, a defect in which the neuronal developmental process arrests and results in dys- or rather aplastic cortical architecture. In contrast, although the cortical architecture is often distorted, neuronal maturation in cases of cranium bifidum cysticum is primarily complete, demonstrating normal cerebral cortical layers and NSE positive neural elements. In conclusion, it is strongly suggested that anencephaly is the most severe form of cranium bifidum, as is myeloschisis in spina bifida, without supportable neuronal maturation. Recognizing these points in the developmental stages with regard to the degree of neuronal maturation, a reclassification of cephalic dysraphism is proposed. The embryopathogenetic and epidemiologic aspects of this proposal of reclassification are discussed.

Occipital encephalocoeles in 57 Nigerian children: a retrospective analysis

Fifty-eight patients with occipital encephalocoeles were retrospectively examined. These comprised about one-half of the cases seen and evaluated for treatment at the University College Hospital, Ibadan, Nigeria, between January 1973 and December 1987. There was a female-to-male preponderance of 2:1. Of the patients 91% were treated during infancy. Only one patient was precluded from surgery because his large ulcerated lesion was associated with severe microcephaly and neonatal sepsis, to which he succumbed. About four-fifths of the lesions exceeded 5 cm in diameter. The operative mortality was 6%, all deaths occurring in patients who were neonates at the time of surgery and whose hernia sacs contained brain substance. Clinically apparent hydrocephalus was more frequently encountered postoperatively, than preoperatively. Developmental delay was apparent in 5 of the 13 patients in whom developmental milestones were assessed during follow-up. For most patients, the follow-up period was short, possibly a reflection of the poor prognosis of the disease.

Cranium bifidum in northern Nigeria

At the Ahmadu Bello University Hospitals in northern Nigeria, 35 girls and 29 boys with cranium bifidum were registered between 1971 and 1985. Only 25% of the patients were brought to the hospital within 24 h of life; 57 had been delivered at home without the mothers having received formal prenatal care. The majority (69%) of the lesions occurred in the occipital region; 52 (81%) were encephaloceles while the others were cranial meningoceles. Resection and repair were performed in 35 patients by general surgeons who had no facilities for vertriculoperitoneal shunts in infants with hydrocephalus. Thirteen children died in the hospital, 3 moved away, and 34 were lost to follow-up at 1 month. Only 3 were seen up to 1 year. Improvement in the care of such patients is expected since a neurosurgical unit has now been established.

Subtorcular occipital encephaloceles. Anatomical considerations relevant to operative management

Three cases of occipital encephalocele, one with associated myelomeningocele, are presented. All received preoperative evaluation with magnetic resonance imaging. Such studies provide optimal demonstration of the cerebral and hindbrain anatomy to guide operative treatment and formulate prognosis. Review of available radiographic, operative, and pathological information suggests that most, if not all, occipital encephaloceles are associated with an anomaly of the hindbrain, and the usual anomaly is a rhombic roof encephalocele. In such cases, the site of cranial herniation is caudal to the torcula, regardless of the presence or absence of occipital lobe tissue within the sac. Experimental and clinical analysis suggests that occipital encephaloceles most likely arise from abnormalities in the development of the skull base.

Central nervous system malformations induced by triamcinolone acetonide in nonhuman primates: pathology

Triamcinolone acetonide (TAC) was administered to pregnant macaques (Macaca mulatta [15] and M. radiata [7]) during gestational days (GD) 23 to 41 using various dosing schedules. A daily dose of 10 mg/kg is approximately equal to 100 x the human dose equivalent. The brains of the fetuses and infants were studied grossly and histologically. All cases displayed either the mild form of the TAC-induced syndrome (craniofacial dysmorphia, cranium bifidum occultum, meningocele, and mild distortion of the midbrain) or the more severe form (occipital encephalocele, hydrocephalus, severe distortion of the midbrain or midbrain "beaking," shunting of cerebrospinal fluid, and craniofacial malformations). The dysmorphology was dose-related, with severity increasing at higher doses or with increased numbers of treatments. Individual cases were assessed for the severity of the syndrome by comparison of like components between groups. The lesions observed were morphologically comparable to those described in spontaneous human cases; the TAC-induced occipital encephaloceles were associated with brainstem and cerebellar abnormalities, and, with the less severe form of the syndrome, brainstem abnormalities were occasionally present, with occipital meningoceles. Controversy exists concerning the significance and temporal development of the midbrain changes. However, the associated alteration in aqueduct conformation may have been responsible for functional compromise and ensuing hydrocephalus.

Central nervous system malformations induced by triamcinolone acetonide in nonhuman primates: pathogenesis

The pathogenetic sequence for TAC-induced encephalocele is in agreement with hypotheses proposing that neural tube closure is followed by protrusion of the mesencephalon, with subsequent growth and development resulting in herniation of the cerebrum and cerebellum. This model could serve to clarify the pathogenesis of encephalocele and to stimulate further study in comparing this defect to other dysraphic states. Triamcinolone acetonide (TAC) was administered intramuscularly (10 mg/kg) to 16 pregnant rhesus monkeys (Macaca mulatta) for 5 alternate days of pregnancy, beginning on gestational day (GD) 23. Conceptuses were removed by hysterotomy at GD 35, 42, 50, or 70 and examined grossly and histologically. Length, area, and perimeter of the tectum and aqueduct area and perimeter were measured with an image analyzer. Changes in treated specimens were suggestive of forces within or ventral to the tectum resulting in dorsal protrusion, rostral-posterior stretching, and attenuation. The angle of the cephalic, pontine, and cervical flexures was also measured. The more acute angle of the cephalic flexure and less acute cervical flexure of treated specimens could represent altered orientation secondary to a mesenchymal deficiency. However, the less acute angle of the pontine flexure in treated specimens suggests an intrinsic alteration in the neural tube. This suggests that encephalocele may result from a combination of mesenchymal and neural tube abnormalities.

The amniotic band syndrome as a cause of anencephaly. Report of a case

The gross and microscopic features of a 28-week-old stillborn female infant with severe cranial, facial, and cerebral malformations due to amniotic bands are described. The structure of the cerebral remnant is similar to that found in dysraphic anencephaly, but collateral evidence of amniotic band can usually be found. It is important to differentiate between the two conditions, since unlike neural tube defects, anencephaly due to amniotic band does not imply a risk of recurrence in subsequent pregnancies.

Fourth ventriculoceles with extracranial extension

The radiographic features and long-term clinical outcome in three patients who presented at birth with a cystic suboccipital mass in direct communication with the fourth ventricle are reviewed. The pathological findings in a fourth infant who died are also discussed. All surviving infants were treated with cyst excision and diversion of cerebrospinal fluid. The prognosis in these children, followed from 6 to 20 years, surpasses that of the more common occipital encephalocele, for which this entity could be mistaken. The morphogenetic implications relative to more common congenital lesions in this location are discussed.

Axial skeletal malformations associated with cranioschisis aperta and exencephaly. The result of experimental intervention after the neural tube closure in rats

Maternal administration of a single dose of cyclophosphamide (20 mg/kg) after the neural tube closure (on day 12) resulted in exencephaly and cranioschisis in 100% of rat fetuses at term. Extensive hemorrhages and edema were regularly associated with these defects. Alizarin-red stained skeletal preparations revealed absence of skull vault, premature closure of basicranial synchondroses, exaggeration of the craniovertebral angle, and agencies and hypoplasia of the vertebrae, ribs and sternum. It is suggested that failure of the neural tube to close is not the primary cause of axial skeletal malformations and even after closure, the axial skeletal anlagen remains susceptible to teratogenic insult.

Occult cranium bifidum. Radiological and surgical findings

Six cases of congenital subscalp nodule associated with underlying cranium bifidum are reported. A plain skull roentgenogram showed a midline bone defect in the parieto-occipital region near the lambda. CT scan demonstrated neither brain malformation nor ventricular deformity except for the high position of the straight sinus. Cerebral angiography revealed an elongation of the vein of Galen and anomalous upward course of the straight sinus. At surgery, the tumor was solid and connected to a cord which extended intracranially via the cranium bifidum and blended with thickened arachnoid membrane either on the dorsal aspect of the midbrain or at the surface of the anterior vermis. Histologically, the tumor consisted in all cases of arachnoid cells and fibrous tissue with immature glial cells in one case. Possible pathogenesis of these tumors could be a result of the fetal nuchal bleb.

Cephaloceles: clinical and neuroradiological appearance. Associated cerebral malformations

Cephaloceles are congenital malformations with herniation of intracranial structures through a defect in the cranium. On the basis of a review of the literature and 31 personal observations the authors discuss the clinical and neuroradiological presentation of their various anatomical locations: sphenoidal, ethmoidal, frontal, occipital and parietal.

Heterotopic growth of dysplastic cerebellum in frontal encephalocele in an infant of a diabetic mother

An infant of a diabetic mother lived 13 days after birth. She had a small dysplastic 12.6 gm. brain, partly forming a frontal encephalocele. The uncleaved forebrain contained a mass of poorly organized heterotopic cerebellar cortex. The cerebellum itself had normal lamination, but was small and continuous with the dysplastic tissue. The ventricular system was absent except for a few midline ependymal rosettes, and the cerebral cortex was not developed. The cerebellar dysplasia resembled a proliferative and invasive lesion by its rostral extension.

Agenesis of the vermis with fusion of the cerebellar hemispheres, septo-optic dysplasia and associated anomalies. Report of a case

Agenesis of the cerebellar vermis with fusion of the dentate nuclei and cerebellar hemispheres (rhombencephalosynapsis) is a rare cerebral malformation. We report the case of a 7-h-old girl whose mother had taken the drug phencyclidine during the first 6 weeks of pregnancy. Absence of septum pellucidum, hypoplasia of the commissural system, optic nerves, chiasm and tracts, moderate hydrocephalus, and agenesis of the posterior lobe of the pituitary were also found. Extracranial congenital anomalies involved the cardiovascular, respiratory, urinary, and musculoskeletal systems. Death was secondary to severe respiratory distress and bradycardia. The literature concerning this rare form of cerebellar malformation is reviewed.

The pathology of occipital encephalocoele and a discussion of the pathogenesis

The midline structures of 5 infant brains with occipital encephalocoeles were studied. Though all cases showed a uniform finding of a midline defect of the occipital bone with a herniated mass, the extent of the bony defect, the amount and nature of the herniated tissues and the degree of distortion of the intracranial structures varied markedly. Two of the cases showed multiple associated systemic malformations and another case had an inverse cerebellum. In spite of such a marked variability in presentation, the common findings of herniation and displacement of the mesencephalon and roof of the diencephalon allowed us to postulate that the primary defect of occipital encephalocoeles is an abnormality of the tissues overlying the mesencephalon of the developing brain. Initial herniation of the mesencephalon and subsequent movement of the rest of the brain as a result of growth determines the tissues present in the hernia sac at birth. Other theories of genesis of occipital encephalocoeles are discussed.

Developmental arachnoid cysts of the posterior fossa-an analysis of 13 cases

Thirteen cases of posterior fossa arachnoid cyst are described. Presenting features were usually headache, vomiting, lethargy, and delayed development in infants. Association with congenital defects is sometimes noted. The cysts are considered to result from abnormalities occurring in foetal life.

Occipital encephalocele: a pathologic and anatomic analysis

The brain from an infant with a cystic occipital mass present at birth is examined in serial section. The occipital mass proved to be a rhombic roof ventriculocele. Within the posterior fossa, it was bound to an occipital lobe encephalocele which issued as a diverticulum of the left lateral ventricle through a microgyric cortical defect in the territory of the left posterior cerebral artery. The posterior medial aspects of both cerebral hemispheres were herniated downward into the widened tentorial gap. Craniolacunae were prominent on the inner aspect of the skull. The aqueduct and central canal of the spinal cord were widely dilated, although the lateral ventricles were collapsed. It is suggested that hydrocephalus secondary to obstruction to flow of CSF through the rhombic roof entrained a sequence of events giving rise to the rhombic roof ventriculocele and causing occlusion of the posterior cerebral artery and subsequent diverticulation of the lateral ventricle through an infarcted region of the posterior-medial hemisphere.