Cysts of the central nervous system : a clinicopathologic study of 145 cases.
Non-neoplastic, non-inflammatory cysts of the central nervous system may cause symptoms because of pressure, rupture or secondary inflammation. A total of 145 cases of cysts were reviewed during a study period of 12 years. The clinical details and histological features were noted in all cases. During this period 53 epidermoid cysts, 16 dermoid cysts, 38 colloid cysts, 23 arachnoid cysts, 5 neurenteric cysts, 5 ependymal and glial cysts, 2 Rathke's cleft cysts and 3 unclassified cysts were encountered. The possible histogenesis is also discussed.
Cysts of the central nervous system (CNS) are usually benign and may be detected incidentally at autopsy. They become symptomatic either because of pressure, rupture or secondary inflammation. Though generally of good prognosis, they may recur due to rupture or incomplete surgical removal. The age, site, cyst wall and cyst contents provide an insight into their histogenesis and embryology. In this paper, we have reviewed the clinical and pathological features of cysts of CNS and discussed the possible histogenesis.
The cysts of CNS diagnosed between January 1987 to December 1998 from the records of surgical pathology of our institute, were reviewed. Cystic degeneration in tumours or inflammatory lesions was excluded from the study. The age, site, cyst wall lining and nature of cyst contents were noted in all cases. These included epidermoid and dermoid cysts of brain and spinal cord; colloid cysts of 3rd ventricle, arachnoid cysts of brain and spinal cord, neurenteric cysts of spinal cord, ependymal and glial cysts of brain, Rathke's cleft cysts and cysts that could not be classified.
There were 145 cysts in the study period of 12 years. The details about age, sex and site were given in [Table I] and [Table II]. Majority of intracranial cysts presented with signs of raised intracranial pressure. Patients with epidermoid cysts had additional cerebellar signs, visual disturbances and deafness. Seizures and increased head circumference was common in arachnoid cysts. The spinal cysts of all types presented with cord compression.
Epidermoid and Dermoid Cysts : The epidermoid cysts were more common in the brain and dermoid cysts in the spinal cord. The commonest location of epidermoid cysts in the brain was cerebellopontine angle followed by suprasellar area and others. Brain stem was the site in two cases. In the spinal cord, they were intradural in 8 and intramedullary in one. The spinal forms had associated anomalies in the form of spina bifida in one and kyphoscoliosis in another. Histology showed cyst wall lined by stratified squamous epithelium [Figure. 1] and the cyst contents were keratinous flakes. Rupture of the cysts with foreign body giant cell reaction/abscess was seen in 10 cases and calcification in one. Recurrence was noted in 3 cases. The intracranial dermoid cysts most cord compression and were associated with spina bifida in 4 cases, dermal sinus in 2 cases and hypertrichosis in 2 cases. One had intramedullary and nine had intradural cysts. Cysts were lined by stratified squamous epithelium and hair follicles in 14 out of 16 cases and sebaceous glands in 2 out of 16 cases [Figure. 2]. The contents included keratin flakes, hair or sebum. Rupture with foreign body giant cell reaction/abscess was seen in 4 cases.
Colloid cysts : All colloid cysts were located in anterior 3rd ventricular region and all of them presented with raised intracranial pressure. The cyst wall was lined by cuboidal to columnar epithelium with or without cilia, supported by delicate collagenous stroma [Figure. 3]. Squamous metaplasia was seen in 3 cases, goblet cells in 3 cases, haemorrhage, haemosiderin pigment and xanthogranulomas in 8 cases, and calcification in one. Cyst contents were mucoid or thin gelatinous.
Neurenteric cysts : All cases presented with cord compression. One was associated with hemivertebra. Cysts were lined by columnar epithelium with few goblet cells and subepithelial mucous glands [Figure. 4]. Alcian PAS stain showed positivity in all the cases. Foci of ulceration and inflammation was seen in one case. Contents were mucoid or clear fluid.
Arachnoid cysts : These cysts, both in brain and spinal cord, presented with pressure symptoms. Histology showed cyst wall lined by flattened to cuboidal cells supported by collagen [Figure. 5]. Foci of calcification were seen in two cases. Contents were clear fluid like CSF.
Ependymal and glial cysts : Cysts presented with signs of pressure. Ependymal cysts were not in communication with ventricle or CSF spaces and the glial cysts were intracerebellar. The cysts were lined by columnar cells or glial cells [Figure. 6].
Rathke's cleft cysts : Cysts were suprasellar in location. The cyst wall was lined by columnar to cuboidal epithelium. The contents were greenish and oil like.
Cyst not otherwise specified : All these cases were spinal, presenting with cord compression. The cysts were intradural in two cases and intramedullary in one case. The cyst wall was lined by flattened cells. The contents were scanty and clear. There was no clue as to the diagnosis of the cyst from either the cyst wall or contents.
All the cysts were benign and there was no malignant transformation in any of them.
Cysts of CNS are usually maldevelopmental in nature.
A few are acquired. The recognition of these cysts radiologically and confirmation by pathology is essential for prognostic purposes.
Epidermoid cysts account for 0.2 to 1.8% of all intracranial tumours and less than 1% of all intra spinal tumours. Cranial sites outweigh spinal sites by 14:1.,, The present series shows an approximate 5:1 ratio. Intracranial epidermoids usually occupy the cerebellopontine angle, parasellar/suprasellar region, cerebral and cerebellar hemispheres, ventricular region and pineal region.,,,, Epidermoid cysts of the brainstem are extremely rare., The present series shows similar incidence with two epidermoid cysts in the pons. Intraspinal epidermoids are usually intradural and extramedullary and only rarely intramedullary in location. Roux et al reviewed the published 47 cases of intramedullay epidermoid cysts. In our series, there was one intramedullary cyst in dorsolumbar location. Dermoid and epidermoid cysts present either due to mass effect of the slowly enlarging lesion or to the immunological response to the rupture of cyst contents. In our series 10 out of 53 epidermoid cysts and 4 out of 16 dermoid cysts presented with rupture, foreign body giant cell reaction and abscess formation. Dermoids, typically become symptomatic during the first 2 decades of life and epidermoids because of their slower production of compact keratin tend to present between the ages of 20 and 50 years.
Epidermoid and dermoid cysts represent nests of cutaneous tissues misplaced during embryogenesis and are found along lines of ontogenic neurocutaneous differentiation. These ectodermal inclusions occur between 3rd and 5th weeks of embryonic life. This inclusion can result in heterotopia of these elements. The median location of these tumours can be explained by the separation of neuroectoderm and its cutaneous counterpart which occurs dorsally along the midline. Laterally situated lesions may result from inclusion of ectoderm at a later stage of embryogenesis, especially during the formation of secondary otic and optic cerebral vesicles.,,, Acquired epidermoid cysts in the lumbar area are due to repeated lumbar punctures or incidental formation of a skin pocket by suturing. In our series, we have not encountered even a single case of acquired epidermoid cyst, probably because of better technique during lumbar puncture.
Colloid cysts of the third ventricle constitute 0.25 -0.5% of all intracranial tumours. It is a well established clinicopathologic entity as defined by its location, contents and histology. Because of its strategic location allowing it to obstruct one or both foramina of Monro, the commonest clinical presentation is acute hydrocephalus as seen in all our cases. With the advent of CT and MRI, it is more readily diagnosed and the characteristic radiologic appearance is due to its high cholesterol content. Rupture with haemorrhage, haemosiderin laden macrophages and xanthogranuloma formation were seen in 8 out of 38 of our cases. Although generally regarded as a malformation, its lineage is controversial. A neuroectodermal origin is preserved because of its location. Recent immunohistochemical studies showed a pattern identical to that of foregut derived epithelium showing endodermal or ectodermal differentiation and to respiratory tract mucosa. Epithelium of colloid cyst is immunohistochemically different from that of normal or neoplastic choroid plexus epithelium. These findings indicate an endodermal rather than neuroepithelial nature for colloid cysts.,
Rathke's cleft cysts are small and asymptomatic and are found in 2-26% of routine autopsy series., With the modern neuroimaging technology, they are being diagnosed much more frequently. Symptomatic Rathke's cleft cysts were reviewed by Voelker et al. These cysts are thought to originate from thought to originate from remnante of Rathke's pouch. It is the persistence and enlargement of the cleft from posterior wall of the pouch that produces Rathke's cleft cyst.,, Immunohistochemical studies confirm the foregut derivation of Rathke's cleft cysts similar to colloid cysts and enteric cysts. Both our cases are symptomatic.
Neurenteric cysts are rare and are most frequently encountered in the posterior mediastinum or as spinal intradural extramedullary location causing progressive myelopathy.,, They are usually associated with complex malformations in neonates. One of our cases was associated with hemivertebra and all of them were intradural extramedullary, two cervical and three thoraco lumbar. These neurenteric cysts are believed to originate from embryonal dysgenesis. Normal development in the 3rd week of embryonal life involves closure of the neurenteric canal and notochordal separation from the primitive gut. Disruption of this process may lead to the inclusion of endodermal tissue and cystic formation. Such cysts of the central nervous system (CNS) have been reported to occur from the level of posterior fossa to the lumbosacral region. Wilkins and Odom classified these cysts into three groups according to histological features. The simplest type of enterogenous cyst, group A is lined by a layer of single cuboidal or columnar epithelial cells with or without cilia, as seen in all of our 4 cases. The more complex type, group B, has additional elements found along the course of the gastrointestinal tract or tracheobronchial tree, including the mucous glands and smooth muscle, as seen in one case in our series. Group C has ependymal or glial tissue in addition to the feature of group B. Immunohistochemical studies confirm the endodermal origin of these cysts.
Arachnoid cysts are benign developmental cysts that occur in the cerebrospinal axis in relation to the arachnoid membrane. The lining epithelium as seen in our cases shows collagen and meningothelial cells. The cysts contain clear, colourless fluid resembling normal CSF as seen in all our cases. Despite several studies, the mechanism of formation of these cysts is not completely understood. In autopsy cases, it can be seen that the arachnoid membrane at the margin of the cyst splits and the cyst is actually contained between the outer and inner layers of the arachnoid. The spinal arachnoid cysts are true arachnoid cysts, located subdurally.
Intraparenchymal cysts lined by ependymal cells probably arise embryologically by budding or cell displacement from the forming ventricular system. All cases of ependymal cysts in our series had no communication with ventricular system. Intraparenchymal glial cysts of the cerebellum as seen in our series are reported earlier. The aetiology and pathogenesis of such cysts are not clear. Probably such cysts are formed by budding from the ventricular system like ependymal cysts with subsequent loss of ependymal cells by stretching or pressure effects.
In our series, we could not categorise 3 spinal cysts into any category as there was no clue from cyst wall or cyst contents. Immunohistochemical or ultrastructural studies would have thrown light in such cases.
These benign cysts of the CNS are being recognised more frequently with the advent of modern neuroimaging. An understanding of the pathogenesis and embryology explains the location, presentation and histology in such cases. They are usually maldevelopmental and malignant transformation of the lining epithelium is extremely rare.