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Neuroradiology Case of the Week

Case 103

Igor Mikityansky, MD and PL Westesson, MD, PhD, DDS

Clinical Presentation: A 3-year-old male presented with increased seizure activity and EEG showing left frontal slowing.

Radiological Findings: There are two large symmetric extra-axial cystic lesions in the middle fossa. They displace temporal horns of the lateral ventricles and hippocampus medially. There is minimal residual volume of the temporal lobe parenchyma bilaterally. The middle cerebral arteries are also slightly stretched at the superior aspect of the lesions. There is increase in extra-axial fluid compartment, especially along the left hemisphere, more so in the frontal region, due to brain tissue loss with secondary widening of the left frontal horn. In the left temporor-occipito-parietal region the grey matter has an abnormal appearance suspicious for polymicrogyria. Corpus callosum appears thin in its entire course.

Diagnosis: Bilateral arachnoid cysts, left-sided polymicrogyria

Discussion: Arachnoid cysts represent intra-arachnoid cerebrospinal fluid-containing cysts that do not communicate with the ventricular system and usually are not associated with brain maldevelopment. They constitute approximately 1% of intracranial masses, can be found at any age and have 4:1 M:F gender distribution. About 50-60% of arachnoid cysts are located in middle cranial fossa, 10% in cerebellopontine angle, 10% are in suprasellar region. The latter include noncommunicating, cyst of the membrane of Liliquist, and communicating, cystic dilatation of interpeduncular cistern, cysts. Finally, 10-20% of cysts are in miscellaneous locations, such as convexity, quadrigeminal region, perivermian etc. Arachnoid cysts usually are translucent and sharply demarcated. They displace cortex and “buckle” gray-white interface [1, 2].
     MRI T1WI usually demonstrate sharply-marginated extra-axial fluid collection isointense with CSF and “Mickey Mouse ears” appearance of suprasellar cyst and lateral ventricles together. T2 and PD/Intermediate WI again demonstrate isointensity of the cyst with CSF. The signal in the cyst is usually completely suppressed on FLAIR. T2* GRE images usually show no blooming, unless hemorrhage is present. DWI exhibits no restriction. The cysts do not enhance. On MRA cortical vessels can be seen being displaced away from calvarium. MRV can demonstrate anomalies of venous drainage. MRS can predict pathology in >90% of similar-appearing intracranial cystic lesions. Phase-contrast cine MR by quantifying flow can distinguish arachnoid cysts from enlarged subarachnoid spaces [1].
     Galassi classification divides arachnoid cysts in three types. Type 1 is small, spindle-shaped and is limited to anterior middle cranial fossa. Type 2 usually has its superior extent along the Sylvian fissure with displacement of the temporal lobe. Type 3 is huge, filling the entire middle cranial fossa with fronto-temporo-parietal displacement [1].
     Differential considerations include epidermoid cyst, chronic subdural hematoma, subdural hygroma, other nonneoplastic cysts. Epidermoid cyst usually has scalloped margins and insinuating growth pattern with creeping along into CSF cisterns and engulfing, rather than displacing vessels and nerves. It does not suppress on FLAIR and shows restricted diffusion on DWI. Chronic subdural hematoma has signal that is different from CSF. It is often bilateral and lentiform-shaped and may show enhancing membrane. Subdural hygromas are often bilateral and have crescent or flat configuration.
     Porencephalic cyst, neurenteric cysts and glioependimal cysts can mimic arachnoid cysts. However, porencephalic cysts are usually surrounded by gliotic brain without any evidence of cortex compression. Patients most of the time have history of trauma events or strokes. Neurenteric cysts are rare. They are mostly seen in spine or posterior fossa and often contain proteinaceous fluid. Glioependymal cysts are rare and usually intra-axial [1].
     Most likely, arachnoid cysts expand when hydrodynamic pulse waves of cerebrospinal fluid (CSF) become entrapped in arachnoid locations [1, 2]. Alternative hypothesis states that they form secondary to failure of the embryonic frontal and temporal meninges (endomeninx) to merge as Sylvian fissure is formed [1]. The cysts may be unilocular or loculated by septations. The wall of the cyst is usually smooth. Most cysts are filled with clear colorless fluid of low protein content comparable to CSF. A few cysts may contain elevated protein content. Microscopic examination of arachnoid cysts shows that the walls are formed from a splitting of the arachnoid membrane, with an inner and outer leaflet surrounding the cyst cavity. The cyst wall consists of fibrous connective tissue slightly denser than normal arachnoid tissue, with hyaline change at times. No epithelial lining is present. The outer wall of the cyst adheres loosely to the dura. The cyst wall is devoid of blood vessels, and changes of inflammation or hemorrhage seldom occur [2].
     Arachnoid cysts are usually sporadic and non-syndromic, though rare familial cases have been described in the literature [3]. Arachnoid cysts are also seen in some disorders of metabolism, such as mucopolysaccharidoses [1]. Lutcherath and his colleagues state that bilateral temporal arachnoid cysts are common in patients with Glutaric aciduria type1, a rare autosomal recessive deficiency of glutaryl-CoA-dehydrogenase leading to abnormal metabolism of lysine, hydroxylysine and tryptophane. Patients usually present with encephalitis-like picture which is followed by hypotonia, dystonia, choreo-athetosis, and seizures. Occasionally, they demonstrate picture of cerebral palsy or no symptoms at all. Their urine has high level of glutaric acid and the enzyme defect can be demonstrated in fibroblasts. Imaging often discovers subdural hygromas or hematomas. Surgical intervention usually triggers severe catabolic state, which can be prevented by switching to special low protein diet [4]. Arachnoid cysts are found in several syndromes but data are not sufficient to indicate whether the association is typical or fortuitous. Arachnoid cysts can be associated with Cockayne syndrome and Menkes disease [2]. They are seen in 1/3 of Acrocalosal syndrome, as well as in Aicardi and Pallister-Hall syndromes [1].
     Acquired arachnoid cysts may develop following surgery, trauma, subarachnoid hemorrhage, neonatal infections and can occasionally occur in association with extra-axial neoplasm. Arachnoid cysts associated with tumors develop as a consequence of CSF loculation surrounded by arachnoid scarring, with expansion of osmotic filtration or via a ball-valve mechanism. These acquired arachnoid cysts are frequently referred to as acquired secondary or leptomeningeal cysts. The mechanism of enlargement of the arachnoid cysts is not well understood. No inner lining is present through which active transport can take place. Neurosurgeons have observed ostia with pulsating fluid in exposed cysts suggesting a hydrodynamic flap-valve or ball-valve mechanism [2].
     Arachnoid cysts often are an incidental finding on imaging, and patients usually are asymptomatic even if the cyst is quite large. The most common associated clinical features include headache, calvarial bulging, intracranial hypertension, craniomegaly, developmental delay, visual loss, precocious puberty, and seizures, with focal neurologic signs occurring less frequently. Arachnoid cysts are known to rupture into the subdural space or undergo intracystic hemorrhage. Middle cranial fossa arachnoid cysts also have been linked to ipsilateral temporal lobe agenesis. Middle cranial fossa cysts are linked to ipsilateral chronic subdural hematomas. Rarely, they may communicate with the subdural space, forming a slitlike extension over the hemispheric surface [2].
     Controversy surrounds the treatment of arachnoid cysts. Some clinicians advocate treating only patients with symptomatic cysts while others believe that even in asymptomatic patients, cysts should be decompressed to avoid future complications. The most effective surgical treatment appears to be excision of the outer cyst membrane, which can be endoscopic, and cystoperitoneal shunting. Occasionally fenestration is performed [1, 2].
     Large arachnoid cysts should undergo serial scans, since they may show progressive enlargement, and patients may become candidates for surgical consideration [2].
     Polymicrogyria, also known as cortical dysplasia, is a malformation that occurs due to abnormality in late neuronal migration and cortical organization. Neurons reach the cortex, but distribute abnormally, forming multiple small undulating gyri. It has predelection for perisylvian regions and represents syndromes when it is seen bilaterally. Polymicrogyria can be caused by intrauterine infection, ischemia, or toxin exposure. However, deletions of 22q11.2 (DiGeorge critical region), Xq28 and 16q12.2-21 have been linked to it as well [5].
     Morphologically it represents small irregular gyri, normal or thick cortex and indistinct cortical-white matter junction. Microscopicaly, there is a range of histology reflecting derangement of the six layered lamination of the cortex. It can present as inlayered or four layered cytoarchitecture. Most involved cortical layers are the fourth and the fifth. Leptomeningeal embryonic vasculature is seen overlying the malformation. The degree of myelination of subcortical and intracortical fibers varies. [5]
     On T1WI polymicrogyria demonstrates irregular cortical surface with isointensity of cortex to gray matter and indistinct cortical-white matter interface. There are two patterns of polymicrogyria on T2WI: for <12 months brain it is seen as small, fine undulating cortex with normal thickness (3-4mm); for >18 months brain, it is seen as thick, bumpy cortex (6-8mm), with or without hypomyelination and cortical infolding. T2*GRE can demonstrate hypointense foci at sites of periventricular calcifications in cases related to CMV infection. Post-contrast images amplify dysplastic leptomeningeal veins overlying regions of polymicrogyria and MRV demonstrates persistent embryonic veins overlying abnormal cortex. MRS detects low NAA at seizure-precipitating, atrophic and/or hypomyelinated sites.
     Clinically, polymicrogyria can present as faciopharyngoglossomasticatory diplegia, developmental delay, seizure, and hemiparesis. Onset and severity of symptoms are related to the extent of the malformation. The treatment usually involves medical management of seizures and supportive care. Corpus callosotomy can be considered if bilateral or diffuse unresectable lesions and intractable epilepsy are present.

References:

1. Osborne AG. Diagnostic Imaging: Brain. Amirsys Inc: Altona, 2004, 1st ed., pp. 7:4-7.
2. http://www.emedicine.com/radio/topic48.htm
3. Pomeranz S, et al. Familial Intracranial arachnoid cysts. Child’s Nerv Syst (1991) 7:100-2.
4. Ltcherath V, et al. Children with bilateral temporal arachnoid cysts may have Glutaric Aciduria Type 1 (GAT1); operation without knowing that may be harmful. Acta Neurochir (Wien) (2000)142:1025-30.
5. Osborne AG. Diagnostic Imaging: Brain. Amirsys Inc: Altona, 2004, 1st ed., pp. 1:62-69.