Applied Radiology

CASE SUMMARY

A 43-year-old male physician with a history of mild hypertension and a 20-year history of migraine headaches presented with a headache that seemed to him to be different in character and much more intense than usual. Medications included atenolol 50 mg QD and ibuprofen PRN. Laboratory data and physical examination were unremarkable. An MRI of the brain was performed (figures 1 and 2).

IMAGING FINDINGS

The sagittal TI-weighted MR image (figure 1) demonstrates a low-signal intensity, homo-geneous, extra-axial collection in the left frontal region with mild mass effect on the adjacent brain (arrows). The axial T2-weighted MR image (figure 2) shows the lesion as homogeneous high-signal intensity. The signal intensity of the lesion exactly follows that of CSF on all pulse sequences. Also seen on both images is characteristic bony remodeling of the inner table of the overlying calvarium, indicating the extremely slow growth of the lesion. No other abnormalities were noted. After consulting a neurologist, the patient started propranolol 80 mg QD and naproxen sodium PRN. Six months later, the patient reports no subsequent headaches.

DIAGNOSIS

Arachnoid cyst

DISCUSSION

True arachnoid cysts are the most common congenital cystic lesions in the brain, accounting for approximately 1% of intra-cranial mass lesions. ¹ Secondary arachnoid cysts have been reported after trauma and infection, forming when an inflammatory process causes arachnoiditis and subsequent encystment of the subarachnoid space. ² In most cases, however, the pathogenesis remains obscure.

About 50% of such lesions occur in the middle cranial fossa, the most common location overall. There is an association with temporal lobe hypoplasia when arachnoid cysts occur in this region, although the exact relationship is controversial. ¹ About one third occur in the posterior fossa, particularly the retrocerebellar, cerebellopontine, and quad-rigeminal plate cisterns. Posterior fossa cysts should be differentiated from a mega cisterna magna and Dandy-Walker malformation. Approximately 10% of these lesions are found in the suprasellar region, where they can mimic an enlarged third ventricle. An ependymal cyst within the third ventricle could produce an identical image. Occasionally, cysts are found in the perisellar region and over the convexities. A large convexity arachnoid cyst can resemble a chronic subdural hematoma. Rare intraventricular cysts have been reported. ³ CT imaging features include a CSF density mass that effaces adjacent sulci and remodels bone. Small cysts may not be seen due to partial volume averaging, especially when located in the middle cranial fossa, and expansion of the sylvian fissure or bony scalloping may indicate the presence of a cyst.

On MRI, the arachnoid cyst appears as an extra-axial mass that follows CSF intensity on all pulse sequences. Occasionally, the signal intensity of an arachnoid cyst can be similar or identical to that of an epidermoid tumor. ¹ Smooth erosion of the inner table of the skull is seen on both CT and MRI, which is thought to be the result of extremely slow growth and transmitted CSF pulsations. ² There is no enhancement. It is unclear if the arachnoid cyst in this case contributed to the patient's symptoms, even though the headaches improved with a change in medication. Arachnoid cysts are usually regarded as incidental findings of no clinical significance. Symptoms can occur when cysts enlarge and exert sufficient mass effect to result in CSF obstruction and subsequent hydrocephalus, a situation commonly seen in children with large posterior fossa cysts.

Symptoms associated with arachnoid cysts include headache, seizures, mental retardation, and cognitive impairment. ^3,4 However, since these nonspecific symptoms represent the most common indications for neurologic imaging, a causal relationship may not exist. Therefore, it is understandable that therapy of headaches and seizures in a patient with a concomitant arachnoid cyst is controversial. ⁴

Cysts in a suprasellar location are associated with symptoms specific to their location, including endocrine abnormalities and damage to the optic chiasm. Sudden expansion of cysts has been reported and can result in hydrocephalus and symptoms of elevated intracranial pressure. ⁵ Our patient had no prior imaging studies that would have been helpful to confirm cyst chronicity without a significant recent change in size. In addition to sudden expansion, other reported complications include hemorrhage and secondary infection of an arachnoid cyst in the setting of meningitis. ^1,2 In the presence of acute hemorrhage, the original arachnoid cyst may be difficult to detect, resulting in the appearance of an unusual looking hemorrhage. Therefore, awareness of this complication is important. Middle cranial fossa cysts have a much higher association with hemorrhage than cysts in other locations.

Indications for surgical intervention include hydrocephalus and elevated intracranial pressure. ^1,2 Surgery for asymptomatic cysts is controversial. Surgical options include marsupialization, fenestration, and cystoperitoneal shunting. ⁴ Spontaneous rupture of arachnoid cysts with development of subdural hygromas has been reported. ⁶ In addition, total disappearance of arachnoid cysts has been reported after minor head trauma, presumably due to a tear in the cyst membrane, leakage, and subsequent resorption of cyst fluid. ⁶

In conclusion, arachnoid cysts typically represent benign incidental lesions with characteristic imaging features. Occasionally, they can become symptomatic and require surgical intervention. Arachnoid cysts should be included in the differential diagnosis of cystic extra-axial lesions of the brain.

REFERENCES

1. Flodmark O: Neuroradiology of selected disorders of the meninges, calvarium, and venous sinuses. AJNR 13:483-491, 1992.

2. Goldberg HI, Lavi E, Atlas SW: Extra-axial brain tumors. In: Atlas SW, (ed): Magnetic Resonance Imaging of the Brain and Spine, pp 471-472. Philadelphia, Lippincott-Raven, 1996.

3. Maiuri F, laconetta G, Gangemi M: Arachnoid cyst of the lateral ventricle. Surg Neurol 48:401-404, 1997.

4. Koch CA, Voth D, Kraemer G, Schwarz M: Arachnoid cysts: Does surgery improve epileptic seizures and headaches? Neurosurg Rev 18:173-181, 1995.

5. Callaway MP, Renowden SA, Lewis TT, et al: Middle cranial fossa arachnoid cysts: Not always a benign entity. Br J Radiol 71:441-443, 1998.

6. Mori T, Fujimoto M, Sakae K, et al: Disappearance of arachnoid cysts after head injury. Neurosurgery 36:938-942, 1995.

Prepared by Glenn G. Gray, MD and Lewis Rothman, MD, Department of Radiology, Lenox Hill Hospital, New York, NY.