British Journal of Radiology 75 (2002),489-490 © 2002 The British Institute of Radiology
A new lump
S M Williams, MRCP, FRCR
1 and
A M Dean, FRCR
2
1 Department of Radiology, Norfolk and Norwich University Hospital, Norwich NR4 7UY and 2 Department of Radiology, Northampton General Hospital, Cliftonville, Northampton NN1 5BD, UK
Correspondence: S M Williams
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Introduction
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A 7-year-old boy was referred to hospital for assessment of a lump in the left temporal region. 6 weeks previously he had caught his head on a metal pole, sustaining a blow just above his left ear. There had been no loss of consciousness. His mother had noted a hard swelling in this area and had become concerned when it persisted unchanged following the trauma. The boy had remained well with no complaints of headache or clumsiness. His mother was adamant that no scalp lump had been palpable prior to the head injury. The boy was developmentally within normal limits with no other significant history. Clinical examination was unremarkable apart from a non-tender, non-fluctuant 4 cm x 5 cm hard swelling above the left ear.
A skull radiograph (Figure 1
) and, subsequently, CT of the head (Figure 2) were obtained. What are the radiological findings?
From the history one might suspect a simple scalp haematoma or possibly a leptomeningeal cyst. However, despite the history of trauma and a newly palpable lesion, the plain radiographic findings were those of a long-standing bone abnormality. There was a well defined lucent lesion arising from the left temporal region. No fracture or features of bone destruction are present. The lesion was solitary. The bones appeared otherwise normal, in particular there were no features to suggest fibrous dysplasia. The key finding is that of scalloping of the inner table of the skull within the lesion. This suggests pressure erosion secondary to a long-standing/slow growing intracranial mass lesion. Unenhanced CT (Figure 2
) showed a large, well defined low attenuation lesion arising from the left temporal fossa with some displacement of midline structures to the right. The appearances were those of a large middle cranial fossa arachnoid cyst. MRI subsequently confirmed these findings, with a cerebrospinal fluid (CSF) signal lesion separated from the trigone by a thin membrane (Figure 3). In view of the degree of mass effect, an arachnoid cyst fenestration procedure was performed in order to establish drainage between the cyst and ventricular system.
Arachnoid cysts are congenital lesions of the arachnoid membrane that expand by CSF secretion [1]. The arachnoid-rich sylvian fissures are the most common location, accounting for over half of all arachnoid cysts [2]. The majority of the remainder are related to the basal cisterns. Arachnoid cysts usually occur sporadically, but have a higher incidence in individuals with adult polycystic kidney disease. Many arachnoid cysts are asymptomatic and are discovered incidentally. Large lesions may be associated with hydrocephalus or seizures. Of those arising within the middle cranial fossa, 20% are small and of no clinical significance, 50% are of moderate size with some expansion of the temporal fossa and 30% are large with a marked associated mass effect that may include expansion of the hemicranium and midline shift. The distortion of the cortical veins by large cysts results in an increased risk of intracyst and subdural haemorrhage, which can occur following trauma, often of a relatively minor nature. Intracystic bleeds can result in some heterogeneous signal, not of CSF intensity, on MRI [3]. There was no evidence of secondary haemorrhage in our patient, despite an episode of trauma being the reason for him coming to medical attention.
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Acknowledgments
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We would like to thank Dr Fiona Thompson, Consultant Paediatrician, Northampton General Hospital, for her permission to use the case.
Received for publication December 8, 2000.
Accepted for publication January 24, 2001.
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References
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Barkovich AJ. Hydrocephalus. In: Barkovich AJ. Pediatric Neuroimaging (3rd edn). Philadelphia, PA: Lippincott Williams & Wilkins, 2000:592–5.
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Wester K. Peculiarities of intracranial arachnoid cysts: location, sidedness and sex distribution in 126 consecutive patients. Neurosurgery 1999;45:775–9.[Medline]
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Ibarra R, Kesava PP. Role of MR imaging in the diagnosis of complicated arachnoid cyst. Pediatr Radiol 2000;30:329–31.[Medline]
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