A midline prepontine cyst: Serial magnetic resonance imaging over 20 years shows very slow growth after its rapid shrinkage

Abstract

An otherwise healthy 22-month-old boy suffered high fever, irritability, nausea, dysphagia, dysarthria and right hemiparesis. Magnetic resonance imaging showed a cystic mass, 15 mm in diameter, with surrounding oedema in the base of the lower pons. The symptoms subsided in about 10 days after onset, followed by a rapid decrease of the cyst size to 5 mm. Thereafter, the patient’s psychomotor growth has been normal. Annual follow-up magnetic resonance imaging scans showed very gradual enlargement of the cyst located on the ventral surface of the pontomedullary junction, reaching 16 mm in diameter in 21 years after onset. It was hyperintense on T1-weighted and isointense on T2-weighted magnetic resonance imaging. No haemosiderin deposition or gadolinium enhancement was seen. This is a rare report of a two decade longitudinal follow-up of a midline prepontine cyst showing asymptomatic and very slow growth. The possible nature of the cyst includes neurenteric, dermoid and epidermoid cyst.

Introduction

The prepontine space is a frequent site of cystic lesions including epidermoid, dermoid and neurenteric cyst.^1,2Such lesions generally manifest a gradual progression of cranial nerves or brainstem signs.^1,2These may, however, present with acute onset of the signs and inflammatory symptoms due to leakage of the cyst content.^3,4We report here on a patient with a midline prepontine cyst who presented with acute symptoms in early childhood. Longitudinal follow-up over two decades, which is rarely reported, revealed subclinical and very slow growth of the cyst after the remission of initial symptoms. We also discuss the differential image diagnoses of the prepontine cyst.

Case report

A 22-month-old boy without significant past medical history was brought to a nearby paediatrician with irritability, high temperature of 39℃ and vomiting (day 1). He was found to have right hemiparesis, dysphagia and dysarthria and was sent to a general hospital. On admission, he was alert but showed signs of meningeal irritation. Computed tomography (CT) scan found a low density lesion in the brain stem. On day 10, magnetic resonance imaging (MRI) showed a cystic mass, 15 mm in diameter, in the base of the lower pons (Figure 1) attached to the ventral pontine surface. It caused oedema in the lower half of the pons. The cyst wall was thickly enhanced. No abnormal signal intensity was seen in the cerebrospinal fluid (CSF) space. With the suspicion of malignant tumour or brain-stem encephalitis, a biopsy was considered. However, his inflammatory signs and neurological symptoms subsided quickly without any specific treatment. He was discharged on day 20 without sequel.

Figure 1.

Magnetic resonance imaging 10 days after onset. (a) Axial T1-weighted image (T1WI); (b) axial T2-weighted image (T2WI); (c) sagittal T1WI; (d) axial gadolinium enhanced (GE) image; (e) coronal GE image; (f) sagittal GE image. Cystic mass with low signal intensity on T1WI (a, c) and high signal intensity on T2WI (b), surrounded by oedema, was seen in the base of the lower pons. The cyst wall, thickly enhanced by gadolinium, was attached to the ventral surface of the pontomedullary junction.

Follow-up MRI on days 43 and 98 showed shrinkage of the cyst and improvement of the oedema (Figure 2(a–d)). On day 146, the size of the cyst had decreased to 5 mm (Figure 2(e,f). His psychomotor development has been normal since then. A nearby neurosurgeon followed him up under a tentative diagnosis of pontine cavernous angioma.

Figure 2.

Magnetic resonance imaging within 5 months after onset. Upper row: axial T1-weighted images (T1WIs); lower row: sagittal T1WIs. (a) and (b) Day 43; (c) and (d) day 98; (e) and (f) day 146. The cyst, hyperintense on T1WI, rapidly decreased its size.

Sixteen years after onset, the patient at his first grade of college was referred to our outpatient clinic due to a gradual increase of the cyst size. It was 8 mm at 6 years, 11 mm at 10 years and 14 mm at 16 years after onset. A 3-Tesla MRI in our hospital showed the globoid cystic mass, 14 mm in the maximal diameter, attached to the anterior surface of the pontomedullary junction. The cyst content was hyperintense on T1-weighted imaging (T1WI) and isointense on T2-weighted imaging (T2WI) and diffusion-weighted imaging (DWI) (Figure 3). No haemosiderin deposition or enhancement of the cyst wall was seen. The probable image diagnosis of neurenteric cyst was made according to these MRI findings. The patient was advised regular follow-up. Annual follow-up MRIs have also shown a very gradual increase of size, 16 mm at the latest follow-up (Figure 4). Interestingly, a bright spot in the cyst on T1WI has changed its location on every MRI study (arrows): implicating movement of a small part with different content from that of the main part. Twenty-one years after onset, the patient is now 23 years old and currently working as a computer engineer. He has preferred observation with follow-up MRI in spite of a suggestion of surgical decompression.

Figure 3.

Magnetic resonance imaging at 16 years after onset. (a) Axial T1-weighted image (T1WI); (b) axial T2-weighted image (T2WI); (c) axial diffusion-weighted image (DWI); (d) axial gadolinium enhanced (GE) image; (e) coronal GE image; (f) sagittal GE image. The cyst, hyperintense on T1WI (a) and isointense on T2WI (b) and DWI (c) located on the ventral surface of the pontomedullary junction. Enhancement of the cyst wall was absent (d–f).

Figure 4.

Follow-up T1-weighted magnetic resonance imaging 17–21 years after onset. (a) and (b) 17 years; (c) and (d) 19 years; (e) and (f) 21 years after onset. The cyst has very slowly increased in size during the time course. Note the change of location of the bright spot in the cyst (arrows).

Discussion

Various cystic lesions arise in the prepontine space, such as arachnoid cyst, epidermoid, dermoid and neurenteric cyst.^1,2,5Neuroradiological clues to differentiate these cysts have been intensely discussed.^1,2,5The parameters for differentiation include CT density and signal intensities on MRI provided by various pulse sequences.

Typically, an arachnoid cyst is composed of cyst contents with uniform intensities compatible with CSF without internal architecture and identifiable cyst wall on MRI.^2,5

An epidermoid cyst generally presents with a hypoattenuated mass on CT scan. The content is hypo to isointense on T1WI and hyperintense on T2WI compared to brain parenchyme. The hyperintensity on DWI, due to high protein content, is a characteristic feature of epidermoid cysts.^2,5Cyst walls are rarely enhanced.

A dermoid cyst generally shows imaging characteristics of lipid reflecting liquid cholesterol in the cyst; typically very hypodense on CT scan and strongly hyperintense on T1WI and DWI.^1,2It shows a heterogeneously mixed intensity on T2WI. If it ruptures, wide dissemination of lipid droplets in the CSF space is commonly observed.

The content of neurenteric cysts is usually iso to slightly hyperintense on T1WI and iso to hyperintense on T2WI. The intensity of the cyst on MRI may vary depending on the protein content of the cyst fluid.¹It rarely shows internal architecture. It generally shows mild or no restriction on DWI, differentiating it from epidermoid and dermoid cysts.^1,2

In our case, recent MRIs showed homogeneous hyperintensity on T1WI and isointensity on T2WI and DWI.^1,2,5No haemosiderin deposition was seen. These features are strongly against the possibility of arachnoid cyst and cavernous angioma. The absence of a lipid signal on T1WI and hyperintensity on DWI seem also not to favour the possibility of epidermoid and dermoid cysts, and suggest neurenteric cyst as the most probable image diagnosis. The midline location of the lesion also supports this presumption.^1,6

The neurenteric cyst is believed to be a slow-growing lesion, which shows slight changes over the years as a result of balanced secretion and absorption of cyst contents.^1,2,7It generally manifests the signs of cranial nerve involvement, brain stem compression and increased intracranial pressure.^1,2However, they occasionally cause chemical meningitis due to rupture of the cyst wall and leakage of the cyst contents, presenting with signs of meningeal irritation.^3,4The reduction of cystic size after chemical meningitis has been reported, as is seen in the first year of onset in our case.⁴Thick enhancement of the cyst wall and oedema in the brain stem revealed on MRI 10 days after the symptomatic onset also seems to be caused by the inflammation. The rare enhancement of the cyst wall seems compatible with that seen in some of Rathke’s cleft cysts with inflammation, thought to be the result of leakage of the contents.⁸

The intensity of the cyst contents has remained unchanged during follow-up in our case. Some authors, however, have reported changes in magnetic resonance intensity of cyst contents during long-term follow-up, 33 months to 7 years, which was accompanied with a significant increase in size.^9,10,11These changes were accompanied by squamous metaplasia of the epithelium and chronic inflammation of the cyst wall.^9,10,11,12The authors speculated that the alteration of magnetic resonance signal intensity was due to mechanical or chemical stimulation such as cyst rupture or surgical impact. Thus the changes of intensity can be a forewarning of a rapid change of the cyst size.

Although the lesion so far has followed an indolent course, we are keeping an eye on the neurological condition, size of the cyst, and magnetic resonance intensities to determine the pertinent timing for surgical intervention.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Conflict of interest

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.