Pilocytic astrocytoma of the optic pathway

6-year-old boy with intermittent headache, nausea, and vomiting in the past 2 months. No relevant personal or family history. He was taken to the emergency room due to symptoms of intracranial hypertension where a brain CT is performed and, after that, was referred to our neuroradiological team to perform an MRI examination.

Emergency brain CT showed a solid right hemispheric intraparenchymal lesion, with a rounded morphology and well-defined edges (47 mm of maximum diameter). No signs of bleeding or calcium. It produced an important mass effect on the midline and third ventricle, causing a severe dilation of the lateral ventricles (Figure 1).

The MRI confirmed a heterogeneous lesion with a solid enhancing component and peripheral cystic areas, with no diffusion restriction (Figure 2 and 3). It was located immediately superior to the optic band and chiasm, and also compressing the basal ganglia and lateral ventricles (Figure 4). It did not show a direct relationship with vascular structures. In the tractography study it was possible to see how it depended on the right optic band and displaced it caudally (Figure 5). This study also confirmed the signs of ependymal transudation from the lateral ventricles due to obstructive hydrocephalus (Figure 6).

Background

Pilocytic astrocytoma (PA) is a slow-growing glioma, classified as grade I by the World Health Organisation (WHO), typically appearing in children [1]. In fact, it is the most common primary brain tumor in childhood. The posterior fossa (cerebellum) is the most common site of origin, however, it can appear anywhere along the entire neuraxis, with a preference for the optic path, hypothalamus, brainstem, thalamus and basal ganglia [2].

Clinical Perspective

Optic pathway gliomas (OPG) are relatively rare. Two forms have been described, the sporadic one, as the patient we present here, and the one related to neurofibromatosis type 1 (NF1) (15-50% of children with NF1 develop OPG) [3,4]. The form of presentation is slightly different; in children with NF1 the most frequent site of presentation is the optic nerve, while in sporadic cases it usually presents in the optic chiasm or tract and associates hydrocephalus more frequently. Also, in cases associated with NF1, the tumors usually present as a thickening of the nerve, without cystic areas, while sporadic cases usually present the typical solid-cystic image [5,6].

Symptoms vary depending on location and extension of the lesion and the structures involved. Headache and vomiting are the main onset symptoms in all cases. In case of optic pathway involvement clinical presentation can vary from visual loss, bitemporal/homonymous hemianopsia, diplopia, papilledema and nystagmus.

Imaging Perspective

The typical presentation of a PA consists of a well-circumscribed round mass with solid and cystic components. The solid components are usually iso-hypointense on T1 and hyperintense on T2/FLAIR with heterogeneous enhancement after contrast administration. On DWI PA has high ADC values as other low-grade tumors. Diffusion tensor imaging and tractography are especially useful for supratentorial PA for surgery planning [7].

Treatment and Outcome

As with posterior fossa and cerebral tumors, PAs that affect the chiasm or the optic tract are usually treated surgically, with complete resection being the main prognostic factor. Gliomas of the optic nerve (usually NF1-related) are usually closely observed due the indolent behavior [8]. This patient was treated by surgery with practically complete resection and has been followed up for 8 years without signs of progression.

All patient data have been completely anonymised throughout the entire manuscript and related files.