A 3-Year-Old Girl with Brain Imaging Findings Suggesting Arachnoid Cyst, and Biopsy Diagnosis of Extra-Axial Multicystic Pilocytic Astrocytoma

Background

Arachnoid cysts, benign fluid-filled sacs within the arachnoid membrane, are a relatively well-understood entity. They are typically associated with congenital or acquired origins and often present with symptoms reflecting their location within the intracranial space. In contrast, pilocytic astrocytomas are low-grade brain tumors primarily affecting pediatric and young adult populations. Arising from astrocytes, these tumors exhibit variable clinical manifestations and radiological features that can pose diagnostic challenges. Importantly, their management and prognosis diverge significantly from arachnoid cysts [1].

The difficulty in accurately diagnosing pilocytic astrocytomas stems from their atypical radiological characteristics, which can occasionally mimic other more common intracranial lesions, such as arachnoid cysts and schwannoma [2–4]. This case report highlights one such challenging scenario in which a 3-year-old girl, who initially received a diagnosis of an arachnoid cyst, presented with symptoms indicative of an alternative intracranial pathology. The case underscores the need for vigilance and a multidisciplinary approach when encountering neuroimaging findings that deviate from typical patterns.

Case Report

The patient was a 3-year-old girl who was admitted to our hospital with concerns of persistent headaches, vomiting, and difficulty walking. The patient had no significant past medical history and was previously healthy. Her parents reported that the symptoms had been present for the past 2 weeks and had progressively worsened.

On physical examination, the patient was alert and oriented, with a Glasgow Coma Scale score of 15. She had papilledema on ophthalmoscopic examination, and a right-sided cerebellar ataxia was observed. Magnetic resonance imaging of the brain revealed a well-circumscribed cystic lesion measuring 3 cm in diameter at the right cerebellopontine angle. The lesion had a signal intensity similar to that of the cerebrospinal fluid on all sequences. These findings were consistent with an arachnoid cyst (Figure 1). Electroencephalography examination was not conducted in this case. The decision was based on the patient’s clinical presentation, radiological findings, and the urgency to address the patient’s symptoms promptly.

The patient underwent a right-sided retrosigmoid craniotomy and total excision of the lesion. Intraoperative findings revealed a cystic lesion without a solid mural nodule at the right cerebellopontine angle, adherent to the surrounding cranial nerves. The lesion was excised completely, with preservation of the adjacent neurovascular structures, including the cerebellar structures.

The postoperative course was uneventful, and the patient was discharged on postoperative day 5. The patient’s headaches and ataxia resolved completely, and she had no further neurological deficits.

The histopathological examination of the excised tumor revealed a pilocytic astrocytoma with characteristic features, including a biphasic pattern with elongated spindle-shaped cells and piloid cells with hair-like processes. The tumor also contained microcysts filled with clear fluid, Rosenthal fibers, and occasional hemosiderin deposits. The Ki-67 labeling index was low, indicating a low proliferative rate of the tumor (Figure 2).

Extra-axial pilocytic astrocytoma is a rare type of tumor that arises outside of the brain parenchyma. In this case, the initial impression was that the patient had an arachnoid cyst, which is a more common diagnosis than an extra-axial pilocytic astrocytoma. However, the histopathology examination of the tumor revealed the presence of pilocytic astrocytoma.

Discussion

Pilocytic astrocytoma is a type of brain tumor that is typically located in the cerebellum and affects children and young adults. It is considered to be a low-grade tumor, and patients with pilocytic astrocytoma usually have a good prognosis following surgical resection of the tumor. However, extra-axial pilocytic astrocytoma is an uncommon subtype of the tumor, and there are limited case reports published in the literature [2,5–8].

The initial radiological assessment of the cerebellopontine angle lesion in this case suggested an arachnoid cyst. The differential diagnosis included several conditions, such as epidermoid cysts, dermoid cysts, and even other tumors that can mimic arachnoid cysts. This highlights the complexity of distinguishing between different intracranial lesions based solely on radiological characteristics, as these conditions can share similarities in their appearance on imaging. This case underscores the inherent limitations of radiological assessments when dealing with atypical cases. Despite significant advances in neuroimaging that have improved the accuracy of diagnosis, certain lesions, such as extra-axial pilocytic astrocytomas, can challenge both radiologists and clinicians owing to their potential to resemble more common entities. In clinical practice, it is imperative for neurosurgeons, radiologists, and pathologists to collaborate closely, especially when confronted with challenging radiological cases, to ensure accurate diagnosis and appropriate treatment.

The diagnosis of extra-axial pilocytic astrocytoma is challenging, as the radiological features of the tumor can be similar to those of other more common lesions, such as arachnoid cysts [9]. In this case, the radiological features of the lesion closely resembled those of an arachnoid cyst, leading to its initial misdiagnosis. This scenario underlines the practical and theoretical difficulties associated with preoperative diagnoses in cases with atypical radiological findings. The uncommon prevalence of extra-axial pilocytic astrocytomas further complicates matters, as there is limited literature available to guide clinicians in differentiating them from more common lesions.

While not all arachnoid cysts require surgical intervention, in this case, the decision to proceed with surgery was primarily driven by the patient’s clinical presentation and imaging findings [1]. The patient presented with persistent and worsening symptoms, including headaches, vomiting, and difficulty walking. These symptoms, along with the radiological characteristics of the lesion, which initially suggested an arachnoid cyst, raised concerns about the potential for mass effect and neurological compromise. Given the urgency to address the patient’s condition, a surgical intervention was deemed necessary. Importantly, it was only postoperatively that the true nature of the lesion was revealed as a pilocytic astrocytoma.

The treatment of extra-axial pilocytic astrocytoma involves surgical resection of the tumor, which is typically curative for low-grade tumors [3,9]. In the present case, the patient underwent a left suboccipital craniotomy with total excision of the tumor, which resulted in a complete resolution of the patient’s neurological symptoms. The patient’s symptoms improved gradually, and she was discharged on the 5th postoperative day.

While the surgical intervention described in this case report involved a traditional craniotomy approach, it is worth considering the potential for minimally invasive surgical techniques in the management of extra-axial pilocytic astrocytomas [10]. Minimally invasive approaches, such as endoscopic procedures, stereotactic biopsies, or keyhole surgeries, have gained prominence in neurosurgery and can offer benefits in terms of reduced morbidity and shorter hospital stays. In select cases, these techniques could be explored to achieve tumor excision while minimizing the impact on the patient’s overall well-being. However, the suitability of such approaches should be evaluated on an individual basis, considering factors such as tumor location, size, and the patient’s clinical condition.

Long-term follow-up is essential for patients with extra-axial pilocytic astrocytoma, as there is a risk of recurrence or the development of new lesions [9]. The patient in this case was scheduled for regular follow-up appointments with the neuro-surgeon and pediatrician, and a long-term follow-up plan was established to monitor for any recurrence or complications.

Conclusions

In conclusion, the presented case underscores the practical and theoretical difficulties associated with diagnosing extra-axial pilocytic astrocytomas, especially when their radiological features closely mimic more common intracranial lesions like arachnoid cysts. Extra-axial pilocytic astrocytoma is an uncommon type of pilocytic astrocytoma that can be challenging to diagnose. However, a histopathology examination of the tumor is necessary to confirm the diagnosis and plan appropriate treatment. Surgical resection of the tumor is typically curative, and long-term follow-up is necessary to monitor for any recurrence or complications.