Neurofibromatosis type 1 (NF-1) is a multisystem genetic disorder characterized by cutaneous, neurological, and skeletal manifestations. Sphenoid wing dysplasia is a characteristic but relatively uncommon osseous abnormality in NF-1 and may result in herniation of intracranial contents into the orbit, producing an orbital encephalocele and progressive proptosis. In routine clinical practice, imaging evaluation of proptosis in NF-1 often prioritizes exclusion of optic pathway glioma or optic nerve sheath meningioma, which may lead to under-recognition of sphenoid wing dysplasia and its complications.
Consequently, orbital encephalocele may be misdiagnosed as an arachnoid cyst.
This report describes two patients with NF-1—one pediatric and one adult—both presenting with unilateral proptosis and previously misdiagnosed as having arachnoid cysts. Through these cases, characteristic CT and MRI features facilitating accurate diagnosis are highlighted, emphasizing the importance of careful skull base assessment to avoid this important diagnostic pitfall.

Keywords: Neurofibromatosis type 1; Sphenoid wing dysplasia; Orbital encephalocele; Proptosis; Arachnoid cyst; Diagnostic pitfall

Neurofibromatosis type 1 (NF-1) is a common autosomal dominant neurocutaneous disorder caused by pathogenic variants in the NF1 gene on chromosome 17, with an estimated incidence of approximately 1 in 3,000 individuals. The disease is characterized by marked phenotypic heterogeneity and involves multiple organ systems, including the skin, nervous system, and skeleton. While cutaneous manifestations and optic pathway gliomas are well recognized and frequently emphasized in clinical practice, skeletal abnormalities—despite being part of the diagnostic criteria—are comparatively under-recognized.

Sphenoid wing dysplasia is a characteristic but relatively uncommon skeletal manifestation of NF-1, reported in approximately 5–10% of affected individuals. Partial or complete absence of the sphenoid wing results in loss of the normal osseous barrier between the orbit and the middle cranial fossa, predisposing to herniation of cerebrospinal fluid (CSF) and brain parenchyma into the orbit. This may lead to the development of an orbital encephalocele, presenting clinically with progressive, often unilateral proptosis and potential optic nerve compression.

In routine imaging evaluation of proptosis in patients with NFNeurofibromatosis 1, diagnostic attention is frequently directed toward excluding optic pathway glioma or optic nerve sheath meningioma, given their strong association with the disease. This diagnostic bias may result in under-recognition of sphenoid wing dysplasia and its complications. Furthermore, because orbital encephaloceles often demonstrate CSF-like signal characteristics on MRI, they may be misdiagnosed as arachnoid cysts, particularly when osseous abnormalities are subtle or not adequately assessed.

This case series describes two patients with NF-1—one adult and one pediatric—both presenting with unilateral proptosis and initially misdiagnosed as having arachnoid cysts. Through detailed CT–MRI correlation, this report highlights characteristic imaging features that facilitate accurate diagnosis, underscores an important diagnostic pitfall, and emphasizes the clinical and management implications of early recognition.

Written informed consent was obtained from both patients and/or their legal guardians for publication of clinical details and imaging findings. All procedures were conducted in accordance with institutional ethical standards and the principles of the Declaration of Helsinki.

Case 1
A 39-year-old man with a known diagnosis of neurofibromatosis type 1 presented with progressively increasing left-sided proptosis over several years. The patient reported mild visual discomfort but no acute visual loss, diplopia, or pain. There was no history of trauma or prior orbital surgery. Clinical examination revealed non-pulsatile left proptosis without signs of acute inflammation.
MRI of the brain and orbits with contrast demonstrated hypoplasia of the left sphenoid wing with herniation of brain parenchyma and CSF into the superior aspect of the left orbit, consistent with a left

orbital encephalocele. This resulted in left-sided proptosis with compression and mild medial deviation of the intraorbital segment of the left optic nerve. Marked narrowing of the left superior orbital fissure was noted, with optic nerve compression at this level.
Prominent arachnoid spaces were seen in the left anterior temporal region secondary to the encephalocele, with medial displacement of the left middle cerebral artery. No abnormal enlargement or postcontrast enhancement of the optic nerve was identified, effectively excluding optic nerve glioma and optic nerve sheath meningioma. Additional findings included multiple scalp neurofibromas and T2/FLAIR hyperintense foci involving the bilateral thalami and periventricular regions, consistent with focal areas of signal intensity (FASI). The left sphenoid sinus was hypoplastic.
CT of the skull base confirmed hypoplasia of the left sphenoid wing with loss of the normal bony separation between the middle cranial fossa and the orbit. Prior imaging studies had described the lesion as an arachnoid cyst, leading to delayed recognition of the underlying pathology.
The patient was referred for multidisciplinary evaluation, and given the slowly progressive nature of symptoms and preserved vision, conservative management with clinical and imaging followup was advised.

An 8-year-old boy, born to a mother with neurofibromatosis type 1, presented with gradually progressive left-sided proptosis noted by caregivers over the preceding year. There was no history of trauma, pain, or visual complaints. Ophthalmologic evaluation revealed mild asymmetry of the globes without acute visual deficit.
MRI of the brain and orbits with contrast, supplemented by CT imaging, revealed dysplasia of the left sphenoid wing with anterior herniation of the left temporal lobe into the posterior aspect of the left orbit, consistent with an orbital encephalocele. This resulted in leftsided proptosis and mild asymmetric enlargement of the left globe.
The left medial and lateral rectus muscles were enlarged and demonstrated post-contrast enhancement, suggestive of extraocular

muscle neurofibromas, with mild surrounding orbital fat stranding. The intraorbital segment of the left optic nerve appeared mildly tortuous but showed no enlargement or abnormal enhancement. CT demonstrated absence of tram-track calcification, excluding optic nerve sheath meningioma. The left superior orbital fissure was asymmetrically enlarged, and the left sphenoid sinus was hypoplastic.
MRI of the brain demonstrated multiple non-enhancing T2/ FLAIR hyperintense lesions involving the bilateral globus pallidi, thalami, cerebellum, and cerebral peduncles, characteristic of FASI. No diffusion restriction or mass effect was observed. As in the first case, the lesion had previously been interpreted as an arachnoid cyst.
Given the patient’s age and absence of visual compromise, a conservative approach with close ophthalmologic and radiologic surveillance was recommended.