Optic Nerve Cysts

Etiology

The cysts may be caused by a variety of etiologies including idiopathic, associated with meningiomas, meningoceles, arachnoid cysts, neuroepithelial cysts, and post-traumatic or postoperative cysts. Specifically, optic nerve sheath meningocele is a dilation of the optic nerve sheath, and expansion of the subarachnoid or subdural cerebrospinal fluid (CSF)-filled space around the optic nerve. An arachnoid cyst is a benign proliferation of normal fibrovascular tissue of the leptomeninges around the optic nerve. Neuroepithelial cysts are rare lesions that typically present in the ventricles or cerebral parenchyma. They have rarely been reported in the intracranial portion of the optic nerve.[1]

Cysts may also be iatrogenic. For example, Naqvi et al.[2] reported 2 cases of cysts that formed after optic nerve sheath fenestration.

Epidemiology

The frequency of cysts is difficult to determine given the varied etiologies of the cysts across age groups. The cysts may also be asymptomatic and never diagnosed. For example, per Lunardi et al.,[3]  only 31 cases of optic sheath meningocele are reported in the literature.

Pathophysiology

The pathophysiology of primary arachnoid cysts is unclear. In the setting of traumatic arachnoid cysts or post-optic nerve sheath fenestration iatrogenic cysts, Akor et al.[4] proposed that trauma results in optic nerve sheath entrapment of secretory neuroepithelial cells and cyst formation. In the setting of congenital cysts, there may be entrapment of these cells during embryogenesis. Lunardi et al.[3] suggest that congenital arachnoid cysts develop through a diverticulum or blind pocket within the arachnoid membrane.

Optic nerve sheath meningoceles may be caused by the abnormal bony structure of the orbital apex. The subarachnoid space is narrowest near the optic canal. Congenital narrowing here could cause meningoceles.[3]

In some cases of optic nerve cysts, dilation of the optic nerve sheath complex could be secondary to proximal neoplasm, vascular hamartoma, or cranio-orbital fracture. In these cases, the neoplastic tissue or fracture acts like a ball valve allowing CSF to enter one direction into the sheath.[3]

Neuroepithelial cysts may be caused by developmental abnormalities when there is incomplete closure along the optic fissure. A more severe version of this condition is a colobomatous cyst also known as microphthalmia with a cyst.[1] The superior end of the embryonic fissure does not close at the time of 10 to 13 mm embryo. The edges of the fissure evert as they oppose one another, forming a cystic structure. These cysts' walls have collagenous fibers and poorly differentiated neuroepithelium and neuroglial tissue. Cysts are connected to the sclera. They may be filled with eosinophilic material or photoreceptor segments.[5]

Vision loss is thought to be due to compression of the optic nerve itself.

Histopathology

Histopathology of optic nerve sheath meningoceles and arachnoid cysts shows normal meninges.[6]

A colobomatous optic nerve cyst is composed of 2 layers. The inner layer has retinal architecture and is derived from primitive neuroretinal tissue. The outer layer has vascularized connective tissue continuous with the sclera.[6]

Mehta et al.[1] reported on a case of a neonate with a neuroepithelial cyst that was resected. Histopathology showed the cyst was lined by simple cuboidal epithelium without cilia or goblet cells. No neural tissue was present; however, immunohistochemical staining was positive for S100, a marker of neural tissue.[1]

History and Physical

Patients with optic nerve cysts may be asymptomatic or present with nonspecific orbital or neurological findings. In the setting of arachnoid cysts, visual acuity can range from 20/20 to NLP.

Garrity et al.[7] reported in a case series of thirteen patients with optic nerve sheath meningocele who presented with a headache, decreased vision, proptosis, afferent pupillary defect, enlarged blind spot, optic disc edema, shunt vessels at the optic disc, and tortuous retinal veins.

Colobomatous cyst often involves the optic nerve itself. As the lesion grows, typically inferiorly, it may produce a palpable mass behind the lower eyelid. In some cases, lowering the lower eyelid can reveal a dark uveal pigmentation to the mass. Bilateral cases may be associated with systemic diseases such as chiasmal glioma, polycystic kidney, trisomy, or Edward syndrome.[1]

Evaluation

Lesions that can be localized on history and physical exam to the optic nerve or brain should be imaged with brain and or orbital MRI with and without contrast. Imaging can elucidate the size of the cyst, consistency, and effect on surrounding structures with imaging. Typically arachnoid cysts and meningoceles will have a signal intensity equal to that of CSF, dark on T1 and bright on T2, without enhancement after intravenous (IV) contrast administration.[6]

Optic nerve sheath meningoceles appear as tubular-cystic enlargement of the optic nerve and optic nerve sheath complex on CT and MRI. On coronal MRI both the optic nerve and sheath appear dilated in a "bull's eye" pattern. Off-axis sagittal views are the best for showing widening of the meninges with a fluid-filled space.[6]

In children with microphthalmos, orbital ultrasound should be used to determine the organization of ocular structures. Ultrasound can also be used to visualize cysts in these patients. MRI is more helpful than CT in characterizing the content of the cyst, which is usually similar to vitreous or CSF. MRI also be used to visualize the relationships or connections, communications between the cyst, nerve, and globe. Visual potential can be evaluated with retinal electrophysiology.[5]

Treatment / Management

Given the variety of etiologies of optic nerve cysts, treatment is not uniform or established in the literature.

Our understanding of treatment is mostly based on case studies or case series. The main concern with surgical resection or drainage of these cysts is that it can be associated with significant morbidity due to the risk of optic nerve transection. Additionally, surgeons have to ensure that the cyst itself is not a coloboma in an eye with stable vision as drainage can result in drainage of intraocular contents.

Saari et al.[8] reported an interesting case of arachnoid cyst of the intraorbital optic nerve in 1977 before imaging was available. The patient presented with slight pain on eye movements, and transient attacks of blurred vision, optociliary shunt vessels, significant optic disc edema with hemorrhages and macular edema, and shallowing of the anterior chamber all in the left eye only. The patient had a lumbar puncture with an opening pressure of 150 mm H20. The lesion was thought to be a meningioma. Optic nerve sheath fenestration showed forceful and voluminous egress of CSF. Histopathology of the resected optic nerve sheath was normal. Nine months after the procedure, the left eye was blind with significant optic nerve atrophy. The authors recommend prompt diagnosis and drainage of arachnoid cysts to preserve vision.(B3)

Naqvi et al.[2] reported 2 cases of post optic nerve sheath fenestration optic nerve cysts. The patients had loculated CSF surrounded by fibrous proliferation at the site of previous optic nerve sheath fenestration. One patient presented with pain, proptosis, and visual loss 9 months after the initial procedure. The other patient presented with vision decline and choroidal folds. Both patients underwent repeat fenestration during which the closed sheath with fibrosis and outpouching was visualized. Larger windows were created in the sheaths.(B3)

For optic nerve sheath meningoceles, Lunardi et al.[3] reviewed 31 cases and recommended early surgical management of optic nerve sheath decompression in patients who present with a rapid decrease in visual acuity over 3 to 6 months. They endorsed the improvement of visual function with minimal morbidity.(B3)

Mehta et al.[1] reported a case of neuroepithelial cyst that presented in a 6 week old with proptosis, exotropia, RAPD, and normal anterior and posterior segment ophthalmic exam. The cyst was drained and resected by anterior orbitotomy through an upper eyelid crease incision. The patient's presenting signs including RAPD resolved postoperatively. The authors iterate that the goal of treatment in patients who present with optic nerve cysts is to prevent or reverse vision loss.

Colobomatous cysts can be aspirated if they are cosmetically unacceptable. In many cases, if the cyst returns after repeat aspirations, the eye and cyst may be removed, and the socket may be fitted with prosthetics.[6](B3)

Differential Diagnosis

The differential diagnosis should include optic nerve neoplasms such as meningiomas and optic nerve gliomas, vascular malformations, and systemic conditions such as neurofibromatosis and Von Hippel Lindau. Increased CSF pressure due to a cerebral lesion, vascular lesion, or idiopathic intracranial hypertension should be considered especially if optic nerve sheath dilatation is bilateral.

Prognosis

The outcomes of surgical management versus observation are varied in case reports and case series. Lunardi et al.[3] report that 13 out of 33 patients underwent surgical drainage of optic nerve sheath meningoceles. Five out of 13 had vision improvement.