Optic nerve glioma overview

# Overview
Optic nerve glioma is a slow-growing brain tumor that arises in or around the optic nerves. Optic nerve glioma may extend into the optic chiasm or hypothalamus.  The majority of the optic nerve gliomas are benign but malignant gliomas of the optic nerve can occur. Optic nerve gliomas are rare. The majority of optic nerve gliomas occur in children. They are usually slow-growing and noncancerous and almost always occur before age 20. Optic nerve glioma may be classified into several subtypes based on microscopic histopathology, anatomic location, and association with neurofibromatosis type 1. Optic pathway gliomas are classified according to WHO classification into two subtypes: pilocytic astrocytoma and fibrillary astrocytoma. Optic nerve gliomas may be classified according to anatomic location into two subtypes: anterior visual pathway tumors and posterior visual pathway tumors. Based on whether or not they are associated with neurofibromatosis type 1, they may be classified into two subtypes: optic nerve gliomas associated with neurofibromatosis type 1 and optic nerve gliomas not associated with neurofibromatosis type 1. Genes involved in the pathogenesis of optic nerve glioma include BRAF-KIAA, tumor suppressor genes and chromosomes 7q34 and 17q. On gross pathology, smooth and fusiform intradural lesions are characteristic findings of optic nerve glioma. On microscopic histopathological analysis, low grade spindle shaped pilocytic astrocytes & glial filaments with the presence of numerous Rosenthals fibers are characteristic findings of optic nerve gliomas. There is no established cause for optic nerve glioma. Optic nerve gliomas affects females and males equally. Optic nerve gliomas commonly affects individuals younger than twenty years of age. There is no racial predilection to the optic nerve glioma.
Optic nerve gliomas affects girls and boys equally. There are no established risk factors for optic nerve gliomas. Symptoms of optic nerve glioma include proptosis, unilateral or bilateral visual impairment, nystagmus, squinting, obstructive hydrocephalus and diencephalic syndrome. On head and neck CT, optic nerve glioma is characterized by variably enlarged and elongated optic nerve with kinking or buckling. On head and neck MRI, optic nerve glioma is characterized by isointense to hypointense mass on T1-weighted MRI, and hyperintense mass on T2-weighted MRI. The mainstay of therapy for optic nerve glioma  is chemotherapy and radiation therapy. Chemotherapy is used in children to delay radiation therapy, and to reduce the severity of radiation therapy induced side-effects. Radiation therapy is not generally used in children less than five years of age and usually avoided in children between five and ten years of age. Radiation therapy is delayed in young children to reduce the risk of significant side- effects such as developmental and neurocognitive delay. Chemotherapy using vincristine, actinomycin D, bevacizumab, etoposide, and other agents has also been reported to be effective in patients with progressive hypothalamic/chiasmal gliomas.  Surgery is not the first-line treatment option for patients with optic nerve glioma. Surgical excision is usually reserved for patients with either progressive proptosis, blindness, exophytic chiasm tumor causing mass effect, hydrocephalus, or with increased intracranial pressure.

# Historical Perspective
Optic nerve glioma was first described in 1816 by Scarpa.

# Classification
Optic nerve glioma may be classified into several subtypes based on microscopic histopathology, anatomic location, and association with neurofibromatosis type 1. Optic pathway gliomas are classified according to WHO classification into two subtypes: pilocytic astrocytoma and fibrillary astrocytoma. Optic nerve gliomas may be classified according to anatomic location into two subtypes: anterior visual pathway tumors and posterior visual pathway tumors. Based on whether or not they are associated with neurofibromatosis type 1, they may be classified into two subtypes: optic nerve gliomas associated with neurofibromatosis type 1 and optic nerve gliomas not associated with neurofibromatosis type 1.

# Pathophysiology
Genes involved in the pathogenesis of optic nerve glioma include BRAF-KIAA, tumor suppressor genes and chromosomes 7q34 and 17q. On gross pathology, smooth and fusiform intradural lesions are characteristic findings of optic nerve glioma. On microscopic histopathological analysis, low grade spindle shaped pilocytic astrocytes & glial filaments, with the presence of numerous Rosenthals fibers are characteristic findings of optic nerve gliomas.

# Causes
There is no established cause for optic nerve glioma.

# Differential Diagnosis
Optic nerve glioma must be differentiated from other diseases that cause optic nerve enlargement such as optic nerve sheath meningioma, orbital pseudotumor, optic neuritis, orbital lymphomas, metastasis, fibrous dysplasia, paranasal mucocele, rhabdomyosarcoma, neurofibromatosis, perioptic haemorrhage, Erdheim-Chester disease, juvenile xanthogranuloma, medulloepithelioma, retinoblastoma, Krabbe disease, optic nerve and chiasm glioma such as germinoma and sarcoidosis, and optic chiasm glioma extending into the hypothalamus such as pituitary adenoma, craniopharyngioma, malignant astrocytoma, dermoid cyst, chordoma, colloid cyst, histiocytosis X, tuberculous  granuloma, and hemangloendothelioma.

# Epidemiology and Demographics
Optic nerve gliomas affects females and males equally. Optic nerve gliomas commonly affects individuals younger than twenty years of age. There is no racial predilection to the optic nerve glioma.

# Risk Factors
Common risk factor in the development of optic nerve glioma is neurofibromatosis type 1.

# Screening
According to the United States Preventive Services Task Force, screening for optic nerve glioma is not recommended. It is recommended that all children with NF-1 have their vision checked every year by an ophthalmologist to screen for the development of eye tumors, including optic nerve glioma.

# Natural History, Complications and Prognosis
Most optic nerve gliomas are benign and produce slowly progressive visual loss associated with variable proptosis and anterior or posterior optic neuropathy. If left untreated, less than 5 percentage of patients with optic nerve gliomas may progress to develop blindness. Common complications of optic nerve glioma include decreased vision, blindness, growth hormone deficiency, precocious puberty, and hydrocephalus. Prognosis is generally good in most patients with optic pathway gliomas.

# History and Symptoms
Symptoms of optic nerve glioma include proptosis, unilateral or bilateral visual impairment, involuntary eye ball movement, squinting, obstructive hydrocephalus and diencephalic syndrome.

# Physical Examination
Common physical examination findings of optic nerve glioma include nystagmus, strabismus, proptosis, visual impairment, afferent pupillary defect, edema and/or pallor of optic disc, torticollis and deficits of cranial nerve II.

# Laboratory Findings
There are no diagnostic laboratory findings associated with optic nerve glioma.

# Electrocardiogram
There are no electrocardiogram findings associated with optic nerve glioma.

# Chest X Ray
There are no chest x ray findings associated with optic nerve glioma.

# CT Scan
On head and neck CT, optic nerve glioma is characterized by variably enlarged and elongated optic nerve with kinking or buckling.

# MRI Scan
On head and neck MRI, optic nerve glioma is characterized by isointense to hypointense mass on T1-weighted MRI, and hyperintense mass on T2-weighted MRI.

# Echocardiography
There are no echocardiography findings associated with optic nerve glioma.

# Other Imaging Findings
Other imaging studies for optic nerve glioma include cerebral angiography, which may show a space-occupying mass.

# Other Diagnostic Studies
Other diagnostic studies for optic nerve glioma include visual field tests, biopsy, and cerebral angiography.

# Medical Therapy
The mainstay of therapy for optic nerve glioma  is chemotherapy and radiation therapy. Chemotherapy is used in children to delay radiation therapy, and to reduce the severity of radiation therapy induced side-effects. Radiation therapy is not generally used in children less than five years of age and usually avoided in children between five and ten years of age. Radiation therapy is delayed in young children to reduce the risk of significant side- effects such as developmental and neurocognitive delay. Chemotherapy using vincristine, actinomycin D, bevacizumab, etoposide, and other agents has also been reported to be effective in patients with progressive hypothalamic/chiasmal gliomas.

# Surgery
Surgery is not the first-line treatment option for patients with optic nerve glioma. Surgical excision is usually reserved for patients with either progressive proptosis, blindness, exophytic chiasm tumor causing mass effect, hydrocephalus, or with increased intracranial pressure.

# Primary Prevention
Effective measures for the primary prevention of optic nerve glioma include regular eye exams in patients with neurofibromatosis type 1.

# Secondary Prevention
Secondary prevention strategies following optic nerve glioma include lifelong follow-up care which further includes a visit to a clinic every year for screening of tumor recurrence, management of disease complications, and management of side-effects of treatment.