The optic nerve contains retinal ganglion cell axons that extend posteriorly from the globe, through the orbit and optic canal, to reach the optic chiasm. The total length of the optic nerve averages 50 mm: 1 mm for the intraocular segment, 25 mm for the intraorbital segment, 10 mm for the intracanalicular segment, and 14 mm for the intracranial segment. An injury to the optic nerve anywhere along this pathway by an extrinsic lesion is termed compressive optic neuropathy (CON). The optic nerve is most vulnerable to injury by a compressive force where it is adjacent to bone or in a confined space (eg, orbital apex, optic canal). 
The clinical hallmark of compressive optic neuropathy is slowly progressive vision loss. Characteristic findings on clinical examination include reduced visual acuity, dyschromatopsia, a relative afferent pupillary defect, visual field defect, and optic atrophy (or edema). A delay in the diagnosis of compressive optic neuropathy is not uncommon since the vision loss is insidious, and the clinical findings may be missed or misinterpreted as a cataract, maculopathy, glaucoma, or optic neuritis.
Clinicians should consider compressive optic neuropathy in the differential diagnosis of unexplained, asymmetric visual acuity, normal or low-tension glaucoma, or visual symptoms responsive to corticosteroids. Similarly, a workup of incidentally discovered optic atrophy should include a neuroimaging study (eg, magnetic resonance imaging) to rule out a compressive lesion. The management of compressive optic neuropathy is often difficult, given the proximity of compressive lesions to critical neurovascular structures in the orbit and intracranial space. Many of the conditions causing compressive optic neuropathy are also resistant to radiotherapy at doses tolerated by the globe and anterior visual pathways.
Optic nerve compression by an extrinsic lesion has been postulated to cause atrophy of ganglion cell axons either through ischemia or mechanical disruption of axonal transport. Rarely, an intrinsic lesion of the optic nerve (ie, optic nerve glioma) can cause damage to the individual axons due to slow compression of the fascicles within the tumor.
Compared to other ophthalmic conditions, compressive optic neuropathy (CON) is relatively rare, with an estimated incidence of about 4 cases per 100,000 individuals per year. The majority of cases encountered in clinical practice are due to Graves' orbitopathy, with compression of the optic nerve by the enlarged extraocular muscles at the orbital apex. 
Compressive optic neuropathy (CON) typically causes permanent vision loss, particularly if optic atrophy is evident at the time of diagnosis. However, with certain conditions such as Graves' orbitopathy and pituitary adenomas, significant visual recovery may occur after surgical intervention. 
When CON is due to a malignant lesion, the patient's systemic prognosis may be affected.
Tumors compressing the optic nerve in the intracranial space may cause additional neurologic morbidity (eg, endocrine dysfunction, cranial nerve palsies, papilledema, stroke).
Tumors within the orbit causing CON may cause disfiguring proptosis and permanent diplopia. 
Treatment of CON may have serious side effects, ranging from profound vision loss (surgery-induced ischemia or radiation toxicity) to permanent brain injury (eg, stroke, radionecrosis).
There is no sex predilection for CON with the exception of Graves' orbitopathy and meningiomas, two conditions which are more common in women than in men.
What would you like to print?