Compressive Optic Neuropathy Clinical Presentation

Patients with compressive optic neuropathy (CON) typically present with slowly progressive or chronic vision loss in one or both eyes. Bilateral cases can result from midline lesions (eg, pituitary adenoma, craniopharyngioma, meningioma) or from bilateral lesions (eg, thyroid eye disease).

Patients may complain of blurred vision, dimness of vision, or an inability to read with the affected eye. Peripheral vision loss, including nerve fiber layer defects, junctional visual field loss, or bitemporal hemianopsia, can result from compressive lesions of the anterior visual pathway. The duration of the vision loss may be difficult to determine; if optic atrophy already is present, the axonal injury occurred at least 6 weeks prior to presentation.

CON may be found incidentally during a routine eye examination or refraction when a patient is documented to have unexplained visual acuity loss. Another common presentation is in patients who are referred for an evaluation after a failed driving test.

Rarely, patients with CON will have sudden visual loss due to an intralesional hemorrhage (eg, pituitary apoplexy). These patients can present with the complaint of “worst headache of my life.”

The patient or the patient's relatives may note the development of proptosis or exophthalmos when the compression is secondary to an intraorbital space occupying condition such as from thyroid orbitopathy, hemangioma etc.

In rare cases, patients may complain of a transient loss of vision. This clinical scenario typically is caused by an orbital apex tumor, and the vision loss occurs only in certain positions of gaze (ie, gaze-evoked amaurosis). This presentation for CON is thought to be related to either direct pressure on the optic nerve or a temporary interruption of the blood supply by the tumor in certain positions of gaze. ^[4]

Sudden or rapidly progressive vision loss due to CON is rare, but trauma would be the main inciting factor in these circumstances. History reveals either blunt or penetrating injury. Trauma can occur along any portion of the optic nerve; however, the intracanalicular portion of the optic nerve is especially susceptible to indirect traumatic forces. Stress forces from trauma transmitted to the skull cause injury to the intracanalicular segment in particular because it is tightly adherent to the periosteum in this location. ^[7]

In general, an 8-point eye examination should be performed in every patient undergoing ophthalmologic evaluation. This examination includes an assessment of visual acuity, pupils, extra-ocular motility and alignment, intraocular pressure, confrontational visual fields/perimetry, external examination, slit-lamp examination, and funduscopic examination. Further workup for CON should include a full neurologic examination, including testing the function of all cranial nerves. ^[8]

The findings of CON in terms of (1) central visual acuity, (2) color vision, (3) pupillary function, and/or (4) visual field (ie, perimetry) are discussed below. Depending on the location of the lesion, additional clinical findings may include proptosis, strabismus, eyelid malposition, anosmia, facial numbness, or pituitary dysfunction. ^[4]

Optic nerve function

Visual acuity

Visual acuity is typically reduced in the affected eye, although the deficit may be moderate. Mass lesions in the orbit compressing the globe may result in a hyperopic shift.

Color vision

Dyschromatopsia commonly occurs in CON. The Ishihara test plate is a simple method to assess color vision in the office setting. Other testing methods for dyschromatopsia include pseudoisochromatic color plates and Farnsworth-Munsell test. ^[9]

Desaturation of color hue: When a bright color (eg, red) is viewed by each eye individually, the patient will describe the color to be less vivid or washed-out in the eye with CON.

Pupils

Because CON is typically a unilateral process, examination of the pupils could reveal a relative afferent pupillary defect on the affected side (ie, Marcus Gunn pupil). ^[4]

Visual field examination

As the ganglion cell axons within the optic nerve travel toward the chiasm, the inferior and superior topographic organization from the optic disc is preserved, but the fibers serving central vision move toward the interior of the optic nerve. This arrangement of fibers within the optic nerve produces the characteristic visual field defects associated with CON.

The majority of fibers within the optic nerve are concerned with central vision. Therefore, CON almost always causes a central scotoma, usually with some peripheral constriction. Altitudinal or arcuate visual field defects mimicking glaucoma may be seen in some cases. The location of the lesion does not always correspond with the topographic location of the visual field defect; for example, a superior orbital mass may not always cause an inferior visual field defect. Enlargement of the blind spot may be noted when optic disc edema is present.

When compression of the optic nerve occurs near its junction with the optic chiasm, perimetry may demonstrate a central scotoma in one eye and a temporal visual field defect in the other eye. This combination of visual field findings suggests intracranial localization at the anterior aspect of the chiasm (ie, junctional scotoma). Tumors at the optic chiasm, commonly pituitary adenomas, often cause a bitemporal hemianopsia that respects the vertical midline. ^[4]

Additional clinical findings

Proptosis

Proptosis is defined as pathologic, forward displacement of the globe in the orbit.

Formal assessment of proptosis: The distance from the lateral canthal rim to the corneal surface is measured with a Hertel exophthalmometer. Proptosis can be assessed qualitatively by comparing the projection of the two globes with the observer positioned above the patient's forehead. ^[10]

If more than 2 mm of proptosis exists, a space-occupying lesion of the orbit is suspected.

Slowly progressive proptosis often is undetected by patients and their families.

Extraocular motility

Ocular motility abnormalities may occur if cranial nerve III, IV, or VI is involved, either through mechanical factors or direct compression of cranial nerves at the superior orbital fissure. ^[4] However, ocular motility may be normal if the orbital mass is moderate in size and the growth rate is slow.

Relative resistance to retropulsion of the globe: Pressing on the globes through the closed lids may reveal an asymmetric resistance to retropulsion, suggesting the presence of a space-occupying lesion in the orbit.

Dilated fundus examination

Funduscopic examination may reveal an optic disc that appears normal, edematous, or atrophic.

Disc edema typically is seen with orbital processes. Optic disc edema in CON is thought to occur from a mechanical blockage of axonal transport by the tumor or mass lesion. In clinical practice, chronic, unilateral optic disc edema is much more likely to be related to optic nerve compression than papilledema, which almost always is a bilateral process, although occasionally it is asymmetric.

If axonal damage has been present for longer than 6 weeks, optic atrophy can be detected clinically as a variable degree of optic disc pallor.

When CON causes optic atrophy, occasionally it is accompanied by optic disc "cupping". It is not known why some cases of CON are associated with an enlargement of the optic disc cup, but clinicians should be aware that rare cases of low-tension glaucoma may be due to a compressive lesion. ^[11]

The combination of ipsilateral optic atrophy and contralateral disc edema is known as the Foster Kennedy syndrome. This clinical picture results from compression of one optic nerve and papilledema involving the other optic nerve from an intracranial lesion (eg, meningioma). ^[12]

Fundus examination images are shown below.

Vascular abnormalities of the optic disc

CON may cause the appearance of optociliary shunt vessels, which are collateral channels connecting the retinal circulation to the choroidal circulation. These vessels become dilated when the venous return within the optic nerve becomes obstructed by a compressive force. Blood is then shunted to the choroidal circulation where it exits through the vortex veins.

The four most common causes of optic disc shunt vessels include retinal vein occlusion, optic nerve sheath meningioma, optic nerve glioma, and chronic papilledema.

The triad of optic atrophy, optociliary shunt veins (optochoroidal collaterals), and progressive visual loss initially was described in patients with optic nerve sheath meningioma (ie, Hoyt-Spencer triad). ^[13]

Orbital tumors compressing the globe may result in choroidal folds. The folds or striae can occur within the macula or adjacent to the optic disc.

Causes of compressive optic neuropathy include the following:

• Inflammatory/autoimmune diseases

  • Thyroid orbitopathy (ie, Graves' orbitopathy)

  • Orbital pseudotumor

  • Sarcoidosis

• Orbital tumors

  • Benign

    • Cavernous hemangioma

    • Schwannoma

  • Malignant

    • Lymphoma

    • Sarcomas (eg, rhabdomyosarcoma)

    • Mesenchymal (eg, fibrous histiocytoma)

    • Metastatic (eg, breast, prostate, lung [most common])

• Optic nerve tumors

  • Optic nerve glioma

  • Optic nerve meningioma

• Vascular lesions

  • Aneurysms (or dolichoectatic vessels)

  • Lymphangioma

  • Orbital varix

  • Arteriovenous malformation

  • Orbital hemorrhage

• Intracranial tumors

  • Meningioma

  • Pituitary adenoma

  • Metastatic

• Congenital lesions

  • Dermoid cyst

  • Teratoma

• Trauma

• Other

  • Cholesterol granuloma

  • Mucocele ^[14]

  • Arachnoid cysts of optic nerve sheath

  • Bony tumors or compression (osteomas, osteopetrosis, fibrous dysplasia, Paget disease)

  • Encephalocele

  • Hypertrophic or granulomatous cranial meningitis

  • Low folate and indoor pollution also may be risk factors as was found in a Tanzanian study conducted by Hodson et al. ^[15]

A general physical examination is followed by a detailed ophthalmologic evaluation. Compression of the optic nerve would be expected to compromise the function of the optic nerve and hence needs assessment of visual acuity, color vision, and contrast. A detailed external exam of the eye and orbit is completed; look for any proptosis followed by ocular motility exam. Pupil exam is important to assess for afferent pupillary defect by the swinging flash light test. Fundus exam helps compare the optic nerve appearance which could be normal when the compression is subtle, show edema in the acute/subacute phase, or even appear atrophic in chronic compression.