Compressive Optic Neuropathy - Causes, Pathophysiology, Diagnosis, and Treatment

Introduction:

The optic nerve serves one of the most important functions of visual perception. It begins from the optic disk in the retinal ganglion cells at the back of the eyeball, which connect directly to the brain. Damage to this nerve may lead to temporary or permanent loss of vision. As the name suggests, compressive optic neuropathy damages the optic nerve due to compression. This nerve compression may be due to external lesions such as tumors.

Other than compression of the optic nerve, damage to this nerve can also be caused by demyelination (damage of the protective covering called myelin sheath of the nerve cells), ischemia (restricted supply of blood and oxygen to the nerve), metabolic, and traumatic insult.

What Is the Optic Nerve?

The optic nerve is the second cranial nerve (nerves that arise from the spinal cord). The main function of the optic nerve is to enable vision. It transmits the visual information recorded in the retina of the eye to the brain, where it is processed. The optic nerve is the only nerve that leaves the cranial cavity and can be visualized clinically. It is formed by the axons (head of the nerve cell) that arise from the retinal ganglion cells at the back of the eyeball.

What Is the Pathophysiology of Compressive Optic Neuropathy?

The nerves of mammals cannot regenerate axons and facilitate axonal growth. When the optic nerve is subjected to damage, the glial cells (which form the structure of the central nervous system and regulate its function) form glial scars that prevent the diffusion of growth factors. The growth factors are -

• Nogo (inhibitory proteins of myelin).

• Myelin-associated glycoprotein.

• Decreased expression of growth factors and lack of laminin also interfere with the regrowth of the nerve cells.

Compression of the adjacent blood vessels restricts the blood supply to the nerve causing ischemia of the optic nerve, thus resulting in apoptosis, that is, programmed cell death. This leads to impairment in axonal transport and transmission of signals.

What Causes Compressive Optic Neuropathy?

Compression of the optic nerve occurs when there is an abnormal mass formation in the brain. This abnormal mass formation may be in the form of a tumor, pus-filled lesion, or any other fluid accumulation in the cranial cavity. The pressure exerted by the external lesion on the optic nerve will result in its damage causing neuropathy. The compression on the optic nerve may be intrinsic or extrinsic.

The following are the causes of compressive optic neuropathy -

• Thyroid Orbitopathy in Grave’s Disease - Grave’s disease is an autoimmune condition where the autoantibodies target the thyroid-stimulating hormone-producing cells resulting in excess thyroid hormone production. Excessive secretion of thyroid hormone causes hyperthyroidism and complications associated with the orbit of the eye called orbitopathy. The most common sign of this disorder is eyelid retraction accompanied by an attentive gaze, also known as the Kocher’s sign. Eyelid retraction is an abnormally high resting position of the upper eyelid. Due to this, it always seems that the eyelid of the person is stretched upwards. Corticosteroids are useful for treating inflammation, followed by radiation therapy and immunosuppressants.

• Craniopharyngioma - These are rare benign (non-cancerous) tumors that grow near the pituitary gland. They develop as solid tumors or cysts.

• Fibrous Dysplasia - It is a benign condition of the bones where fibrous tissue starts growing in the place of the bone.

• Orbital Hemangioma - Also known as orbital cavernous venous malformation. It is a benign tumor that grows in the orbit of the eye.

• Pituitary Adenoma - It is a benign tumor that grows on the pituitary gland.

• Optic Gliomas - Optic nerve glioma is a slow-growing tumor that affects children.

• Cerebral Aneurysms - An aneurysm is a bulge inside an artery. This bulging aneurysm puts pressure on the nerve or brain tissue.

• Trauma - Traumatic injury to the eyeball or near the eye can cause compressive optic neuropathy.

• Infections - Aspergilloma fungal infection is associated with compressive optic neuropathy.

What Are the Symptoms of Compressive Optic Neuropathy?

Compressive optic neuropathy is usually characterized by chronic progressive vision loss in one or both eyes. A sudden loss of vision may be rare except in cases of trauma. A blunt trauma or penetrating injury can cause an abrupt loss of vision.

The following symptoms may accompany along with vision loss -

• Diplopia (Double Vision) - The person can see two images of everything.

• Dyschromatopsia - Abnormal color vision.

• Exophthalmos - Also known as proptosis, the protrusion of the eyeball anteriorly outside the orbit. It may involve one or both eyeballs.

• Afferent Pupillary Defect - The retina or optic nerve disease causes an afferent pupillary defect. It is a condition in which the pupils respond differently to light stimuli in one eye at a time. It is the difference in overall light sensitivity in one retina compared to the other.

• Photophobia - It is discomfort in the eyes when exposed to bright light.

• Red-Eye - The eyes appear red, irritated, and bloodshot. This redness is due to inflammation of the blood vessels under the eyes.

• Headache - Patients with tumors suffer from severe headaches.

How Are Optic Neuropathies Diagnosed?

• A complete ophthalmological examination follows a complete neurological examination.

• Magnetic resonance imaging MRI scans of the brain and orbit will help visualize the optic nerve, brain tissue, and other contents of the orbit.

• A CT (computed tomography) scan will help demonstrate fractures near the orbit in traumatic injuries.

• Compression of the optic nerve will result in reduced visual activity; various visual tests are carried out to examine this. One of which includes the Ishihara test. This test is carried out to check if a color deficit is present in the vision. It can determine if the patient is suffering from only red and green color blindness. This color vision test makes use of numerous colored dots. One color is used to make dots that create a number, and the other is used for the dots surrounding the number. The patient has to identify the number written in colored dots. This is the most widely used color deficiency test worldwide by ophthalmologists and optometrists.

• Blood Tests - A complete blood count is carried out. Thyroid function tests are also carried out to assess thyroid hormone secretion.

How Are Optic Neuropathies Treated?

The management of this condition depends upon the causative agent.

• Corticosteroid medications are useful in treating inflammation.

• Radiation therapy for the treatment of tumors. The surgeon has to be very careful while using radiation therapy because it can cause irreversible damage to the optic nerve.

• Surgical orbital decompression can help in some cases; however, it is a high-risk surgery.

Conclusion:

The prognosis of compressive optic neuropathies depends upon the severity of compression, time of diagnosis, and initiation of treatment. Complete recovery of vision has been observed as early as one week too. However, in most cases, vision recovery is slow and partial. Some tumors can be excised easily without damaging the optic nerve, while some cause permanent damage to the optic nerve resulting in partial or complete loss of vision. Prompt diagnosis and treatment play a critical role in managing this disorder. It is treatable, and patients can live their lives normally after treatment.