eye health

Optic neuritis

Generality

Optic neuritis consists of inflammation of the optic nerve. This condition, which can lead to partial or complete loss of vision, is generally caused by infections, autoimmune diseases or damage to the optic nerve (from compression, tumor or ischemia). The onset of optic neuritis is typically characterized by a triad of signs clinical: reduced visual acuity, eye pain and impaired color perception.

In most cases, inflammation is unilateral, although both eyes may be involved at the same time.

Causes

The optic nerve transmits visual information, coming from the ganglion cells of the retina (located at the bottom of the eyeball), to the visual cortex located in the occipital lobe (area of ​​the brain responsible for processing electrical stimuli in visual images). When the optic nerve is inflamed, its function is compromised; in particular, optic neuritis causes reduced vision due to swelling and degeneration of the myelin sheath that covers and protects the optic nerve. Under normal conditions, this sheath isolates the optical fibers, preventing the electrical impulse that runs through them to disperse. Myelin damage therefore alters the normal transmission of electrical signals from the retina to the brain.

Optic neuritis can arise when the immune system mistakenly attacks the myelin sheath, resulting in its partial or total destruction. Even a direct axonal injury can contribute to optic nerve damage and compromise its ability to conduct electrical impulses. However, the most common cause of optic neuritis remains multiple sclerosis (MS), a disease in which the immune system attacks the myelin sheath that covers the nerve fibers in the brain and spinal cord, resulting in inflammation and damage to the nerve cells involved. Optic neuritis is the most common visual disorder associated with multiple sclerosis and is often the first symptom that appears in a subject with this demyelinating disease. Optic neuritis is a strong predictor of MS development; in fact, almost half of patients with optic neuritis have lesions of the white matter of the brain, which can be detected by magnetic resonance and are consistent with the clinical picture of multiple sclerosis.

Other autoimmune diseases that can cause inflammation of the optic nerve are:

  • Optic neuromyelitis (or Devic's syndrome, affects the optic nerves and the spinal cord, but does not cause brain damage such as multiple sclerosis);
  • Systemic lupus erythematosus;
  • Behçet's disease;
  • Sarcoidosis.

The infections that can cause inflammation of the optic nerve are:

  • Lyme disease;
  • Syphilis;
  • Meningitis;
  • Viral encephalitis;
  • Chickenpox;
  • Rubella;
  • Measles;
  • Mumps;
  • Shingles;
  • Tuberculosis.

Other causes of optic neuritis may include:

  • Nutritional deficiencies (example: vitamin B12);
  • Drug toxicity or toxins (such as amiodarone, isoniazid etc.);
  • Head radiotherapy;
  • Drug-induced vasculitis (chloramphenicol, ethambutol, etc.);
  • Temporal arteritis;
  • Diabetes.

Finally, any process that causes inflammation, ischemia or compression of the optic nerve, including primary and metastatic tumors, can interfere with the ability to adequately conduct electrical impulses through the optical pathway.

The disease mainly affects adults, aged between 18 and 45 years.

Symptoms

To learn more: Symptoms Neuritis Ottica

The main symptoms of optic neuritis may include:

  • Gradual or sudden loss of vision (partial or total), usually in one eye;
  • Pain, which worsens with the movement of the affected eye;
  • Discromatopsia: reduction of color perception, which appear faded and lacking in vivacity (especially red);
  • Severe blurred or "fogged" vision, which can evolve into temporary blindness;
  • Low contrast sensitivity;
  • Visual phenomena, such as perception of luminous flashes in the absence of light (phosphenes), flashing lights and spots in the visual field (floaters, also called "flying flies" or "moving bodies");
  • Slow response to light variations.

Simultaneous vision loss in both eyes is rare, but can occur during recurrent episodes of optic neuritis. In some people, symptoms may temporarily worsen as body temperature rises, in case of exercise, hot and humid weather, sun exposure or fever (Uhthoff phenomenon). This effect is determined by the inhibition of nerve conduction in partially demyelinated axons. The Uhthoff phenomenon is resolved as soon as the body temperature returns to normal.

After the onset, the symptoms may worsen over the course of a few days or weeks, and then gradually resolve. If these do not improve after 8 weeks, a condition other than optic neuritis may be present.

Complications

The possible complications of this condition may include:

  • Decreased visual acuity : most people recover normal vision within a few months. Sometimes, vision loss can persist even after improvement of optic neuritis.
  • Optic nerve damage : most patients have permanent damage to the optic nerve after an episode of optic neuritis, but may not have symptoms resulting from this condition.
  • Treatment side effects : long-term use of corticosteroid drugs can cause bone thinning (osteoporosis) and make the body more susceptible to infection. Other possible side effects of the therapy include weight gain, mood changes, stomach disorders and insomnia.

Diagnosis

Optic neuritis is suspected in patients who present a typical triad of clinical signs: reduced vision, eye pain and impaired color perception. The eye and neurological examination can provide a correct diagnosis: the characteristic findings may include a reduced peripheral vision, an alteration of the pupillary reflex, a decrease in the perception of brightness in the affected eye and a swelling of the optic disk.

An ophthalmologist can carry out the following investigations to establish the diagnosis of optic neuritis:

  • Eye examination : the ophthalmologist will monitor visual acuity and color perception. Visual ability is compromised in proportion to the magnitude of optic neuritis. In many cases, only one eye is affected and patients may not be aware of the altered color perception, at least until they are asked to close or cover the healthy eye.
  • Ophthalmoscopy : this test allows to examine the structures present in the back part of the eye (ocular fundus), using an ophthalmoscope. The ophthalmologist will pay particular attention to the optic disc, the area of ​​insertion of the optic nerve onto the retina. Although this sign is not always detectable, in about 33% of people with optic neuritis, the optic disc appears swollen (anterior papillitis) and there may be dilation of the blood vessels around the nerve. However, the optic nerve often shows no abnormal appearance visible on ophthalmoscopy, since the inflammation is entirely retrobulbar (retrobulbar optic neuritis).
  • Pupillary reflex to light : the exam measures the integrity of the sensory and motor functions of the eye. The ophthalmologist moves a lamp in front of the eyes to assess the reaction of the pupils to the light source. The optic nerve is the afferent pathway of the pupillary reflex, that is, it perceives the incoming light. If this is affected by inflammation, a defect in the reaction of the pupils may be found. The impairment of the optic nerve causes, in fact, a lower pupillary narrowing in response to the luminous stimulus, since the signal collected by the damaged optic nerve cannot reach the brain.
  • Visual evoked potential : the doctor places small surface electrodes on the patient's head, to evaluate the brain's electrical reaction to certain visual stimuli (it is recorded in a similar way to the electroencephalogram). During this test, the subject sits in front of a screen that shows a moving checkerboard pattern. The visual evoked potential allows us to highlight a reduced electrical conduction of the optic nerve, which can represent damage. Optic neuritis can also occur without any symptoms. In other words, inflammation of the nerve pathways can occur without the patient noticing changes in vision. For this reason the visual evoked potential can be useful in formulating the correct diagnosis.
  • Optical coherence tomography (OCT) : it is another painless and non-invasive test able to evaluate the health of the optic nerve. Optical coherence tomography can detect evidence of demyelination of the optic nerve and allows the doctor to know if damage has occurred due to previous inflammatory processes.

It is important to remember that optic neuritis can be an initial symptom of certain pathological conditions. A complete medical examination can help to rule out any related illnesses.

This may include:

  • Blood analysis : blood tests allow the detection of the presence of inflammatory parameters, such as the ESR or the C-reactive protein. A high erythrocyte sedimentation rate (ESR) can help determine whether optic neuritis is caused by inflammation of the cranial arteries (temporal arteritis); furthermore, blood tests allow to detect the presence of anti-myelin antibodies (to investigate autoimmune diseases) and signs of any viral and bacterial infections.
  • Magnetic resonance : a patient who presents a first episode of optic neuritis, in general, undergoes an MRI scan to look for possible lesions on the central nervous system. This imaging test allows a detailed scan of the structures involved in optic neuritis to be performed. During the investigation, a contrast agent can be injected to assess any alterations present at the level of the optic nerve and the brain. MRI, for example, can determine if myelin has been damaged and can help diagnose multiple sclerosis, demonstrating the presence of characteristic abnormalities. If the patient has atypical symptoms or if optic neuritis is associated with other neurological or ocular signs, the procedure can exclude or confirm the presence of tumors and other conditions that can mimic optic neuritis (ischemia or compression due to various types of swelling) .

Treatment

In most cases, the prognosis is good: the disorder is only transient and the vision improves spontaneously within a few weeks or months, unless an underlying condition is the cause of optic neuritis. Patients may regain normal vision, but contrast sensitivity and color perception may remain slightly altered.

If optic neuritis is determined by a specific infectious cause, appropriate therapy may be prescribed; the eradication of the underlying infectious agent usually prevents further episodes.

When optic neuritis is related to multiple sclerosis, vision returns to normal within 2-12 weeks in the absence of treatment, but may also progress to a state of permanent low vision or blindness.

A therapeutic regimen with intravenous corticosteroid drugs (such as methylprednisolone) may be prescribed in order to accelerate recovery; however, high doses of corticosteroids should be used with caution to avoid possible side effects.

The intravenous therapy can be followed by a gradual reduction of the dose of corticosteroids, which can be taken orally for about 11-14 days (example: prednisone). In cases arising from demyelinating diseases, such as multiple sclerosis, optic neuritis may be recurrent.

To reduce the incidence of future attacks an immunomodulator (interferon, natalizumab etc.) or an immunosuppressive therapy (cyclophosphamide, azathioprine or methotrexate) may be prescribed.

Continue: Medicines for the Care of Optical Neuritis »