Optic nerve injury: symptoms, diagnosis and treatment of optic neuritis
Optic nerve injury – known in medical terms as optic neuritis – is an inflammation that, if left untreated, could lead to partial or total loss of visual ability
This is because the optic nerve is the fundamental structure in the visual process responsible for transporting the electrical impulse generated on the retina by light from an object within the visual field to the brain.
In the visual process, the eye receives information from outside, which is then, in the form of the aforementioned electrical impulses, sent to the brain, which is then able to decipher it and return the image.
This information is carried by the optic nerve, which, if damaged, may not communicate it to the brain correctly or completely.
In most cases, the lesion presents as unilateral, although both eyes may be involved to varying degrees and not at the same time.
What are the causes of optic nerve injury?
Optic nerve injuries can be grouped, depending on the underlying cause, into injuries caused by infections, injuries caused by autoimmune diseases or injuries caused by physical damage to the optic nerve.
Injuries caused by infections
Some of the infections that can cause damage or injury to the optic nerve are Lyme disease, syphilis, meningitis, viral encephalitis, varicella, rubella, measles, mumps, shingles, tuberculosis.
Injuries caused by autoimmune diseases
Optic neuritis can occur when the immune system mistakenly attacks the myelin sheath, totally or partially destroying it.
The most common cause, however, of optic nerve lesions is a demyelinating disease known as multiple sclerosis: the immune system mistakenly attacks the myelin sheath that covers the nerve fibres of the brain and spinal cord and, consequently, also that covering the optic nerve.
Optic neuritis is one of the most common symptoms of multiple sclerosis and is often the first symptom to appear as an alarm bell.
Other autoimmune diseases that can lead to optic nerve damage are neuromyelitis optica, systemic lupus erythematosus, and sarcoidosis.
Injuries caused by physical damage to the optic nerve
Injuries to the optic nerve caused by physical damage are usually the result of compression, usually due to a primary or metastatic tumour or cerebral ischaemia, which pressing on the optic nerve prevents the proper transmission of electrical impulses to the brain, or in the case of severe contusions of the eyeball with direct trauma to the portion of the optic nerve that passes through the orbital cavity.
Other causes that can lead to optic nerve injury are nutritional deficiencies, (especially of vitamin B12), high toxicity of drugs or toxins, such as amiodarone or isoniazid, radiotherapy, drug-induced vasculitis and certain diseases with specific vascular damage such as temporal arteritis and diabetes.
Optic nerve injury: recognising the symptoms
As already mentioned, in most cases optic neuritis presents as unilateral, although both eyes may be involved in presenting symptoms.
The pathology, at its onset, is accompanied by a well-defined triad of clinical signs: reduced visual acuity, ocular pain noted in movement and visual field alteration.
Other symptoms that may accompany or may follow those just mentioned are altered vision of the colour range, reduced contrast sensitivity, and the appearance of visual phenomena such as bright flashes in the absence of lights, flashing lights or blotches.
If left untreated, optic nerve injuries may worsen to such an extent that vision is permanently impaired, even to the point of losing it completely.
Diagnosing an optic nerve injury
From the moment the patient begins to experience a noticeable decrease in vision accompanied by some of the symptoms described above, it is necessary to visit an ophthalmologist for a specialist consultation so that the right treatment can be started as soon as possible so that the damage is not severe and permanent.
During the specialist examination, the ophthalmologist – after taking a thorough anamnesis to highlight any other pathologies or infections in progress – will immediately proceed with the prescription of some specialist tests useful for investigating the real nature of the symptoms complained of by the patient.
Among the tests that might be performed is an eye test, useful for investigating visual acuity and colour perception.
Ophthalmoscopy, a specialised test that uses an instrument – the ophthalmoscope, which projects a beam of light through the pupil onto the retina and through this is able to provide information about the internal structures of the patient’s eye, especially if these structures are altered, torn or damaged.
If it is an optic nerve injury, the optic disc usually appears swollen or there is vasodilation of the blood vessels around the optic nerve.
The pupillary reflex to light is a test that measures the integrity of the sensitive and motor functions of the eye: if the pupil, when subjected to light stimulus, reacts incorrectly, this could be a symptom of an impairment of the optic nerve.
The visual field test, which shows any alterations in the visual field, helps confirm the diagnosis of optic neuritis.
Optical coherence tomography (OCT) is a test specifically designed to assess the health of the optic nerve by obtaining very precise scans of the optic nerve fibres at its emergence within the eye (optic papilla).
Optic nerve injury: the most appropriate treatment
In most cases, optic nerve injury – if treated properly – is only temporary.
Consequently, the patient will be able to recover lost visual acuity spontaneously within a period of a few weeks to a few months.
What may not be fully recovered, however, may be contrast sensitivity and colour perception, which may remain slightly impaired.
If the optic nerve injury was caused by a specific infectious cause, the most appropriate therapy consists of treating the infectious agent.
This usually succeeds in curing the lesion and preventing further episodes.
When optic neuritis is related to an autoimmune disease – such as, for example, multiple sclerosis – vision returns to normal within a few weeks but, in the absence of treatment, can also worsen to a state of permanent low vision or blindness.
The most widely used and effective treatment is drug therapy with intravenous corticosteroids; this must then be gradually reduced to oral corticosteroids.
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