The VEP is particularly useful in detecting past optic neuritis.

This refers to inflammation of the optic nerve, associated with swelling and progressive destruction of the sheath covering the nerve, and sometimes the nerve cable.

As the nerve sheath is damaged, the time it takes for electrical signals to be conducted to the eyes is prolonged, resulting in an abnormal VEP. This may be seen in multiple sclerosis - one of the most common causes of optic neuritis (as above). Abnormal VEP's are seen in multiple sclerosis patients due to the presence of optic neuritis.

The following are less easily differentiated but may cause abnormal VEPs:

1. •Optic neuropathy - this can be due to damage of the optic nerve from a number of causes, including: a blockage of the nerve's blood supply, nutritional deficiencies, or toxins. As the nerve is damaged, electrical signals do not conduct properly. Examples include diabetes in the advanced stages which can be associated with damage to the blood vessels and nerves supplying the eyes, or toxic amblyopia which is a condition of the eyes associated with decreased vision, due to a toxic reaction in part of the optic nerve.
   

2. •Tumours or lesions compressing the optic nerve - if the optic nerve is compressed, the pathway for conduction is affected and an abnormal VEP is seen.
   

3. •Glaucoma - patients who suffer from glaucoma have increased intraocular pressure (ie pressure inside the eye). This can result in damage to the optic nerve, leading to prolonged VEPs.
   

4. •Ocular hypertension (high pressure) - this refers to any situation in which the pressure in the eye is higher than normal. There are no signs of glaucoma, but patients may be at increased risk of developing glaucoma later in life.
   

Clinical usefulness of the VEP

1. •The VEP is a standardised and reproducible test of optic nerve function
   

2. •It is more sensitive compared to magnetic resonance imaging (MRI) in detecting lesions affecting the visual pathway in front of the optic chiasm (area in the optic pathway where the optic nerve crosses sides)
   

3. •It is usually less costly compared to other investigations such as MRI
   

4. •If results of the VEP are negative, this can be useful in excluding certain disorders.
   

Summary

The VEP is an important test that is very good at detecting problems with the optic nerve and lesions in the anterior part of our visual pathway, before the optic nerves merge.

However, it is a non-specific test and to determine the exact underlying problem in each patient, a good history and examination is also very important.