Inflammation affecting a nerve or nerves which may be localised to one part of the body – as, for instance, in SCIATICA – or which may affect many nerves, being then known as POLYNEURITIS. Owing to the fact that the most peripheral parts of the nerves are usually affected in the latter condition (i.e. the fine subdivisions in the substance of the muscles), it is also known as peripheral neuritis.
In cases of localised neuritis, the fibrous sheath of the nerve is usually at fault, the actual nerve-fibres being only secondarily affected. This condition may be due to inflammation spreading into the nerve from surrounding tissues; to cold; or to long-continued irritation by pressure on the nerve. The symptoms produced vary according to the function of the nerve, in the case of sensory nerves being usually neuralgic pain (see NEURALGIA), and in the case of motor nerves some degree of paralysis in the muscles to which the nerves pass.
In polyneuritis, the nerve-fibres themselves in the small nerves degenerate and break down. The condition is protracted because, for recovery to occur, the growth of new nerve-fibres from the healthy part of the nerve has to take place. The cause of polyneuritis may be infection by a virus – for example, HERPES ZOSTER – or a bacterium, as in LEPROSY. Neuritis may also be the result of agents such as alcohol, lead or products from industrial or agricultural activities. ORGANOPHOSPHORUS is believed by some to be a factor in neuritis and other neurological conditions.