Postherpetic neuralgia (PHN) is the most common complication of herpes zoster or shingles. It is defined broadly as any pain that remains after healing of herpes zoster lesions or rash, usually after a three month period.
Shingles (also known as herpes zoster) is a skin rash that can be very painful. It is a viral infection of the nerves, and the nerves affected by shingles can occasionally continue to cause severe pain after the shingles have cleared. Shingles appear as a rash, patch, or a line of painful blisters which arise on the skin over a nerve in the shape of a band. This band follows the distribution of a specific nerve where the virus has been living before its spread to the skin. Shingles affects both men and women equally. This disease is caused by the same virus that causes chicken pox (varicella zoster). Chicken pox usually affects young children, and its symptoms involve itchy blisters all over the body. Once these symptoms go away, the varicella zoster virus stays in the nerves near the spine. It is almost as if the virus "goes to sleep". It will "wake up", however, when the body's immune system is weakened. The virus then grows in the nerves, causing pain. When the virus reaches the skin, it causes the shingles rash.
This acute phase lasts until the lesions are healed, usually several weeks after the onset of the rash. Postherpetic neuralgia (PHN) refers to pain that persists after the acute phase of the illness passes. The exact point at which acute herpes becomes PHN is arbitrary. Some authors have proposed that PHN starts when the pain lasts more than three months after the acute lesions are healed or persists for more than 3-6 months from the beginning of the skin eruptions.