Herpes Zoster and Post Herpetic Neuralgia
Herpes zoster ‘shingles’ is the clinical manifestation of the reactivation of a life long latent varicella zoster virus from dorsal root or cranial nerve ganglia, normally contracted after a chickenpox infection ‘varicella’ in early life. After the primary infection the virus is possibly frequently reactivated, but competent cell mediated immunity prevents clinical manifestation.
These asymptomatic reactivation and contact with people infected with chicken pox may enhance the immunity. The commonest cause of viral recurrence is decline in cell-mediated immunity related to age. Reduced immunity associated with some malignancies, chemotherapy, radiotherapy, HIV, and immunosuppressant drugs are also risk factors.
Herpes-zoster affects the sexes equally and its commonest symptom is pain. A prodromal phase may include itching and paraesthesia. Other symptoms include feeling of helplessness, depression, flu-like symptoms. insomnia, limitation of movement, and post herpetic neuralgia (pain after the cessation of the rash). The most commonly affected dermatomes are the thoracic (45%), cervical (23%), and trigeminal (15%). It may affect one or more dermatomes at a time. Secondary bacterial infection may also occur. Varicella zoster virus tends to be reactivated once in a lifetime, with the incidence of second attacks being at < 5%
Pain is a well-known complication of herpes zoster. It may precede the onset of the herpetic rash and, rarely, can occur without the development of a rash. The pain can be classified to acute herpetic pain and post herpetic neuralgia. Acute herpetic pain is characterized by severe, debilitating, burning, aching like pain, or unbearable itching in association with the outbreak of a herpes zoster rash. Descriptions of deep soreness and stabbing pain are common.The pain continues for several weeks after the lesions crusted, gradually fading over weeks to months. Post herpetic neuralgia (PHN) on the other hand, is the persistence of pain of after resolution of the rash. However, the time interval used in the definition of PHN varies in the literature from 1 to 6 months with 3 months being the most generally accepted time period. It is relatively common, affecting 10 to 15% of those with herpes zoster. PHN is associated with scarring of the dorsal root ganglion and atrophy of the dorsal horn on the affected side, which follows the extensive inflammation that occurs during herpes zoster.
With the development of PHN many patients will note a definite change in pain character to typical neuralgia; a combination of continuous deep burning aching pain, paroxysmal lancinating pain like electric shock or sharp stabs and allodynia. The pain can be complicated by dysesthesia, paresthesia, hyperalgesia, and hyperesthesia. PHN is normally located in the same unilateral, dermatomal distribution of the herpes zoster outbreak. The thoracic nerves and the fifth cranial nerve are the most commonly affected. However, it is not unusual to have pain in several dermatomes above and below the original herpes zoster region, or to report sensations in areas anatomically unrelated to the herpes zoster outbreak. Some patients with PHN find themselves comfortable only while calm and relaxed, others find that constant activity and distraction are needed to relieve their pain.
The duration of PHN is highly variable. In a longitudinal study of those who developed PHN, only 48% were symptomatic 1 year after onset.4 A prospective study performed through a network of primary care providers in Iceland from 1990 to 1995, showed that 14 of the 25 who developed PHN were symptomatic 12 months after onset. Thus, the natural history of resolution of PHN over time can cause a problem when evaluating a treatment efficacy and may limit the ability to generalise the results of controlled clinical trials in this population.
Administration of antiviral agents within 72 hours of the onset of herpes zoster may reduce the intensity and duration of acute illness. In some cases adding corticosteroids (prednisone, prednisolone) to the antiviral agent can be beneficial. Although these modalities offer considerable relief to many individuals with acute herpes zoster, none of them prevent PHN. Once this develops, there is no cure, but a variety of medications exist, including anticonvulsants, tricyclic antidepressants (TCAs), opioids and topical treatment modalities, which may provide some pain relief.
The first group of medications are the anticonvulsants, which include Gabapentin and Pregabalin. These provide moderate pain relief to some patients with PHN. It is important to mention that they can be associated with adverse events and their use should be closely monitored.
The second and third groups of medication are the tricyclic antidepressants TCAs and the opioids. Both are widely used and have demonstrated efficacy in clinical trials. Both TCAs and opioids offer significantly greater pain relief than placebo. There is a small tendency for the reduction in pain ratings to be greater with an opioid than with a TCA. On the other hand, opioids were associated with a high incidence of constipation, nausea, and drowsiness. However, the pain relief they offer was such that this benefit outweighed the side effects.
Another modality of treatment is the topical application of Lidocaine-containing patch 5%. These offer significant pain relief in comparison with both no patch and vehicle patch. Topical reactions to the patch application and removal are generally mild.
Other treatment modalities have also been reported to be beneficial in the medical literature with no strong supporting evidence including: Capsaicin cream, TENS application, epidural injection of local anaesthetic and steroid, prostaglandin E2, botulinum toxin, ATP, and intrathecal baclofen. These treatment modalities could be explored if the well-evidenced modalities fail to relief PHN pain.
Prevention and vaccination should also be seriously considered. Vaccine markedly reduces morbidity from herpes zoster and PHN among older adults and is proven to be cost effective for adults above 65 years of age.
It is clear from the above that PHN is a known complication following herpes zoster. Pain could be very severe and even debilitating. Different treatment modalities are available but herpes zoster vaccination among older adults could be the best strategy to prevent the morbidity of these conditions