Post-herpetic neuralgia (PHN) is defined as a chronic neuropathic pain that persists for three or more months following acute Herpes zoster (shingles) infection. It is a severe painful secondary phase, which may develop three to six months after the resolution of initial phase of Herpes zoster (HZ) infection. Pain has a dermatomal distribution and is confined to same dermatome as the rash. Herpes zoster infection results from activation of the varicella zoster virus (VZV) which remains latent in dorsal root ganglia since the first infection of chickenpox (varicella). Pain frequently precedes Herpes zoster infection (pre-herpetic neuralgia), and outlasts the herpetic rash by some weeks. It is more common in older individuals (develops in up to 75% of patients over 70 years of age) and is less common in individuals younger than fifty years of age.
Post-herpetic neuralgia subsides eventually on its own. However, it may make patients suffer for years and may leave a band of anaesthetic skin. It may lead to muscle weakness as well, if it affects major motor nerve.
Post-herpetic neuralgia is associated with persistent and, very often, refractory neuropathic pain. The neuralgia is excruciating with superimposed lancinating paresthesias. Pain may be constant or intermittent, aggravated by minor stimulus (allodynia) such as heat or touch, and is worse at night. The agony of pain leads to insomnia, anorexia, and changes in mood (severe depression).
Post-herpetic neuralgia may produce multiple types of pain in patient e.g.
Post-herpetic neuralgia is a psychosocial condition with a significant impact on the psychological, social, and physical functioning of an individual.
The crusts of Herpes zoster falls away on healing, and leave pink scars in the distribution of rashes. The scars gradually become hypo-pigmented and atrophic.
Symptoms of PHN are
1. Constant aching, deep, or burning pain
2. Paroxysmal lancinating pain
3. Hyperalgesia (increased sensitivity to pain)
4. Allodynia (pain associated with non-painful stimuli)
1. Hypo-aesthesia: There is hypo-aesthesia or decreased sensations in the scarred region in patients with PHN. Greater the degree of hypo-aesthesia, longer is the time nerve block treatment needs to be given.
2. Allodynia and hyperaesthesia: Allodynia and hyper-aesthesia, are frequently noted in the scarred skin.
Chronic PHN is commonly shows psychiatric co-morbidity such as low mood and tendency to social isolation.
In most of the patients, pain and sensory abnormalities resolve as the skin lesions heal over time. In some, pain may persist in spite of healing.
Risk factors for developing PHN are
PHN is a painful condition, which develops following acute Herpes zoster infection. Pain frequently precedes Herpes zoster (pre-herpetic neuralgia) and usually outlasts the rash of HZ by some weeks.
Acute Herpes zoster infection or shingles results from activation of the varicella zoster virus (VZV) infection, which is latent in the dorsal root ganglia since the initial infection (varicella). The virus replicates in the ganglionic neurones, and infects other neighbouring cells and then transmitted down the axon of nerve to infect the skin where it results in blister formation. To a limited extent, virus also travels centrally and causes inflammation of the meninges and spinal cord.
The diagnosis of PHN is by clinical examination in most of the patients.
AHZ is characterised by acute pain lasting for about two to four weeks and it precedes rash by about seven to ten days. The typical rash present as red maculopapular eruption that changes into vesicles, pustules, and finally crust formation. Normally the symptoms of AHZ resolves within two to four weeks, and only about ten percent of patients develop PHN. PHN is the frequent chronic complication of HZ and the most common neuropathic pain resulting from infection.
Testing is, generally done to identify coexisting treatable diseases such as vertebral compression or any underlying disease responsible for immunocompromised state of patient. Immunocompromised state predisposes to the infection with Herpes zoster infection.
PHN is an exceptionally complex drug resistant neuropathic pain. It results from changes in central and peripheral nervous system somatosensory processing.
Aim is to treat patient with Post-herpetic neuralgia within first 72 hours of the rash, in order to reduce both acute neuralgia (AN) and PHN.
PHN is one of the most difficult pain syndromes to treat. The reason why PHN occurs in some patients and not in others, is not known. The condition occurs more frequently in older patients, and also following acute Herpes zoster infection of the trigeminal cranial nerve, as compared to acute Herpes zoster involving thoracic region. It is said that aggressive treatment of acute Herpes zoster infection helps in avoiding PHN.
A live attenuated vaccine to boost immunity to VZV and reducing the risk of HZ, is recommended for adults older than 60 years of age. It significantly reduces both HZ and PHN.
Due to the associated side effects of systemic therapy, the focus has shifted to topical therapy. It is a convenient and pain-free self-administration by the patient. There is reduced frequency of administration, reduced systemic exposure due to limited absorption from the skin surface, and enhancement of patient compliance.
Local topical therapy includes
Surgical approaches such as stereotaxic trigeminal tractotomy and dorsal root entry zone (DREZ) treatment procedures have been described in the treatment of PHN.