How to treat postherpetic neuralgia?🔨🔨🔨

Herpes zoster is an infectious disease caused by a virus. The varicella-zoster virus can remain latent in the human body for a long time. They can be reactivated to cause infectious skin disease. Postherpetic neuralgia is one of the complications of shingles. Pain that persists for 1 month or more after clinical healing of the shingles rash is considered postherpetic neuralgia. Its prevalence ranges from approximately 9% to 34%. Patients with postherpetic neuralgia suffer from severe pain for a long time, which can easily lead to depression and their quality of life will be seriously reduced.

What are the clinical manifestations of postherpetic neuralgia?

1. Clinical manifestations of pain:

The clinical presentation of postherpetic neuralgia is complex and varied. It can be continuous or discontinuous. It has the following characteristics:

• Pain location: common in trigeminal nerve (mainly ophthalmic branch), neck or unilateral chest. About 50% of the patients occurred in the chest. Occurrence in the head and face, neck and waist accounted for 10% to 20% respectively. Occurs in the sacrum about 2% to 8%. Other parts are less than 1%. The painful area of postherpetic neuralgia is usually larger than the area of herpes zoster. Bilateral herpes is rarely seen in patients.
• The feeling of pain: Different patients experience different pain sensations. It can be like burning, knife cutting, needle sticking, shocking or tearing. Patients may experience one type of pain or possibly multiple pains at the same time.

2. Features of Pain:

• Spontaneous pain: Pain occurs in the rash distribution area and the surrounding area without any stimulation.
• Hyperalgesia: The patient's response to a noxious stimulus is prolonged or intensified.
• Allodynia: Nonnoxious stimuli cause pain in the patient. Such as small changes in temperature, clothing rubbing against the body, etc. will cause pain to the patient.
• Paresthesia: The patient will have some paresthesias such as numbness, itching and tightness in the painful area. Patients may also experience objective sensory abnormalities such as hypoesthesia, abnormal temperature and vibration sensations. 

3. Course of disease:

Postherpetic neuralgia persists for more than a year in 30% to 50% of patients. In some patients, the course of the disease can even reach ten years or more.

4. Other clinical manifestations:

Patients with postherpetic neuralgia are often accompanied by decreased mood, sleep and quality of life. Anxiety, depression, inability to concentrate, etc. are manifestations of moderate to severe disturbance of their emotions. Studies have pointed out that about 60% of patients have had or often have suicidal thoughts. More than 40% of patients have moderate to severe disturbance of daily life and sleep disturbance. Various systemic symptoms such as chronic fatigue, anorexia, weight loss and lack of activity may also appear in patients. 

Drug therapy for postherpetic neuralgia.

Gabapentin: It is a first generation calcium channel modulator. Its pharmacokinetic profile is non-linear and its bioavailability decreases with increasing dose. Its initial oral dose is 300 mg daily and patients need about several weeks to slowly titrate to the effective dose. Its effective dose is generally 900 to 1800 mg daily. The dose of gabapentin should be reduced in patients with renal insufficiency. In order to avoid adverse reactions of dizziness and drowsiness in patients, the dose should be started, slowly increased or gradually decreased at night.

Pregabalin: It is a second-generation calcium channel modulator. It can relieve postherpetic neuralgia. It can also improve sleep and mood disorders. Its pharmacokinetic profile is linear and has a rapid onset of action. Its oral initial dose is 150 mg daily. Its dose can be increased to 300 mg daily within a week and its maximum dose is 600 mg daily.

Amitriptyline: It is a tricyclic antidepressant. It inhibits the reuptake of serotonin and norepinephrine by the presynaptic membrane. It also blocks alpha adrenoceptors and sodium channels. The descending pathway of pain conduction will be regulated by the above mechanism to play an analgesic effect. However, its onset is slow. Its first dose should be taken at bedtime. Take two to three times a day, 12.5 to 25mg each time. The dose can be gradually increased according to the patient's response to amitriptyline. It recommends a maximum daily dose of 150mg, but in some patients up to 300mg is available. Amitriptyline should be avoided in patients with ischemic heart disease or at risk of sudden cardiac death. Patients with high risk of glaucoma, urinary retention and suicide should also use amitriptyline with caution.

5% lidocaine patch: It inhibits voltage-gated sodium channels and reduces ectopic impulses of primary afferent nerves after injury. This reduces pain perception in people with postherpetic neuralgia. Its onset of efficacy is ≤4 hours. Patients can apply 1 to 3 patches to the painful area. Each patch can be used for up to 12 hours.

Tramadol: It acts on μ-opioid receptors, serotonin receptors, and norepinephrine receptors to achieve analgesic effects. It can relieve burning pain, pin prick pain and allodynia caused by postherpetic neuralgia. However, it has no obvious analgesic effect on lightning-like and knife-like pain. Its initial dose is 25 to 50 mg once or twice daily. The maximum daily dose is 400mg. Nausea, vomiting, dizziness, constipation and other adverse reactions are dose-related.

Opioid analgesics such as morphine, oxycodone, and fentanyl are effective in the treatment of postherpetic neuralgia, but their addiction and tolerance limit them to second-line treatment. In addition, neurotrophic drugs can help relieve nerve pain and neuroinflammation. Commonly used drugs are methylcobalamin, vitamin B1 and vitamin B12. They can be given orally or injected intramuscularly.