neuropathic%20pain
NEUROPATHIC PAIN
Neuropathic pain is the sensation of pain due to abnormal discharges of impaired or injured neural structures in the peripheral &/or central nervous system.
It is characterized by hyperesthesia, hyperalgesia and allodynia.
Common neuropathic pain syndromes are central neuropathic pain, painful diabetic peripheral neuropathy, postherpetic neuralgia, trigeminal neuralgia, postsurgical neuropathic pain, HIV-related neuropathy, lumbosacral radiculopathy and complex regional pain syndrome.

Diagnosis

  • History and clinical examination are required to confirm the presence of a neuropathic pain syndrome and also an important step in reaching an etiologic diagnosis for neuropathic pain
  • Rapid and correct diagnosis of neuropathic pain (NP) should be done so that appropriate treatment can be started earlier

Classification

Common Neuropathic Pain Syndromes

Central Neuropathic Pain

  • Pain may be burning, shooting, aching, or pricking and is often associated with dysesthesia, hyperalgesia or allodynia to brush or cold
  • Seen among post-stroke, spinal cord injury and multiple sclerosis patients

Painful Diabetic Peripheral Neuropathy (DPN)

  • Symmetrical sensory loss and burning, shooting, stabbing or severe deep aching pain of both lower extremities among patients with diabetes mellitus
    • Affects about 50% of diabetes mellitus patients
  • Pain may be accompanied by allodynia and hyperalgesia
  • Symptoms may worsen at night

Postherpetic Neuralgia (PHN)

  • Pain persisting for >3 months after herpes zoster skin lesions have healed
  • Risk factors for developing postherpetic neuralgia:
    • All patients >50 year with herpes zoster virus infection
    • Patients with a greater degree of skin surface area involved and more severe pain at presentation of herpes zoster virus infection
    • Patients with herpes zoster virus ophthalmicus
    • Patients who experience sensory dysfunction on the affected dermatome
    • Painful prodrome prior to rash
  • Patients at greatest risk for developing postherpetic neuralgia should be offered antiviral treatment
  • Diagnostic clinical features of postherpetic neuralgia:
    • Pain localized to the dermatome affected by the herpes zoster rash
    • Pain may be described as burning, throbbing, sharp or shooting
    • Allodynia
    • Pain accompanying movement
    • Areas of scarring of hypopigmentation caused by herpes zoster rash
    • Presence of psychosocial stress

Trigeminal Neuralgia (TN)

  • Characterized by intermittent unilateral facial pain following the unilateral sensory distribution of the trigeminal nerve, typically radiating to the maxillary or mandibular area
    • Ophthalmic division pain alone occurs in <5% of patients
  • Pain is quite severe, stabbing electric shock-like sensation
  • Pain may be brief and paroxysmal but may occur in multiple attacks several times a day, with no pain between episodes
  • Typically with normal physical examination findings, although mild light touch or pin perception loss has been described in the central area of the face
  • Average age of pain onset in idiopathic trigeminal neuralgia is usually the 6th decade of life, but may occur at any age
  • Symptomatic or secondary trigeminal neuralgia tends to occur in younger patients
  • Common triggers precipitating pain attack include light touch or vibration and activities such as shaving, face washing or chewing

Postsurgical Neuropathic Pain

  • Peri-incisional sensory loss, pain and allodynia for >3 months after surgery; phantom pain following amputation or mastectomy

HIV-Related Neuropathy

  • Symmetrical painful paresthesias, most prominent in the toes and soles of the feet among HIV patients

Lumbosacral Radiculopathy (Herniated Intervertebral Disc)

  • Lancinating pain radiating into the anterior thigh (L2/3) or lower leg (L4-S1) with motor weakness or sensory loss

Complex Regional Pain Syndrome

  • Regional (eg limb) pain together with edema, cutaneous blood flow and sweating abnormalities

History

History

  • Will help determine whether the character and distribution of pain follow the neuropathic criteria and whether a relevant lesion or disease in the nervous system is probably responsible for the pain

Pain Intensity

  • Can be rated using a valid verbal, numerical or visual analog scale or Numeric Pain Rating Scale such as the Neuropathic Pain Scale and Neuropathic Pain Questionnaire
  • Assessed at each visit to monitor therapeutic response

Description of Sensory Symptoms

  • Quality of pain: burning, sharp, stabbing, cold, allodynia, hyperalgesia, spontaneous, dysesthesia, paresthesia
  • Frequent non-painful sensations: pricking, tingling, aching, numbness, hypoesthesia, anesthesia, hypoalgesia, analgesia
  • Sensory abnormalities and  pain paradoxically co-exist

Temporal Variation of Pain

  • Pain usually becomes worse toward the end of the day
  • Rule out a neoplastic process if pain progressively increases over the recent months

Functional Impact

  • Impact on sleep, self-care, daily activities, work, social and sexual dysfunction, mood and suicidal ideas

Previous Treatment

  • Usually resistant to medications (eg Paracetamol, NSAIDs)
  • Adequate titrated doses of specific drugs should be determined and documented

Alcohol and Substance Abuse

  • History of dependence disorders can affect decision about prescribing opioids and cannabinoids
  • Consider interaction of sedatives and alcohol with other substances

Spontaneous Pain or Sensation

  • Paresthesias (eg tingling, itching, sensation of something crawling on one’s skin, discomfort of one’s foot “falling asleep”)
  • Dysesthesias (eg pricking, electric shock-like, burning or shooting pain)
  • Phantom pain

Stimulus-Evoked Pain or Sensation

  • Allodynia
  • Hyperalgesia
  • Hyperpathia
  • Hypoesthesia/Anesthesia

Physical Examination

Physical Examination

  • Allows integration of the patient’s current symptoms and localization of the involved elements of the nervous system
  • Identifying pain localization, quality, intensity and pattern is essential
  • Reveals the presence of negative (loss of function) and positive (hyperalgesia and/or allodynia) signs for sensory modalities affecting the somatosensory system and relevance to the underlying disease or lesion

Motor Examination

  • May reveal motor weakness in the distribution of the involved nerve

Deep Tendon Reflex

  • May be decreased or absent in the distribution of affected nerve

Sensibility Examination

  • Reduced or absent light touch, pin prick, vibration responses and proprioception in the affected nerve territory
  • Sensory disturbances can expand outside the area of nerve innervation
  • Dynamic allodynia: pain arising from gentle brushing of skin with cotton ball
  • Thermal allodynia: burning sensation due to an ice cube placed on the skin
  • Hyperalgesia to a pin prick test
  • Pain on leg lifting: irritation of lumbar nerve roots
  • Myofascial trigger points: myofascial pain plus neuropathic pain

Skin Examination

  • Changes in skin temp, color, sweating or hair growth (complex regional pain syndrome)
  • Residual dermatomal scars persisting after herpes zoster infection
  • Characteristic skin changes of diabetes mellitus

Laboratory Tests

Ancillary Tests

  • May be conducted to document the presence of a specific underlying neurologic disease or confirm a sensory lesion within the pain distribution

Laboratory Tests

  • Quantitative means to measure objective response

Neurophysiological Testing

  • Standard neurophysiological responses to an electrical stimulus can identify, localize and quantify damage along peripheral or central sensory pathways
  • Pain-related evoked potential: Laser-evoked potentials are the easiest and most reliable methods for assessing function of the nociceptive and A-delta fiber pathways in patients with neuropathic pain
  • Electromyography and Nerve Conduction Velocity (EMG/NCV): Provides objective evidence of nerve injury or dysfunction but primarily evaluates the large myelinated fibers thus small fiber neuropathy may not be ruled out if the result is normal
  • Microneurography: Provides valuable information on the physiology and pathophysiology of all nerve fiber groups but this is not recommended as routine procedure for assessing patients with peripheral neuropathic pain 
  • Pain-related reflexes: Diagnostically useful only for facial pains as in trigeminal pain disorders

Skin Biopsy

  • Best tool for assessing neuropathies with distal loss of unmyelinated nerve fibers
  • Recommended in patients with painful or burning feet of unknown origin and with clinical impression of small fiber dysfunction

Additional Tests to Identify Other Causes of Neuropathies

  • Glucose tolerance test
  • Thyroid function
  • Vitamin B12 levels
  • CD4+ T-lymphocyte counts
  • Lumbar puncture

Quantitative Sensory Testing

  • Psychophysiological measure of perception in response to external stimuli of controlled intensity, which allows documentation of sensory profile
  • Appropriate to quantify positive sensory phenomena like mechanical and thermal allodynia and hyperalgesia which may help characterize painful neuropathic syndromes and predict or monitor treatment effects

Functional Neuroimaging

  • Functional neuroimaging studies are encouraged in patients with neuropathic pain

Computed Tomography (CT) Scan

  • Facilitates specific diagnosis

Three-Phase Bone Scan

  • May help in the diagnosis of complex regional pain syndrome

Positron Emission Tomography (PET)

  • Measures cerebral blood flow or metabolic activity in defined brain regions

Magnetic Resonance Imaging

  • Can identify small patches of inflammation in peripheral nerves

Activation Studies

  • Investigate local synaptic changes specifically associated with a given task or a particular stimulus by comparing statistically activated and controlled conditions

Screening

Screening and Assessment Tools

  • The main advantage is to identify potential patients with neuropathic pain, particularly by non-specialists

Screening Tools

  • Unidimensional scales
  • McGill Pain Questionnaire
  • Douleur Neuropathique en 4 (DN4) Questions
  • ID-Pain
  • Pain DETECT
  • Leeds Assessment of Neuropathic Symptoms and Signs
  • Standardized Evaluation of Pain
  • Chinese Identification Pain Questionnaire

Assessment Questionnaires

  • Brief Pain Inventory
  • Neuropathic Pain Scale
  • Neuropathic Pain Symptom Inventory
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