Paresthesias: a practical diagnostic approach

American Family Physician,  Dec, 1997  by Jerry T. McKnight,  Bobbi B. Adcock

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Paresthesias are abnormal sensations experienced in the absence of specific stimuli.[1(p1234),2] These sensations are usually described as burning, tingling or numb feelings, although they may be described as feelings of cold, warmth, prickling, pins and needles, skin crawling or itching.[2] The most common locations of paresthesias are the hands, arms, legs and feet, although paresthesias can be present anywhere on the body. Paresthesias are contrasted with dysesthesias, which are abnormal interpretations of appropriate stimuli.[1(p5l5),2] Paresthesias are common presenting complaints, and diagnosis is usually assisted by knowing the specific clinical presentations associated with various paresthetic syndromes.

Pathophysiology and Etiology

The basic pathophysiology of paresthesias is an altered nerve or nerve pathway function. Paresthesias are thought to represent abnormal showers of impulses generated from an ectopic focus[3] and can arise from an abnormality anywhere along the sensory pathway, from the peripheral nerves to the sensory cortex.[4] Paresthesias can be caused by central nervous system or peripheral nervous system abnormalities. Central nervous system causes include ischemia, obstruction, compression, infection, inflammation and degenerative conditions (Table 1).

TABLE 1

Selected Causes of Paresthesias

Central                  Inflammatory
Ischemic                  Systemic lupus
  Cerebrovascular           erythematosus
   accident              Nutritional
  Transient ischemic      Vitamin [B.sub.12] deficiency
   attack                Miscellaneous
Structural                Multiple sclerosis
  Tumor                  Peripheral
  Trauma                 Neuropathy (see Table 2)
Infectious
  Brain abscess
  Encephalitis

Proprioception is tested by grasping the sides of the finger or toe being tested and asking the patient, whose eyes should be closed, to indicate whether the digit is moved into an up or a down position.[17] Loss of position sense is associated with a nerve root lesion, a peripheral nerve abnormality or dorsal column disease.

Thermal sensation is tested with test tubes filled with water of various temperatures. Patients with normal thermal sensation should be able to distinguish between stimuli differing by a few degrees.[2] Unfortunately, the examiner is relying on a subjective patient response, which is dependent on the patient's level of motivation and intelligence. A sensory loss may not be present in areas of symptoms; likewise, sensory deficits may be detected in asymptomatic areas.

Abnormal findings should be correlated with reflex or motor abnormalities. These abnormalities may suggest a specific nerve, nerve root or spinal cord lesion. Hyperreflexia is suggestive of upper motor neuron disease, while decreased reflexes are associated with spinal nerve segment disease or peripheral nerve injury. A neurologic examination may also reveal signs of a neuropathy or myelopathy that may help determine the etiology of the paresthesia. The extremities may demonstrate trophic changes, changes in skin color or palpable nerves, such as the superficial radial or the posterior auricular nerve, as occurs in patients with hereditary hypertrophic neuropathy.[18] Muscle groups, especially the distal musculature, should be assessed for strength and signs of atrophy. Abnormalities of specific muscles or muscle groups are associated with specific nerve lesions.

LABORATORY EVALUATION

The laboratory examination (Table 5) is. a vital aspect in the evaluation of paresthesias. Basic laboratory evaluation should begin with a complete blood cell count, a chemistry profile, a urinalysis and determination of thyrotropin-stimulating hormone and sedimentation rate.[19] Depending on the clinical situation, additional serologic tests, including measurement of folate and vitamin [B.sub.12] levels, VDRL, antinuclear antibodies and serum immunoelectrophoresis, should be considered.[19] A purified protein derivative (PPD) tuberculin should be applied, if clinically indicated. Heavy metal analysis of serum for the presence of lead, mercury or arsenic may be necessary. Tests of nerve conduction velocity and electromyography should be ordered if the laboratory tests do not provide a diagnosis.[20,21] Radiologic studies should focus on a specific anatomic area, such as evaluation of a suspected lumbar radiculopathy. Evaluation of a suspected herniated disc should include magnetic resonance imaging (MRI), myelography or computed tomography of the affected area.

TABLE 5

Laboratory and Other Studies Useful in Diagnosing Paresthesias

Initial evaluation Complete blood cell count Chemistry profile Urinalysis Thyrotropin-stimulating hormone Sedimentation rate

Miscellaneous studies Folate Vitamin [B.sub.12] VDRL Antinuclear antibodies Serum immunoelectrophoresis Nerve conduction velocities Electromyography Roentgenograms Magnetic resonance imaging Myelography Computed tomographic scan Lumbar puncture Purified protein derivative (PPD) Heavy metal analysis Nerve biopsy Muscle biopsy