Sympathetic blocks

Blocks of the sympathetic nervous system used to be the mainstay for diagnosing reflex sympathetic dystrophy and causalgia. However, modern research has shown that these conditions, now embraced by the rubric - complex regional pain syndromes - do not necessarily involve the sympathetic nervous system. The diagnosis can be rendered on clinical grounds, without the use of sympathetic blocks. Blocks of sympathetic nerves have been relegated to testing whether or not the pain is sympathetically maintained.

Traditionally, sympathetic blocks have involved the injection of local anesthetic agents on to that section of the sympathetic trunk that innervates the affected region of the body. Such blocks interrupt conduction in sympathetic nerves, and relief of symptoms is taken to indicate that the symptoms are sympathetically mediated. Alternatives have been the use of intravenous injections of drugs that ostensibly block the terminal of sympathetic nerves in the periphery, or the action of their transmitter substances. The validity of these procedures has been challenged.

Table 12.4 Painful neuropathies and the laboratory tests for their diagnosis.

Neuropathy

Diagnostic test

Serum

Urine

Glucose tolerance Liver function tests

B 12

Antinuclear antibodies Sjogren antibodies HIV antibodies Protein electrophoresis 8-galactosidase Globotriasosylceramide Electrolytes Renal function tests

Glucose

Diabetic Alcohol

Vitamin deficiency Vasculitis Sjogren's disease AIDS

Primary amyloid Fabry's disease

Uremic

Porphyric

Tangier's disease

Churg-Strauss Heavy metal Familial amyloid Hereditary

Charcot-Marie-Tooth Cryoglobulinemic

Glucose

Glucose tolerance Liver function tests

B 12

Antinuclear antibodies Sjogren antibodies HIV antibodies Protein electrophoresis 8-galactosidase Globotriasosylceramide Electrolytes Renal function tests

Low cholesterol Low HDLs Low apoprotein Eosinophilia Arsenic, thallium

Glucose

Protein electrophoresis Globotriasosylceramide

8-aminolevulinic acid Porphobilinogen

Biopsy

Biopsy

Biopsy Biopsy Biopsy Biopsy

HDL, high-density lipoprotein.

Since their inception, stellate ganglion blocks have customarily been performed without controls. If a stellate ganglion block abolished the patient's pain, the response was assumed to be genuine and physiological. Indeed, so strongly established has been the faith in stellate ganglion blocks that they were excused challenge with controls. Textbooks that describe these blocks make no mention of the need for controls.70, 71 The face validity and construct validity of the blocks is simply assumed. Only Bonica72 briefly calls for the repetition of blocks with different agents in order to test the validity of the response.

The first controlled study of stellate ganglion blocks appeared some 50 years after their introduction into pain medicine. In patients with complex regional pain syndrome, Price et al.73 injected the ganglion with either normal saline or local anesthetic. With either agent, just as many patients reported relief of pain immediately after the block. The only difference that emerged was that patients who received local anesthetic retained relief the following day; no patient who received normal saline was so relieved. Consequently, the immediate response to sympathetic blocks is not a valid criterion for sympathetically mediated pain. That criterion must be amended to prolong relief lasting to the following day.

Intravenous sympathetic blocks have not withstood challenge with placebo controls. Intravenous guanethi-dine does not have effects distinguishable from those of normal saline;74,75,76,77 nor does phentolamine.78,79,80 This lack of specificity has led some authorities to suggest that the active component of intravenous sympathetic blocks is the application of the sphygmomanometer cuff.77

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