Sympathetic nerve blocks: in search of a role.

OBJECTIVE: The role of sympathetic blocks in pain therapy is examined in the light of changing concepts of pain pathophysiology. A critical review of the literature also sought to develop an evidence-based analysis of outcome studies to provide recommendations for appropriate applications of sympathetic blocks, together with ideas for further clinically based research. METHODS: A focus on the pathophysiology of neuropathic and inflammatory pain disorders was used to help redefine what contribution, if any, was provided by the sympathetic system, to chronic pain states. Validation of nerve block therapies based on historical practices and these newer concepts and outcome determinations has then been used to present an overview of clinical nerve block therapies as applied to the sympathetic nervous system. RESULTS: 1. Pain Diagnosis: A reclassification of reflex sympathetic dystrophy (RSD) to the new taxonomy of complex regional pain syndromes (CRPS) is supported, with evidence that only a questionable sympathetic contribution at the dorsal root ganglion level can be ascribed etiologically to this group of disorders. Sympathetic blocks can establish whether pains may be nonresponsive or variably responsive to such blocks, but are considered inappropriate in determining a clinical diagnosis. 2. Neuropathic Pain Therapy: (a) A critical review of the literature regarding the use of sympathetic blocks in the treatment of acute herpes zoster pain and in the treatment of postherpetic neuralgia found little support for the widely held view that sympathetic blocks reduced either the incidence of long-term reduction of pain in these disorders. Further attempts to reduce PHN by the combination of blocks with aggressive drug therapies during acute herpes infection are suggested. (b) CRPS (RSD) treatments are seen as evolutionary at present, with the role of sympathetic blocks being only part of a balanced pain treatment strategy aimed at getting patients activated under cover of good analgesia and improved function. These proposals come as consensus recommendations but are not substantiated by outcome studies. 3. Ischemic Pain: Permanent sympathetic block with neurolytic or thermocoagulation techniques provides up to 50% long-term improved blood flow and reduction of pain and ulceration for patients with advanced peripheral vascular disease. This is particularly appropriate at lumbar levels in which percutaneous techniques are safe when conducted with real time imaging control. CONCLUSIONS: Changes in the understanding of CRPS disorders and the role of the sympathetic nervous system in neuropathic pain has changed both the diagnostic and management strategies for these pain states. The sensitivity and specificity of response to sympathetic blocks in establishing their value at diagnostic aids will not be fully established without further clinical study. Further use of intravenous regional blocks or diagnostic intravenous infusions remains questionable. Preventive and therapeutic use of sympathetic blocks in herpes zoster pain remains open to well-controlled study.

Epidural space as a Starling resistor and elevation of inflow resistance in a diseased epidural space.

BACKGROUND AND OBJECTIVES: The origin and the presence of negative pressure in the epidural space as well as the relationship of the extent of epidural anesthesia to epidural pressure has long been a subject of controversy. To further elucidate epidural pressure and its time course, the pressure at the needle tip was continuously measured as it traversed the interspinous ligament and the ligamentum flavum. METHODS: In a group of 22 patients, fluid was infused under gravity, and in a second group of 25 patients, boluses of fluid were administered at controlled infusion rates and under gravity. The volume-pressure-flow relationship was thus measured in one of two ways, either with a manual syringe and pressure transducer or with a pressure-monitoring-computer-controlled volumetric infusion pump. RESULTS: Natural pressure, (i.e., pressure in the epidural space before instrumentation is applied) could be approached when the space was first entered before fluid was infused (initial pressure); or after fluid had been infused (residual pressure). Epidural pressure could be extrapolated from the upsweep of the volume-pressure-flow relationship by projecting it back to just before the first injection. The extrapolated pressure lay between the initial and residual pressures. Medicinal solution placed in the barrel of the syringe did not infuse under gravity until the syringe barrel was lifted to a certain height, at which flow began and continued at a perceptible rate, with very little or no further increase in height required to maintain flow. The pressure at which flow began was the critical opening pressure, a characteristic of a Starling resistor. Furthermore, resistance to inflow of fluid was related to the presence or absence of natural or surgical disease in the epidural space. Resistance was significantly higher in the diseased than in the surgical group, at 114 (range, 22-226) mm Hg/L/h versus 46 (range, 8-86) mm Hg/L/h. Three phases were seen in the pressure-time recordings. CONCLUSIONS: Volume-pressure-flow relationships in the epidural space can be explained by a model in which epidural and subarachnoid pressures are inextricably related with the Starling pressure, dependent on the subarachnoid pressure. This model suggests reasons why spread of anesthetics might be difficult to predict.

Nerve blocks in the diagnosis of low back pain.

The use of diagnostic injection techniques in the evaluation of low back pain disorders is a clinical judgment skill, based on applied anatomy, pharmacology of the agents employed, and the neurophysiology of pain. In earlier years, the focus was primarily on the anatomic basis of pain, with sometimes oversimplistic interpretations of the results obtained. Although an anatomic correlation of a subjective complaint remains the same diagnostic objective today, a clearer understanding of pain concepts has also given these injection techniques a greater sensitivity but narrower spectrum in their application. Local anesthetic injections can readily identify sources of pain in soft tissues, scar tissue, nerve injury, and ligaments. They have particular application in this regard in postsurgical and postinjury back pain states in which the normal anatomy or function may be distorted. Local anesthetic blocks can also corroborate or define sites of pain in dorsal facet joints and in torsional disc capsule tears, where sites of multiple imaging or negative imaging findings make for difficult clinical diagnosis. In this context, they can also allow additional injection of deposteroid preparations, extending the procedure to a longer term therapeutic measure. Less specific anatomic information can be developed with respect to contributions from afferent and efferent responses, and in peripheral as compared with central mechanisms subserving the total clinical pain. For disorders of neuropathic and sensitization pain states, the spinal segmental level of signal processing from which conscious projection is derived can be well defined. Planned use of diagnostic blocking techniques is not a shortcut to sophisticated imaging or neurophysiologic studies but readily complements such investigations when conclusions are equivocal. Diagnostic blocks are only as good as the detailed clinical examinations which precede and follow the block itself.

Reversal of midazolam sedation with anexate.

Anexate (Ro 15-1788) a specific benzodiazepine antagonist, was assessed for its action in reversing midazolam-induced sedation. Sixty-five patients undergoing prostatic surgery under subarachnoid anaesthesia received midazolam for intraoperative sedation (mean dose 16 mg) followed by either active drug (anexate) or placebo given as a randomized, double-blind i.v. injection. The anexate dose sufficient to reverse sedation (0.36 +/- 0.09 mg), produced immediate and dramatic improvements in ability to comprehend and obey commands, in orientation in time and space and in degree of anterograde amnesia. These changes remained significantly different from the control group for 60 min after injection. There was no effect on arterial pressure, heart rate or ventilatory rate and no anxiety states were observed. After initial complete awakening, sedation increased gradually in drug-treated patients, while in the control group, sedation scores and cognitive testing scores all diminished over the 4-h study period. Anexate used in doses up to 0.5 mg provided safe and effective antagonism of midazolam-induced sedation in a clinical setting.

Intra arterial ketanserin in peripheral vascular disease.

An open study utilizing a serotonergic S2 antagonist, ketanserin, in a single 10mg intra arterial dose, was undertaken in eleven patients suffering various micro-vascular ischemic diseases. Ipsilateral limb blood flow improved 50%, skin temperatures rose 2.5 degrees C and venous oxygen tensions measured in the diseased foot showed modest elevation. The changes were significant and comparable to those achieved with chemical sympathectomy. These findings suggest ketanserin may have a place in the treatment of microvascular ischemic disease. However, systemic and contralateral flow induced changes suggested regional specificity was not obtained with arterial injection.

Newer methods of delivery of opiates for relief of pain.

Successful pain management using opiates requires both an analgesic with sufficient intrinsic activity and an effective administration system. Most instances of unsatisfactory pain control, however, are due to failure to achieve and maintain adequate blood concentrations of the chosen drug. Newer techniques of administration aim to overcome this problem. Oral opiate therapy with conventional or sustained-release formulations of morphine provide good control of terminal cancer pain provided that a regular dosing pattern is established and reviewed according to the patient's needs. This represents a significant departure from the traditional 'as required' prescription of this type of drug. In the management of acute severe pain, sublingual and intravenous opiates--self-administered as needed, or given by mandatory dosing schedules--have also been shown to overcome the limitations of intermittent intramuscular injections. A further novel development, stemming from basic neuroscience research, is the selective application of opiates to the spinal cord via the epidural or intrathecal route. This controversial technique has led to major improvements in treatment of some types of acute and chronic pain.

Lumbar sympathectomy: a single needle technique.

A single-needle lumbar sympathectomy technique is described which uses the tip of the 12th rib as a marker to determine the appropriate position for the insertion of the needle. Radiological screening is used to control final placement at the body of L3 and to visualize spread of radio-opaque phenol in the correct prevertebral plane. Following sympathectomy, fore-foot temperatures increased by a mean of 3 degrees C and lower leg blood flow doubled, these being accompanied by significant increases in ankle pressure index and venous oxygen tensions in the affected foot. These physiological improvements brought about resolution of ischaemic foot pain in 70% of patients at follow up 1 month later. As these results were not significantly different from those obtained in patients having two-needle sympathectomy, blockade with a single needle under x-ray control is considered to be an effective technique for the performance of neurolytic sympathectomy.

Clinical actions of fentanyl and buprenorphine. The significance of receptor binding.

Receptor binding assays were undertaken in an attempt to elucidate the opioid binding characteristics of fentanyl and buprenorphine, and to investigate some of the differences between them. Buprenorphine showed slow receptor association (30 min), but with high affinity to multiple sites from which dissociation was very slow (T 1/2 = 166 min) and incomplete (50% binding after 1 h). This contrasted with the receptor binding of fentanyl, which achieved rapid equilibrium (within 10 min) and dissociated equally rapidly (T 1/2 = 6.8 min) and completely (100% by 1 h). Competitive displacement showed buprenorphine displacement of fentanyl binding was concentration- and time-dependent over ranges encountered in clinical use, but buprenorphine binding was displaced with only very high concentrations of other opioids. These findings offer pharmacodynamic explanations for the differences in fentanyl and buprenorphine analgesic response profiles and suggest how binding interactions might be applied to therapeutic use.

Comparison of midazolam with thiopentone for outpatient anaesthesia.

The water soluble benzodiazepine, midazolam, was compared with thiopentone as an intravenous induction agent in unpremedicated patients undergoing cystoscopy as out-patients. Induction time was longer with midazolam, though subsequent transition to halothane inhalation anaesthesia was smooth and uneventful. Both drugs had similar effects on the cardiovascular and respiratory systems and were without local irritation following intravenous injection. Recovery was more prolonged with midazolam, tests of memory and unaided mobility showing greater impairment one hour after anaesthesia. Notwithstanding a slower and less predictable onset of unconsciousness and somewhat slower recovery phase, midazolam provides a safe alternative to thiopentone as an induction agent suited to minor surgical procedures.

Analgesic responses to i.v. lignocaine.

The analgesic effect of i.v. lignocaine was evaluated in five patients with clinical neuralgic pain of varying aetiology. The response was compared with that on concurrently-induced ischaemic pain, initially of the same intensity. Following a high dose infusion of 3 mg kg-1 (lignocaine concentrations greater than 3 microgram ml-1) both pains were decreased, clinical pain to a significantly greater extent. Thereafter, at lower doses and blood concentrations, lignocaine was without effect on ischaemic pain, but almost totally suppressed the same patient's clinical pain. The results suggest a divergence in the specificity of the analgesic action of lignocaine i.v. according to the nature of the pain-inducing process. Disorders manifesting as deafferentation or central neuralgias appear to be affected favourably by lignocaine i.v. whereas pain of peripheral origin is unaffected by lignocaine, except at blood concentrations which approach toxic values.

Electrical stimulation in the relief of pain: a pilot study.

A group of 26 patients who suffered from chronic pain resistant to other therapeutic measures, were given electrical stimulation in two forms. The first utilised penetrating acupuncture needles connected to a standard Chinese electrical impulse generator and the second method employed American transcutaneous stimulators with conducting skin electrodes. At long term follow-up 50 percent of patients treated exhibited sustained relief--20 percent when the device was used alone and a further 30 percent when it was combined with other forms of specific pain therapy. There was no difference in the response to needle acupuncture or transcutaneous stimulation. The best response was seen in patients with neuralgic disorders and painful musculo skeletal syndromes affecting the cervico thoracic region.

The abdominal cutaneous nerve entrapment syndrome.

Entrapment of the abdominal segmental nerves within the rectus abdominis muscle may produce abdominal pain simulating surgical or genito-urinary disease. The diagnosis can be made by localising with one finger an area of tenderness close to the lateral border of the muscle. Pressure at this site will usually reproduce the pain and precise injection of local anaesthetic will abolish the pain immediately. Effective long-term relief is usually obtained by the injection of long acting steroid or a small volume of 7 percent aqueous phenol.

Management of intractable pain: the scope and role of nerve blocks: review of one year's experience.

This paper presents the first one year's experience of nerve blocking procedures carried out for the relief of intractable pain. The control of pain is complex and has led to the development of Pain Clinics employing the skills of different specialties. The scope and organisation of such a clinic developed at Auckland Hospital is outlined. Some 103 patients were referred with intractable pain from cancer, musculo-skeletal disorders, neurogenic causes and ischaemic causes and ischaemic problems. Of these, 55 percent derived complete relief. Breakdown of results demonstrate that over 70 percent of patients in the groups of pain of neurogenic and ischaemic origin had complete relief of pain. Recognised complications of major nerve blocking procedures were encountered in a small percentage of patients, but these caused no great concern. Nerve blocks have a definite place in the treatment of chronic pain and form an important adjunct to the overall management.