|ADR Frequently Asked Questions Discuss Failed back syndrome and spinal blocks in the General Discussion forums; Diagnostic Spinal Blocks Introduction The failed back syndrome is often synonymous with the failed diagnosis syndrome. The formulation of a ...|
Diagnostic Spinal Blocks
The failed back syndrome is often synonymous with the failed diagnosis syndrome. The formulation of a rational approach to treatment necessitates that every effort be made to accurately localize the source of pain. In a majority of cases the source of pain arises from multiple structural elements, often at multiple levels. The following chapter outlines the use of a diagnostic block protocol used to help sort out this difficult problem. The approach has evolved over a ten-year period at our institute in which the majority of referred patients are in the category of failed back surgery.
If pain symptom reproduction occurs during the injection of a non-irritating solution of local anesthetic at a known localized site and pain relief is achieved following local anesthetic block, then it is highly likely that the source of symptomatology is related to the structure(s) blocked. Gentle mechanical stimulation or deformation of asymptomatic structures does not typically cause pain nor reproduce symptoms. If the neural threshold of afferent pain fibers is reduced by chronic (or acute) chemical and/or ischemic injury, then gentle movement caused by an injected volume of fluid causes activation of afferent pain fibers with subsequent reproduction of symptoms. Likewise, gentle contact with asymptomatic supporting structures such as the capsular ligaments of posterior joints, disc annulus or longitudinal ligaments is only mildly discomforting. If injured by acute tearing or repetitive over stretching, however, these structures become acutely sensitive to contact with a block or discogram needle. Initial relief is achieved by conduction block following penetration of local anesthetic. Even in the case of epidurally injected anesthetic solution, the initial onset is segmental and most likely involves diffusion through the relatively thin aura and arachnoid membranes in the region of the spinal roots near the junction where they form the spinal nerve. The segmental and, in some cases, differential block by local anesthetic allows one to reasonably conclude that the pain is transmitted at a specific level or specific structure. In addition, the local placement of a steroidal mixture directly on the aural or synovial areas of inflammation/and or edema will result in a more effective action. If a reduction in symptomatology occurs within one week following injection of steroid mixture, this can be used as supplementary evidence for pain localization.
The evaluation of the failed back patient requires differentiation between the psychological and the organic components of pain. The thiobarbiturate (i.e. Pentothal, Brevital) pain study, described by Krempen may be used to more objectively quantify the degree of functional overlay.
At a titrated hypnotic dose of thiobarbiturate, reaction to painful stimuli remains intact and often is facilitated. A more objective estimate as to the amount of pain exaggeration in the particular movement tested can thereby be measured by comparing the response prior to and after induction. In order to interpret the test there must be a symptom-provoking movement which can be tested in a supine or prone position.
The symptom-provoking movement, i.e., straight leg raise is compared against a control stimuli, i.e., trapezius squeeze. The intensity of both the symptom stimulus and control stimulus is graded from zero to three. For example, straight leg raise at 30 degrees = 1, 60 degrees = 2, 90 degrees = 3. Likewise the intensity of the control stimuli is subjectively graded from 0 = no pressure to 3 = maximum intensity. The patient's reaction to each movement is similarly graded; 0 = no reaction to 3 = vocalization, grimace, and withdrawal.
An IV is started with 500 mg of Brevital in 500 cc crystaloid and slowly titrated. The tests are again repeated when the patient is relaxed but still coherent (Stage 1) and at the stage where the patient no longer responds to verbal commands but the lash reflex is still present, (Stage 2). A comparison of the intensity vs. reaction to control vs. symptom stimulus at the three stages is used to grade the central response from zero to 100 per cent.
For example, suppose at stage 0 and 1 a patient screams out in pain at a 30 degree straight leg raise (grade 4 response) and likewise to a level 1-3 trapezius squeeze. If at Stage 2, the same leg can be lifted to 90 degrees with little or no reaction and, at the same time, a grade 3 to 4 response is caused by trapezius squeeze, the patient is graded between 70 to 100 per cent central response.
It is important to remember that Brevital is a thiobarbiturate used as a general anesthetic induction agent. The same qualifications and precautions as those for the administration of a general anesthetic must be observed. Patients need be NPO 4 to 6 hours prior to the test. All of the usual general anesthetic equipment must be available and set up in the testing area. One must be experienced in the management of airways.
Finally, it must be remembered that no conclusions drawn are cast in stone. Every effort is made not to falsely label a patient's pain as psychogenic in origin. As might be expected, the test is most useful at the upper and lower ranges of response.
Indwelling Epidural Block
The indwelling epidural is used to access the patient's placebo response, to identify the general pain level, and to help separate anterior element pain vs. posterior element pain. The use of a catheter allows repeated injections and a more selective placement of anesthetic (and steroid) at the suspected symptomatic level and side. The catheter position (or needle in a single injection technique) very significantly affects the area of local anesthetic spread. This is especially true when space occupying lesions, i.e., scars, stenosis, or bulging-herniated discs have partially or completely obliterated the epidural space. Unfortunately, liquids flow in the route of least resistance which is likely to be away from the area one wishes to block. Therefore, as previously mentioned, the use of fluoroscopy to accurately verify catheter placement and, when indicated, the use of water soluble contrast solutions to define the area of spread, is a prerequisite to diagnosis.
If the pathology is suspected to lie in the L4-S1 segments, (commonly the site of previous surgery), the caudal route is chosen. Using an 18-gauge thin wall Crawford needle and an epidural catheter with a wire stiles or inner wire coil, the catheter can be safely manipulated under direct fluoroscopic vision. It is usually possible to place the catheter in close proximity to the suspected site of maximal pain. Prior surgery, with the resultant obliteration of the epidural space, may unfortunately prevent passage beyond the S1 level. A lumbar route is chosen to evaluate the L1-L3 levels. Entrance is made at the level and side of maximal symptomatology with a Touhy needle. In contrast with the caudal route, the angle of the catheter with the needle precludes safe manipulation of the catheter. The catheter is passed one to two cms and withdrawn.
An initial dose of normal saline is used to access the patient's placebo response. Following this, an initial injection of two to five ccs of local anesthetic, i.e., .5% Marcaine, will commonly give a localized one sided block confined from one to three levels. Failure to obtain pain relief is due to the level blocked being asymptomatic, a failed local anesthetic block due to spread away from the desired level, or a failed local anesthetic block due to poor neural penetration. The two latter situations are usually due to heavy scaring in an area of prior surgery. Increasing volumes are used when necessary, however the localizing value decreases as the levels blocked increases. Posterior element pain, such as caused from segmental instability, is poorly blocked at the lower volumes because of the multisegmental nerve supply of posterior structures. Segmental relief of anterior element pain, but incomplete relief of posterior element pain indicates primary anterior root pain generators located within the area blocked. As with other blocks, a more prolonged decrease in pain symptoms following the local application of a steroidal mixture prior to removing the catheter, is presumptive evidence that there is ongoing chemical and/or mechanical injury in the surrounding area.
We have had one recognized case of infection in over two thousand indwelling blocks. This occurred using a caudal catheter in a insulin-dependent diabetic and responded to intravenous antibiotic treatment. More common, is several days to several weeks of discomfort at the caudal insertion site secondary to periostial inflammation. In contrast, the lumbar epidural block has one potentially dangerous complication. Whereas, frank aural puncture is easy to recognize and thus local anesthetic withheld, an unrecognized aural cut may be manifest as a high or total spinal block. This may not become apparent for up to twenty minutes post injection if a longer onset anesthetic such as Bupivicaine (Marcaine/Sensorcaine) is used.
When the posterior epidural space is obliterated because of underlying stenosis or spondylosis, the aura is pushed against the overlying ligamentum flavum. Even with controlled entrance into the epidural space the block needle may make a small aural or arachnoid cut. The cut is unrecognized because the needle has not fully penetrated the aura and a small test dose may not cause a spinal block. The injection of five to ten ccs of local anesthetic, however, will cause a pressure differential between the attenuated epidural space and the subarachnoid space, and as a result, more than the usual mass of local anesthetic will cross into the subarachnoid space.
Settling of the anterior anterior joint complex, either iatrogenic, as the result of previous discectomy or as the consequence of recurrent disc injury, renders the facet synovium and capsular ligaments more prone to injury. The intraarticular changes that occur within the joint have been well documented by Kirkaldy-Willis, however, the most common source of symptomatology may be secondary to injury/inflammation of the facet capsule itself which is richly innervated by the dorsal ramus of the lumbar spinal nerve. A deep branch of the dorsal ramus loops under the transverse process and supplies the joint capsule of its superior articular face. Another more lateral branch of the same dorsal ramus is directed caudally and sends a branch to the joint capsule of the inferior articular facet. A single lumbar spinal nerve, therefore, supplies two facet joints, and each facet joint has bisegmental innervation. Stimulation of normal joints by pericapsular or intraarticular injection, refers pain to the low back, groin, and upper thigh. It is difficult to delineate unique or consistent referral patterns based on level, possibly due to the overlapping segmental innervation and the poor central discriminatory pattern of the low back. Of particular interest is that stimulation of the upper facet levels L1-2 produces low back pain similar to that of the LS-S1. In contrast, the referral pattern in the symptomatic patient is generally more intense and with a wider distribution of pain which may be into leg and foot. Dermatomal distribution of pain may represent true indirect referral or direct mixed spinal nerve compression and/or irritation by the close proximity of a facet capsule which may enlarge due to synovitis.
Unlike a peripheral joint, one cannot visualize nor selectively examine the facet joints; thus intraarticular and/or pericapsular facet injections offer a unique diagnostic tool. Clinical suspicion arises when facet maneuvers are positive (extension and rotation) and/or paravertebral pressure over the facets elicits reproduction of symptoms.
The patient is rotated into the oblique position until the facet joint is best visualized on the fluoroscope. A mark is made on the skin overlying the visualized opening, and a #22 or #20 gauge spinal needle is advanced to the facet. When the needle contacts the facet capsule, pain reproduction will be stimulated at a symptomatic joint. Usually the needle can be passed into the joint. However, the obliquity and size of the facet opening may technically preclude easy passage, and it is probably not prudent to persist in an attempt to enter the joint. This is especially true at the lower LS-S1 joint. In this case, placing local anesthetic or steroids on the capsule is probably adequate for diagnostic and/or therapeutic purposes.
A normal joint should have a tight capsule and therefore have a firm endpoint at less than one cc of contrast solution. At a degenerated level the facet capsule may have become stretched, and the joint capsule will admit several ccs of the dye which can often be seen to leak from previous capsular tears. The facet capsule is anesthetized with three to five ccs of local anesthetic such as .5% Marcaine; an intraarticular injection consists of 1/2 to two ccs of the same mixture. Steroids are usually added to the local anesthetic if the joint has reproduced symptoms. Because localizing symptoms are often vague, it is usually necessary to block more than one level.
The failure to elicit pain referral when contacting the facet capsule tentatively identifies an asymptomatic joint. Reproduction of symptoms suggests a symptomatic level and, if relief follows local anesthetic injection, that the joint is probably the source of part of the patient's symptomatology. We have generally found that once the joint has been entered, the amount of pain is much less. This strengthens our view that it is the capsule rather than the articular synovium that is the primary source of pain.
As with all blocks, care must be exercised in interpretation. The most common source of error is the spillage of local anesthetic to the spinal root. This is a very common occurrence and inevitably occurs when more than 2 ccs of local anesthetic is injected. Patients must be examined carefully for any signs of anterior ramus block.
Posterior Root Block
The lumbar dorsal ramus syndrome (LDRS) is described by Bogduk. Burton has demonstrated a common clinical distribution of "dorsal ramus" pain generated from facet joints, sacroiliac joints, and other musculoligamentous structures. It is quite likely that much of the "mechanical" low back pain of the failed back patient is mediated via this ramus. The medial and lateral branches of the posterior spinal root can be blocked at the base of the transverse process and can help separate posterior element pain from anterior element pain.
Spinal needles are advanced to the base of the transverse process at the level in question and one level above and below. In the case of the S1 level, the space between the inferior facet of L5 and the posterior sacral foremen is used as a landmark. Reproduction and relief of symptoms are noted after the injection of 3 ccs of .5% Marcaine at each level. If consistent relief of greater than 50% occurs on two occasions and the clinical situation is warranted, I cc of 10% phenol in glycerin can be injected following the local anesthetic. This may give up to nine months of partial to full relief. Extra care must be exercised when using phenol, and special attention should be given at the S1 level, where the posterior foremen can be entered inadvertently.
Selective Nerve Root Block
There are many anatomical and physiologic reasons for specific radicular involvement resulting in a "pain generator." This is especially true where previous surgery has resulted in ischemia secondary to stenosis, immobility secondary to scarring, or compression secondary to a recurrent disc, and thus rendered the nerve root abnormally sensitive to movement. In addition to the above factors, the nerve root becomes a common source of ongoing painful stimuli which produces a degree of instability which may be a common denominator of the "failed back syndrome". Selective nerve root block radiculography has been described by Macnab, Takeshi, Krempen and others as an effective method for confirming and localizing anterior root symptomatology. Saul has demonstrated a high correlation between pain reproduction and relief following selective nerve root block and abnormalities demonstrated on EMG.
Diagnostically, the block is used to help differentiate between anterior vs. posterior causes of radicular symptoms, and to identify the level from which anterior root symptoms originate. In addition, the more specific application of a steroid mixture results in a more favorable location for therapeutic action.
The exact level to be blocked is identified with fluoroscope, and a #20 gauge spinal needle is directed toward the appropriate foremen in between two and eight cm from the midline. In the medial approach the transverse process is used to gauge the depth of penetration. The needle is advanced to the process, the depth noted, and then passed off the caudal edge approximately one cm where it should lie in close proximity to the emerging root. A more lateral approach allows the needle to lay in a more medial position within the foremen. Lateral and AP fluoroscopic imaging is used to confirm placement. If prior fusion bone is present a far lateral approach must be taken, although it is not always possible to position the needle in close proximity to the emerging anterior ramus. The sacral nerve roots are approached via the posterior sacral foremen. The anterior sacral foremen are usually quite apparent on the fluoroscopic image and may be confused with the posterior foremen. The needle is passed through the sacral foremen by feel and the correct placement confirmed by the injection of several ccs of water soluble contrast media. Injection of contrast is followed by one to four ccs of local anesthetic mixed with steroid, i.e., 1% xylocaine + 80 mg Depo Medrol.
Several ccs of water soluble contrast media outlines the course of the nerve root. The movement of the nerve root caused by injection and the mildly irritating effect of the solution will recreate the patient's usual radicular pain if the nerve root is symptomatic. The dye will be seen to travel around the nerve root proximally into the epidural space and distally to the sciatic nerve. The outline of the dorsal root ganglion at the same level may be apparent. Unfortunately, scarring and in some cases foraminal stenosis may prevent good proximal spread. Previous authors have described correlation between the radiculogram and pathology confirmed at the time of surgery. Although we rely more on reproduction and relief of symptoms, it is often possible to identify pedicular or superior facet kinking of the nerve root.
One to three ccs of local anesthetic should anesthetize the single nerve root within three to ten minutes. As identified by the spread of contrast media, three ccs of solution may sometimes spread to more than one level via the epidural space, and therefore careful dermatologic testing must identify the roots blocked. Referred radicular pain, originating from posterior structures is not fully relieved because of multisegmental innervation. Partial relief of back pain, which often occurs, may be due in part to blocking the recurrent sinuvertebral nerve at the same level.
Potentially the nerve root can be damaged by the block needle, however we have encountered no permanent injuries of this type. The most common side effect is several days of increased pain, but this may be followed by more prolonged relief due to decreased edema and inflammation resulting from the injected steroids.
Discography offers a useful method for localizing symptomatic disc level(s) and an invaluable diagnostic procedure for identifying an asymptomatic level. A review by Hudgins discusses a sensitivity (the percentage of positive results obtained when abnormal subjects are tested) of 83%, and the specificity (percentage of negative results obtained with normal subjects) of 78% when using only the pattern of injected contrast media for assessment. More important is the very low incidence (1%) of false positive results meaning that a normal discographic pattern and nonreproduction symptoms essentially rules out the possibility of symptomatic disc disease at that level. A retrospective study by Simmons showed a "diagnostic accuracy" of 91% for cervical and 82% accuracy for lumbar discs. Crock feels discography is indispensable in the modern practice of spinal surgery.
We use discograms to 1) confirm or rule out a discogenic pain generator in any patient who has not responded appropriately to conservative care in which clear cut objective findings are absent, and 2) preoperative assessment of the discs above a planed spinal fusion.
Evaluation is based on the following criteria:
1) Internal architecture outlined by the flow of contrast
2) Pain reproduction during the injection
3) Flow characteristics of the injected dye.
As reported by Erlacher a study of 200 discograms performed on cadavers, the distribution of injected of methylene blue and water soluble contrast medium completely correlated by direct visualization and radiographic interpretation. In a more recent study by Adams et. al., the injection of 139 cadaveric lumbar spines demonstrated a direct correlation between the pattern of injected dye as seen on discogram and the stage of disc degeneration. It was shown that the injected contrast media forms a system of pools within the nucleus by pushing aside existing matrix. As the disc degenerates, the nucleus coalesces into fibrous lumps. Radial fissuring begins to develop, first in the inner annulus, and eventually extending to the outer layers. The discograms are rated by the stage of degeneration, as described by Adams. The contribution of the disc to the patients symptomatology is assessed by the amount and concordancy of pain reproduction during injection.
Patients should experience little discomfort during injection, aside from a feeling of fullness and possible mild abdominal or back ache at end injection. The endpoint is firm at 1-3 ccs and the dye is does not penetrate the annular fibers. The appearance in order of increasing degeneration is as follows:
The fluid is distributed in a uniformly opaque distribution in the center of the nucleus although the shape does not necessarily need to be round nor centrally located.
The typical shape appears to be contained within the nucleus in a hamburger or horseshoe shape as the nucleus is starting to coalesce into fibrous lumps.
The shape has an irregular shape and begins to penetrate the inner annulus.
The abnormal appearing discogram represents the continuum of changes resulting from ongoing internal disruption of the disk.
The contrast reaches to the outer edge of the annulus but is contained by the outermost annular fibers. The dye reaches the outer annulus via raial fissures which are typically located at the posterior or posterolateral disc border. Standing post injection radiographs in the flexion and extension views may accentuate the posterior disc bulging. If abnormal annular sensitivity is present ("internal disc disruption syndrome" described by Crock), severe back pain will be reproduced at less than 1/2 cc of injected dye. More commonly, the patient's exact dermatologic referral pattern will be reproduced at end injection as the injected pressure causes the disc to bulge outward onto the adjacent aura.
The contrast extends to the outer edge of the annulus and leaks though a complete radial fissure.
a) Nuclear Protrusion/Herniation: The contrast medium extends from the confines of the annulus fibrosis more than 3 mm beyond the margin of the vertebral body. In the case of central herniations, the dye may localize beneath the posterior longitudinal ligament. Occasionally a radiolucent defect, seen best in the oblique projection, appears within the pool of contrast media and may represent an extruded disc fragment. The symptoms reproduced are usually intense and often cause nerve root referral patterns.
b) Advanced Degenerative Disc Disease: The disk typically has a flattened fissured appearance. Contrast media may leak profusely into the epidural space through these annular disruptions. Since the disc morphology represents the expected appearance occurring at the end stage of the degenerative process, unless exact reproduction of symptoms occurs, the disc is not considered to be symptomatic.
The patient is placed in a lateral position. In this position the discs can most easily be visualized and more important the inclination of the lower disk angles can be better visualized. A line is drawn on the skin through the center of the disk to a position approximately 10 cms lateral to the midline. After appropriate sterile prep an #18 introducer spinal needle is directed toward the disc and followed by a #22 gauge 10 cm needle use to enter the disc. The needle point should lie midpoint in the disk in both the lateral and anterior-posterior radiographs. Incorrect needle placement can result in an annular (or more rarely a subarachnoid) needle placement. Water soluble medium is injected via a 1-3 cc syringe. The patients response to injection is carefully noted paying particular attention to the concordancy with his usual symptomatology.
Increased back pain is typical for several days following discogram. Chemically induced discitis is the major complication. The latent period by vary between three to twelve weeks and responds to rest and corticosteroid treatment. Crock estimates the complication to occur in 1-2% of cases. Permanent injury to the disc secondary to discography has not been shown to occur in animal experiments, cadaveric experiments, nor in disc material removed from patients at the time of surgery. In our experience, disc space infections are rare, possibly because of the double needle technique.
Since failed back syndromes are inevitably associated with soft tissue and/or neuritic injury, it is not surprising that signs of sympathetic over or under activity are occasionally seen. The hallmark of all the sympathetic dystrophies seems to be the persistent, burning hyperesthetic pain associated with autonomic dysfunction. If suspected, a definitive diagnosis is established with a sympathetic nerve block. Pain relief after a successful lumbar sympathetic block confirms the diagnosis. A series of blocks are done two to three times a week for three weeks. If refractory, a small volume of phenol can be placed on the sympathetic chain.
Temperature monitors are applied to the skin of both extremities prior to the block. The patient is placed in a lateral position on the fluoroscopy table. At approximately five finger breadths from the midline, opposite the midpoint of the L2 or L3 vertebral body, a #22 needle through an #18 introducer needle is advanced to the anterior lateral border of the vertebrae. The positioning is confirmed by fluoroscopy and the injection of contrast media. Ten to fifteen ccs of .25% Bupivicaine (Marcaine) is injected; sympathetic block follows within 10 minutes. If phenol is to be injected, the needles are positioned and documented using CAT scan. In this case, 4 ccs only of 1% xylocaine must produce the desired sympathetic block. Four ccs of 10% phenol in water soluble contrast media is then injected.
The sorting out of difficult diagnostic problems requires an orderly approach. The tree progresses from the general to the specific, using clinical and radiological assessment to determine the path which hopefully will yield the most localizing information. A pre and post block evaluation is recorded on all patients. The activities and range of motion positions which recreate symptoms are measured via a visual analog scale. All blocks are done under fluoroscopy control to accurately localize the area blocked. Water soluble contrast is used to define the spread of local anesthetic.
The desired goals are the clarification of the following:
1) % psychological overlay;
2) % anterior vs. posterior pain generator;
3) symptomatic spinal level(s);
4) specific pain generating structure(s).
In addition, a "feel for the patient" can be achieved during the stress of the procedure which may be indicative of future surgical outcome. In most instances these goals can be achieved, but, as with any diagnostic procedure, the quality of information gained varies directly with the experience and interest of the performer. Finally, all decisions regarding the interpretation must be taken entirely within the clinical context.
ADR Munich 26th July 2002 L5/S1. Aged 75 now
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