Minimally Invasive Approaches Diagnostic Procedures for Low Back Pain

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The precision diagnostic approach was developed to determine in conjunction with other diagnostic tools the cause or the causes of pain in low back pain. By stimulating or anesthetising specific structures, needle procedures can determine precisely the source of the patient's pain (Steindler, 1938). These procedures can target the source of pain and unlike imaging studies determine whether the structure is generating pain or not. This approach is subject to control in order to ensure the validity of the test in each and every patient. The procedure requires fluoroscopy and special skills such as the ability to deliver a needle accurately and safely to the targeted structure.

Epidemiologically, three causes of back pain are predominant with or without surgery. Discogenic pain, Facet joint pain and SI joint pain (Man-chikanti et al., 1999 a). For these three aetiologies, three test procedures are available in the investigation of chronic low back and FBBS pain: Discog-raphy, Medial Branch blocks and Sacro-iliac (SI) joint blocks.

Many FBSS patients present with a mixed clinical picture and multiple tests may be needed to determine the ''pain generator''.

Provocative Discography

Discography involves the injection of radiographic contrast into the nucleus of an intervertebral disc. This invasive procedure is justified only if it provides new information that cannot be obtained by less invasive options. Discography does not compete with CT or MRI in the diagnosis of disc herniation. It is not only an anatomical diagnostic but mainly a functional test.

After a classical evaluation of the patient, including radiological exams, even when diseased structures have been identified with MRI for instance, most of the time, we still don't know which structure causes pain. We still need a way to reproduce the pain as we try to do during the physical exam;

we still want a symptom related response. We then need to target the cause of the pain and its specific origin.

Provocation discography is achieved by distending the disc from the inside using medium contrast. Diseased discs are painful (Walsh et al., 1990). Although originally believed to be due to increased pressure on nerve roots in patients with herniations, pain occurs in patients with no evidence of herniation or disc-bulge and so must arise from the disc itself. Moreover, the reproduction of pain cannot be ascribed to a chemical effect of contrast medium or spillage of contrast medium into the epidural space, for it occurs without spillage, or if normal saline is used instead of contrast medium (Coppes, 1997).

Discography is performed under local anaesthesia; no or minimal sedation is required or desired. Heavily sedated patients may give partial to inadequate answers to the test. The patient, under sterile conditions lies prone. A posterolateral approach is used to enter the disc at the desired level. A well trained operator is necessary to perform a discography; a painful procedure due to inexperience will preclude a good and valid evaluation. A 22 G to 25 G needle 13 to 17 cm is used to enter the disc. Under the C-arm, lateral and a-p views are used to check the exact place of the tip of the needle. The contrast medium is injected into the disc and intensity and quality of pain are recorded as well as the pressure needed to induce pain (McNally et al., 1996). Discography findings are classified in two groups: symptomatic and radiological findings.

Symptomatic findings: An intact disc without any degenerative abnormalities will support pressures as high as 100 pounds per square inch; the injection is not very uncomfortable. In pathological conditions, the pressure needed to induce pain may vary a lot between subjects but should be below 50 PSI. Pain is recorded on a visual analog scale ranging from 0 being no pain to 10 rating unbearable pain. The patient should be blinded to the level of injection, not knowing if the control disc or the suspected disc is injected first. Evaluation must include quality of the pain, it should be similar to the usual patient's complain.

Radiological findings: The procedure must be completed by a post-discography CT-scan. This exam determines the grade of fissure of a disc. A non injected image does not give this information. CT-scan evaluation of a discogram is looking at the repartition and shape of the injected dye. It must be planned immediately after the discography (Bernard et al., 1990).

The disc can be either intact or ruptured or may present with internal disc disruption (IDD). IDD presents in four different stages. Grade I, II, III extend to the inner, middle and outer third of the annulus fibrosus, grade IV also extends circumferentially around the annulus assuming the shape of a ship's anchor (Aprill et al., 1992). These fissures have no relation with degeneration, are not age related. A strong correlation as been dem onstrated between painful discography and IDD (Moneta et al., 1994). The reason why IDD is painful is not clearly established. Probably, in grade II, III and IV, the degradated matrix of the nucleus may chemically irritate the nerve endings of the outer third of the annulus (Heggeness et al., 1993). The second hypothesis is increased mechanical nociception due to mis-distribution of the charges on the diseased disc more sensitive to stress. These theories need further studies to be clearly demonstrated.

Provocative discography like any other diagnostic procedure in low back pain evaluation must not stand alone. It must be interpreted in the light of all the other information about the bio-psycho-social context of the patient. Viewed as an individual exam, this test has its limitations. The important issue is to be able to draw conclusions after a negative or a positive test.

In healthy young subjects with no pre-existing chronic painful illness, the false positive rate is extremely low. Walsh et al. in 1990 reported in a study on 10 volunteers. 16.7% of them had minimal pain on injection, 6.7% moderate pain and 3.3% "bad" pain (Walsh, 1990).

Further studies on older subjects suffering from chronic pain and on patients with significant psychometric features showed, as one would expect, higher false-positive rates. Carragee and al in 2000, conducted a prospective study including 30 patients. Little pain was elicited by low pressure injection of any anatomically normal disc. However, when discs although asymptomatic had fissuring of the annulus, the injection was painful. The main predictors of pain intensity were presence of chronic pain and abnormal psychometric scores (Carragee, 2000). As compared to the Walsh study, 40% of chronic pain group and 80% of the somatization group had at least one positive disc (Carragee, 2000).

In FBSS patients, discography is often used to evaluate recurrent or persistent back pain. Heggeness et al. reported in a retrospective study 83 patients who had undergone discography. 72% of them had a positive concordant pain response on injection of the previously operated disc. This may give a clue about the importance of the discogenic pain in FBSS patients (Heggeness, 1997).

Another study examined post-discectomy patients with or without persisting pain. 40% of the asymptomatic patients had positive injections on the previously operated level as compared to 63% in the symptomatic group. Moreover, considering the psychometric data, the rate of positive injections were the same in the two groups. Operated discs are painful in symptomatic as well in asymptomatic patients. Yet it remains true that concordant pain is reproduced in symptomatic patients. A damaged disc, symptomatic or not is usually painful when injected and according to the presence of associated aggravating factor, however the pain may be more intense (Carragee, 1999).

Future studies that focus on provocative discography should include a control discography on the adjacent level as proposed by the International Spinal Injection Society (ISIS). Since using a control discography with the provocative one, the false positive rate in normal discs is low even in the chronic pain population. Furthermore adding psychometric screening may help in reducing false-positive rate. As false-negative tests do not occur, we may conclude that all pathological discs are sensitive to provocative dis-cography and with a good patient selection, reasonable diagnostic accuracy can be achieved. If the adjacent disc is used as a control, the specificity of this test will increase.

Medial Branch Blocks

Among the workers population, the prevalence of facet joint pain is 15% (Schwartzer, 1995 b). In an older group population, it increases up to 40% (Manchikanti, 1999 a). Not testing patients with back pain for zygoapo-physal joint pain precludes the diagnostics in this proportion of patients and leads to further and perhaps futile investigations. After surgery, this remains valid and although the incidence is lower for other causes overpass this one, a significant proportion of patients will benefit from investigating the Z joint.

The zygoapophyseal joint is innervated by the medial branch of the dorsal rami (Fig. 1).

Provocative saline Z-joint injection has been shown to induce pain in the back, the buttock and even down the leg in healthy volunteers. Anaesthetizing medial branches prevented the induction of pain in similar conditions (Kaplan, 1998).

Medial branch blocks are achieved under fluoroscopy guidance by specifically placing a needle onto the nerve and inject 0,5 ml of local anaesthetic (Bogduk, 1997). Each z-joint is innervated by two nerves blocked separately to anesthetize the joint. Single diagnostic blocks have a 47% false-positive rate. To achieve validity, controlled blocks must be performed with two local anaesthesia agents, a short and a long acting one (15% false positive blocks). If anaesthesia of the joint lasts longer with the second agent, the test is valid (Schwarzer, 1994).

Sacro-Iliac Joint Blocks

The sacro-iliac joint is responsible for 15% of low back pain (Schwartzer et al., 1995 c). S-I joint block is performed under fluoroscopy and a needle is introduced in the joint cavity. A contrast medium is used to insure correct placement of the needle tip (Slipman et al., 2002). A control block is mandatory to reach validity (Maigne, 1996).

Discogram Approach

Fig. 1. Schematic drawing of lumbar spine nerve supply. 1 Medial branch dorsal ramus; 2 intervertebral disc; 3 communicating ramus; 4 sympathetic trunk;

* = mamillo-accessory ligament

Fig. 1. Schematic drawing of lumbar spine nerve supply. 1 Medial branch dorsal ramus; 2 intervertebral disc; 3 communicating ramus; 4 sympathetic trunk;

* = mamillo-accessory ligament

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