Patients with chronic pain can present with two patterns: generalized/diffuse (e.g. fibromyalgia) or localized/regional (e.g. meralgia paresthetica). In the latter case, interventional techniques including nerve blocks can be considered for pain relief or employed as a diagnostic tool to assist in the evaluation of the source of pain or prognosticate the treatment strategies.

Diagnostic block

There are two main roles of a diagnostic block in pain management. The first is to locate the source of the pain. Even though the pain is restricted to a region such as the lower back, the pain generators can be from the facet joints, disc, ligaments, osseous or myofascial components. Using facet joint related pain as an example, the literature suggests that it cannot be reliably diagnosed with clinical assessment or radiologic imaging [1]. By interrupting the nociceptive input from the facet joint, the clinician can determine whether the facet joints are the source of the pain. Another example is the situation when a patient has multiple disc herniations and the clinical assessment is inconclusive of the symptomatic level. A selective nerve root block provides valuable information for determining the symptomatic level [2]. Another role of the diagnostic block is to prognosticate regarding treatment response. A positive diagnostic block increases the likelihood of a positive outcome from a more definitive procedure as in facet joint disease [3].

There are a few basic requirements in considering a diagnostic block. The suspected pain generator should be confined to a region allowing an interventional procedure to interrupt the pain signals. The block can be in form of a nerve block, local infiltration or joint injection. The local anesthetic used for the interruption of the pain signal should be precisely administered to the target. The patient should be cognitively intact to interpret the pain response and document the pain scores over a period of time in a diary.

Based on the pain response following the diagnostic block, the clinician will evaluate the source of pain and plan the management accordingly. When assessing a diagnostic test, a 2×2 table is typically used based on the test response and the presence of disease [4] and from there, the sensitivity, specificity and positive/negative predictive value can be estimated (Figure 22.1). In interpreting the response to a diagnostic test, a similar 2×2 table can help the clinician to understand the complexity of the evaluation (Figure 22.1).

Typically, the clinician dichotomizes response as a positive or negative responder (response A and B in Figure 22.2. A positive response (A) confirms the source of pain is the target of the block. A positive response can, however, be influenced by higher center as a result of expectation or non‐specific treatment effect [5]. Thus, the concept of controlled block is proposed (1 block with local anesthetic and another one with placebo). More commonly, comparative local anesthetic blocks can be done in which case two local anesthetic agents with different durations of action are used [6]. Some investigators may inaccurately use a positive response (A) to placebo to conclude that the complaint of pain is the result of ‘malingering’, hypervigilance or somatization of symptoms when in reality it is simply a placebo response [7]. A negative response (B) generally suggests that the source of the pain is not from the structure targeted by the block. An alternative interpretation is that the target structure is simply a non‐responder to the local anesthetic block discouraging further treatment. This can be explained by the influence of higher centers (depression, anxiety or pain catastrophizing) or the effect of central sensitization [8]. When interpreting the response to the diagnostic block, clinicians make an assumption that the diagnostic block accurately interrupts the pain pathway. This is not always the case (response C and D). In some cases even with the use of image‐guidance one may not be able to block the pain generator and a misguided block can lead to a false negative result [9]. To complicate the interpretation further, a misguided diagnostic block can lead to a positive response from the expectant or non‐specific treatment effect or due to the spilling local anesthetic outside the target [10]. Overall, the clinician who performs the diagnostic block should be aware of these limitations.

Therapeutic intervention/block

The term therapeutic block typically refers to the insertion of a needle into the target site or nerves followed by the administration of local anesthetic with or without steroid. However, over the last few decades, there are additional percutaneous interventions available (Table 22.1).

In terms of nerve management, the nerve entrapment can be managed by hydrodissection [11] or minimally invasive ultrasound‐guided decompression [12]. Different types of neural ablation (chemical, thermal and cryotherapy) techniques [13] as well as different types of neuromodulation (pulsed radiofrequency treatment, peripheral nerve and spinal cord stimulation) have been described.

Injection around the tendon aims at disease around the tendon but it is of minimal value to the intrinsic disease of the tendon such as tendinopathy or calcific tendinitis. Fenestration is a potential treatment for tendinosis or tendinopathy with or without administration of biologic agents (autologous blood or platelet rich plasma) that stimulates the growth [14]. In patients with calcific tendinitis, the calcium crystals can be managed with fenestration and barbotage [15].

In general, interventional procedures can be broadly classified as targeting three anatomic categories: peripheral nerve, neuraxial and musculoskeletal structures. Two examples in each category will be used to exemplify the application of interventional procedure in pain management.

Nerve management	MSK management	Types of injectate
Perineural injection	Tendon	Local anesthetics
Nerve release/ hydro‐dissection Chemical ablation (alcohol, phenol)	Fenestration Barbotage	Adjuvants (e.g. steroids) Hyaluronic acid
Thermal (radiofrequency/RF) ablation	Cyst	Botulinum toxin
Cryoablation	aspiration	Dextrose
Nerve decompression	cyst neck fenestration	Platelet rich plasma
Neuromodulation Pulsed RF Peripheral nerve stimulation Spinal cord stimulation		Biologic agents

Peripheral nerve intervention

Pathologic states (injury or entrapment) of the peripheral nerves can cause persistent pain. These nerves can be the targets for interventional pain management. The prerequisites for performing procedures on these nerves are the ability to precisely identify their location and understand their functions (motor and sensory). Smaller nerve intervention has attracted significant interest in recent years. When the area of pain can be linked to an individual and identifiable nerve, one can perform a diagnostic injection to confirm the hypothesis. If that injection leads to an appropriate response, a definitive treatment can be offered.

Interventional treatment for intercostal neuralgia has been well investigated. Persistent pain after thoracotomy is common, affecting about 67% of thoracotomy patients [16]. Thoracic postherpetic neuralgia affects about 47% of herpes zoster patients older than 60 [17]. Intercostal neuralgia may also develop after rib fractures.

Typically, a diagnostic injection is performed under ultrasound guidance with 1‐2ml of local anesthetic administered to the intercostal nerves in the painful region (Figure 22.2). If this leads to an appropriate response (sensory block in the area of nerve distribution with analgesia duration concordant with the duration of local anesthetic), definitive treatment can follow. Cryoablation is the most commonly studied ablation modality for intercostal neuralgia (Figure 22.2), resulting in 6‐9 months of pain relief in most cases [18–24]. It causes Wallerian degeneration of the axon. As the connective tissue around the nerve such as epineurium remains intact, the nerve can regenerate in the same pathway. The procedure can be repeated if the pain recurs. Another less commonly used type of ablation is radiofrequency ablation [25, 26].

A classic example of small nerve intervention is the treatment for meralgia paresthetica, of which the reported incidence is 4.3 cases per 10,000 patient years in the general population [27]. This condition refers to the entrapment of the lateral femoral cutaneous nerve (LFCN) under the inguinal ligament, causing numbness and painful burning sensation on the lateral aspect of the thigh. The LFCN is a pure sensory nerve, readily identifiable by ultrasound in the fat‐filled grove between the sartorius and tensor fascia‐lata. Easy access and the lack of a motor component make it an excellent target for both diagnostic and therapeutic blocks. The typical appearance of the entrapped LFCN is swollen and enlarged (Figure 22.3

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