Radicular Pain and Radiculopathy

Radicular pain is commonly confused with radiculopathy, and also with somatic referred pain. This article should be read in conjunction with that on referred pain.
== Descriptions ==
=== Radicular Pain ===
Radicular pain is a subset of neuropathic pain, and refers to pain that is evoked with stimulation of the dorsal root or dorsal root ganglion of a spinal nerve - i.e. the peripheral axons or cell bodies rather than the peripheral nerve endings as occurs with nociceptive pain. In radicular pain, the pain is felt in the peripheral innervation of the affected nerve. Therefore it is a form of referred pain, but doesn't involve the convergence mechanism. The pain is felt in the periphery because of stimulation to the nerve proximal to the peripheral distribution. The older now obsolete term for neuropathic pain was "neurogenic pain."
Various authors commonly confuse radicular pain distributions with dermatomes, and also confuse somatic referred pain with dermatomes. There is in fact no good correlation between dermatomes and radicular pain patterns (sometimes called dynatomes), or somatic referred pain and dermatomes. If anything, radicular pain patterns tend to follow myotomal distributions of innervation more than a dermatomal distribution. The concept of dermatomes is more useful in sensory radiculopathy.
It is not possible to differentiate between L4, L5, and S1 lumbar radicular pain patterns alone.Furman MB, Johnson SC. Induced lumbosacral radicular symptom referral patterns: a descriptive study. Spine J. 2019 Jan;19(1):163-170. doi: 10.1016/j.spinee.2018.05.029. Epub 2018 May 22. PMID: 29800710. A segmental diagnosis can only be made with some accuracy in combination with radiculopathy. In the cervical spine there are some differences between nerve dynatomes found in a derivation study.Slipman CW, Plastaras CT, Palmitier RA, Huston CW, Sterenfeld EB. Symptom provocation of fluoroscopically guided cervical nerve root stimulation. Are dynatomal maps identical to dermatomal maps? Spine (Phila Pa 1976). 1998 Oct 15;23(20):2235-42. doi: 10.1097/00007632-199810150-00019. PMID: 9802168. This is especially true of non-adjacent dynatomes, however there is a lot of overlap and the maps hasn't been externally validated as a way of localising the segment.
=== Radiculopathy ===
Radiculopathy is commonly confused with radicular pain. Radiculopathy is distinct, it is not defined by pain, but rather refers to conduction block along a spinal nerve or its roots with some combination of weakness, numbness, and hyporeflexia. With sensory fibre block numbness develops in a dermatomal distribution. With motor fibre block weakness develops in a myotomal distribution. Reduced reflexes can occur through either sensory or motor blockade.
Some level of localisation can be made with patterns of radiculopathy, especially with motor and reflex changes. In the presence of pure sensory changes differentiation between adjacent segments can't be achieved in the cervical spineRainville J, Laxer E, Keel J, Pena E, Kim D, Milam RA, Carkner E. Exploration of sensory impairments associated with C6 and C7 radiculopathies. Spine J. 2016 Jan 1;16(1):49-54. doi: 10.1016/j.spinee.2015.07.462. Epub 2015 Aug 4. PMID: 26253986.; but may be variably possible in the lumbar spine.Nitta H, Tajima T, Sugiyama H, Moriyama A. Study on dermatomes by means of selective lumbar spinal nerve block. Spine (Phila Pa 1976). 1993 Oct 1;18(13):1782-6. doi: 10.1097/00007632-199310000-00011. PMID: 8235861.
While it is true that muscles receive innervation from more than one spinal nerve root, sufficient injury to one root usually still causes significant loss of power. There are two characteristics of the motor findings in radiculopathyMcGee, Steven R. Evidence-based physical diagnosis. Philadelphia, PA: Elsevier, 2018.
# Weakness affects two or more muscles from the same spinal segment but different peripheral nerves. For example C6 radiculopathy can affect muscles supplied by the musculocutaneous nerve and the radial nerve. (all the muscles down the C6 column in the myotome chart)
# There may be weakness of muscles supplied by the proximal nerves (i.e. dorsal scapular, suprascapular, axillary, long thoracic). Proximal nerves originate from the nerve roots but quickly innervate the shoulder muscles, and move away from the course that the peripheral nerves of the arm travel. If there is weakness of both proximal muscles and distal muscles then the lesion is near the nerve roots. For example, a C7 radiculopathy can cause both scapular winging (weak serratus anterior, long thoracic nerve) and triceps weakness. The serratus anterior weakness helps localise the lesion proximal to the radial nerve or brachial plexus.
=== Distinguishing Somatic Referred From Radicular Pain ===
The following table can serve as a guide in differentiating somatic referred from radicular pain. Note, burning is not a helpful differentiating descriptor in lumbar radicular pain.Scholz J, Mannion RJ, Hord DE, Griffin RS, Rawal B, Zheng H, Scoffings D, Phillips A, Guo J, Laing RJ, Abdi S, Decosterd I, Woolf CJ. A novel tool for the assessment of pain: validation in low back pain. PLoS Med. 2009 Apr 7;6(4):e1000047. doi: 10.1371/journal.pmed.1000047. Epub 2009 Apr 7. PMID: 19360087; PMCID: PMC2661253.
It is important to note that patients may have a combined state, experiencing both somatic referred and radicular pain. For example a prolapsed disc may irritate the dura of the nerve root (somatic) as well as the nerve root itself (radicular). 
== Mechanisms ==
In animal studies, squeezing a lumbar dorsal root only elicits a momentary burst of activity in Aδ and C fibres. However, squeezing a lumbar dorsal root ganglion elicits a sustained but temporary response in all fibres i.e. Aβ, Aδ, and C fibres. For sustained activity to occur in squeezing a dorsal root, the root must be previously injured and inflamed. In the presence of an inflamed nerve root there is heterotopic activity in all fibres: Aβ, Aδ, and C fibres, like with squeezing a non-inflamed DRG.Howe JF, Loeser JD, Calvin WH. Mechanosensitivity of dorsal root ganglia and chronically injured axons: a physiological basis for the radicular pain of nerve root compression. Pain. 1977 Feb;3(1):25-41. doi: 10.1016/0304-3959(77)90033-1. PMID: 195255. 
Human studies have found similar results. Squeezing or pulling normal nerve roots doesn't cause radicular pain, pain only occurs if they are inflamed. The DRG compression findings in animals haven't been attempted in humans. The human studies have found further qualitative features of the pain that has contributed to our understanding such as it being lancinating and traveling in a band no more than 2-3 inches wide.Norlen G. On the value of the neurological symptoms in sciatica for the localization of a lumbar disc herniation; a contribution to the problem of the surgical treatment of sciatica. Stockholm 1944.McCulloch JA, Waddell G. Variation of the lumbosacral myotomes with bony segmental anomalies. J Bone Joint Surg 1980; 62B:475-480. DOI. Full Text.SMYTH MJ, WRIGHT V. Sciatica and the intervertebral disc; an experimental study. J Bone Joint Surg Am. 1958 Dec;40-A(6):1401-18. PMID: 13610969.
The Aβ involvement in inflamed nerve roots means that radicular pain is not exactly pure "nociception," and this is probably where the qualitative in the experienced pain differences lie compared to simple nociceptive pain.
The inflammatory soup contains a variety of cells, neuropeptides, and cytokines. Macrophages, lymphocytes, and fibroblasts attend the scene. There is the production of nitric oxide, prostaglandin A2 and E2, TNFα, interleukins 8 and 12, leukotriene B4, thromboxane, interferon γ, metalloproteinases, and immunoglobulins. This leads to the generation of action potentials and pain propagation.
There are no equivalent data for cervical radicular pain. Apply a sufficiently strong stimulus to a normal dorsal nerve root leads to peripheral radiation of pain. While gentle stimulation of dorsal roots that have already been compressed causes pain and paraesthesia. Herniated cervical discs produce nitric oxide, metalloproteinases, interleukin-6, and prostaglandin E2.
One study showed that applying a sufficiently strong stimulus to a normal dorsal nerve root is always followed by a peripheral radiation of pain; but gentle stimulation of dorsal roots previously affected by compressive lesions, evoked a sensation of pain or paraesthesia.FRYKHOLM R. Lower cervical vertebrae and intervertebral discs; surgical anatomy and pathology. Acta Chir Scand. 1951;101(5):345-59. PMID: 14868335. Herniated cervical intervertebral discs have been shown to produce nitric oxide, metalloproteinases, interleukin-6 and prostaglandin E2Kang JD, Georgescu HI, McIntyre-Larkin L, Stefanovic-Racic M, Evans CH. Herniated cervical intervertebral discs spontaneously produce matrix metalloproteinases, nitric oxide, interleukin-6, and prostaglandin E2. Spine (Phila Pa 1976). 1995 Nov 15;20(22):2373-8. doi: 10.1097/00007632-199511001-00001. PMID: 8578386.
== Aetiology ==
Studies often confuse the definitions of radicular pain and radiculopathy and so this clouds the picture as to any potentially differing causes.
Disc herniation is the cause in 98% of cases. The remaining 2% is made up of a large number of other causes which include the various causes of foraminal stenosis, epidural disorders, meningeal disorders, and neurological disorders. See Lumbar Radicular Pain
== Treatment ==
Treatment for lumbar radicular pain has included intramuscular steroids, caudal epidural steroids, interlaminar epidural steroids, transforaminal steroids, conservative therapy, and microdiscectomy. (See Lumbar Radicular Pain)
Treatment for cervical radicular pain has included conservative therapy, transforaminal steroids, and surgery. (See Cervical Radicular Pain)
==Key Reading==
==See Also==
* Referred Pain
* Lumbar Radicular Pain
* Cervical Radicular Pain
== References ==
Category:Pain Types
Category:Radiculopathies