Lhermitte’s sign (also known as Lhermitte’s phenomenon and the barber chair phenomenon) describes a transient sensation of an electric shock extending down the spine and/or extremities upon flexion of the neck, often a sequela of neurologic disease. It was first described by Marie and Chatelin in 1917, was erroneously credited to Babinski and Dubois, and eventually correclty credited to Jean Jaque Lhermitte through the seminal paper Les douleurs à type de décharge électrique consécutives à la flexion céphalique dans la sclérose en plaques: Un cas de forme sensitive de la sclérose multiple (1924) by Lhermitte et al. and Gutre. Lhermitte described it in multiple sclerosis and spinal cord diseases, he further hypothesized it was a result of irritation and inflammation of the cord, likely in the posterior and lateral columns.
Lhermitte's sign is classsified as one of the paroxysmal pain syndromes of multiple sclerosis. Multiple sclerosis is a chronic, predominantly immune-mediated disease of the central nervous system, and one of the most common causes of neurological disability in young adults globally. The new Mc Donald's criteria 2017 there is a clinical and radiographical dissemination of time and space of symptoms, with presence of at least one lesion in at least two out of four CNS areas: Periventricular, cortical or juxtacortical, infratentorial and spinal cord. Additional radiographical and labororatory criteria include: new T2 and/or Gd-enhancing lesion on follow-up MRI, with reference to a baseline scan, irrespective of the timing of the baseline MRI, simultaneous presence of asymptomatic Gd-enhancing and nonenhancing lesions at any time, patients fulfilling disemmination of time and space, presence of OB in CSF. Common initial clinical symptoms include: mononuclear painful visual loss, spinal cord hemiparesis, mono/paraparesis, hypoesthesia, dysesthesia, paraesthesia, urinary and/or sphincter dysfunction, diplopia, oscillopsy, vertigo, gait ataxia, dysmetria, intentional/postural tremor, facial paresis, faciobrachial–crural hemiparesis, faciobrachial–crural hemihypoesthesia, affecting multiple organ systems of the patient. 
Lhermitte's sign or symptom should not to be confused with Uhthoff phenomenon, another often-noted objective finding in multiple sclerosis patient, which is characterized by heat sensitivity after prolonged heat exposure, saunas and hot tubs. Although sometimes bothersome or even frightening, to patients, these events do not prove to be a true advancement of disease conditions (such as in multiple sclerosis) or lead to further injury to the central nervous system (CNS). 
Lhermitte's sign it is not a disease, as a paroxysmal multiple sclerosis-induced neuropathic pain syndrome, it is thought to develop as a direct or indirect demyelinating lesions in the brain and spinal cord. Specifically, it activates ascending spinothalamic tracts at the cervical level that have been sensitized by demyelination. This view is supported by an MRI study of plaque formations in the cervical spine, which were present in 95% of those with a history of Lhermitte’s sign compared with 52% of those who did not report the sign. 
Other etiologies besides MS, includes tumor progression causing spinal cord compression, radiculopathy, cervical spondylitis, transverse myelitis, subcute combined degeneration of the cord, radiation myelopathy, chemoradiation, among others. It seems to be preferentially present in patient with cervical demyelinating lesions and abnormal nerve conduction studies. It is not a sensitive or specific sign for any of the disorders mentioned. Neck flexion irritates demyelinated tracts in the posterior column, causing the electric sensation experience by patients.  
Epidemiological studies of the incidence and prevalence of Lhermitte's sign (LS) are scant. One prospective study showed that the incidence of LS, by self report, was about 16% in almost 700 patient.  An old study reported that that LS was experienced by 33.3% out of 114 patients of MS; and in 16%, it was reported to have been occurred in the first episode of MS.  One study compared the prevalence in patient with MS and Neuromyelitis Optica (NMO). They found that the prevalence of LS among MS patients (4.3%) was significantly lower than NMO patients (20.5%) (P < 0.0001). 5.9% of the MS and 12.5% of the NMO patients had a positive family history of Lhermitte’s sign. It was observed that a higher proportion of patients with NMO rather than MS experienced the sign (20.5% vs. 4.3%). . The overall prevalence of LS range from 9-41 % . Although this symptom is typically self-limiting with spontaneous resolution after some weeks, its frequency and intensity may be troublesome in some patients.  The incidence and prevalence of the sign on rarer causes, including Vitamin B12, Behcet's disease, SSRI discontinuation syndrome, among others has not been studied in the population. There are no available statistics regarding the incidence or prevalence of Lhermitte's sign in today's global population. 
Lhermitte's sign pathophysiology is related to demyelination of dorsal columns of the cervical spine, associated with radiographical demyelinating lesion and electrodiagnostic abnormalities on nerve conduction. It is also associated with compressive myelopathy with reported affect on the dorsal columns of the caudal medulla. As mentioned before it is thought to transiently activate neuropathic pain pathways. Most recent theory is that it implicates glutamatergic signalling and microglial cell activation in the CNS. In the case of Lhermitte's sign, it is thought to result from ectopic firing and hyperexcitability of demyelinated sensory neurones (in cervical regions of spinal cord), involving ascending spinothalamic nociceptive signal transduction and impaired function of inhibitory GABAergic interneurons. Other proposed molecular mechanisms include downstream activated microglia that enhance pro-inflammatory cytokine signalling, activation of proteins like bradykinin with B1 and B2 receptors, upregulation of Wnt signalling, CREB phosphorylation and other transcription factors in the CNS that augment hyperexcitability and pain.  
Neck movements, tiredness, stress, and heat can trigger Lhermitte’s sign. Patients often describe Lhermitte's sign as an electric shock of pain that runs from the head down to the back, and through the arms and legs. It often happens when they bend their head down and touch their chin to their chest or when an examiner elicits it.
Although there are no routine laboratory, radiological, or other tests to assess or manage Lhermitte's sign, there has been some prospective studies linking its presence to radiographic and electrodiagnostic findings. One study showed a correlation between the clinical sign and the presence of a demyelinating lesion on cervical MRI and conduction delay of median and tibial somatosensory evoked potentials (SSEP), which was clinically significant.  This suggests that patients that have this physical exam finding, are more likely to have a cervical spine lesion and abnormal upper and lower extremity nerve conduction due to demyelination.
There is no randomized or double blinded studies level I evidence based treatment for LS, usually it is benign and self-resolves, rarely patients may have severe pain and discomfort. Few reports and anectodal evidence show that carbamazepine, oxcarbazepine, gabapentin, may be beneficial in some patients.  Inhibition of pro-inflammatory cytokine signalling, augmentation of inhibitory cytokine signalling and blockade of chemokine receptor-mediated inflammatory cell recruitment to the CNS, have potential as future strategies for improving relief of MS-associated neuropathic pain, including Lhermitte's. 
Additionally, there are case series using extracranial picotesla range pulsed electromagnetic fields (EMFs), that effectively treated patients with Lhermitte's sign. One theory is that the reduction of axonal excitability occurs through the modulation of ionic membrane permeability. Second theory involves modulating pain control systems through neurotransmitter activity and pineal melatonin functions, as discussed earlier. 
The differential diagnosis of Lhermitte sign, besides MS, has been reported in the literature and includes: tumor progression causing spinal cord compression, radiculopathy, cervical spondylitis, transverse myelitis, subcute combined degeneration of the cord, radiation myelopathy, parasitic invasion of the cord, Arnold-Chiari Malformation, high dose chemoradiation (cisplatin), Trauma, Arachnoiditis, Herpes Zoster toxicity, Syringomyelia, Behcet's disease, vitamin B12 deficiency, nitric oxide toxicity, systemic lupus erythematous, and post-dural puncture headache.
Cisplatin or docetaxel neurotoxicity has been tied to Lhermitte's sign.
Lhermitte's sign or syndrome is not a disease process itself, and it is usually intermittent in nature, only elicited under neck flexion. The prognosis of the disease process underlying the Lhermitte's sign is variable as abovementioned.
There are no known complications related to Lhermitte's sign, as it is not a disease itself. However, it is also occasionally reported as a facet of a discontinuation syndrome related to certain medications. Psychotropic medications such as SSRIs and SNRIs, specifically paroxetine and venlafaxine have been shown to have an association. When being on these medications for some length of time, and then suddenly halting to drastically reducing dosages, some patients enforce experiencing symptoms similar to Lhermitte's sign. Regarding SSRI withdrawal symptoms, fluoxetine, given the extended length of its half-life, can be given at a single small dosage, and as a result, avoid Lhermitte's sign and other similar symptoms. On the other hand, paroxetine withdrawal has been reported in the literature as causing reversible Lhermitte's sign, this is thought to be related to the drug's short half-life. 
In dentistry, there have been studies which found Lhermitte's sign associated with nitrous oxide abusers (believed to be tied back to depleting vitamin B12). It is also occasionally reported as a facet of a discontinuation syndrome related to certain medications. Psychotropic medications such as SSRIs and SNRIs, specifically paroxetine and venlafaxine have been shown to have an association. When being on these medications for some length of time, and then suddenly halting to drastically reducing dosages, some patients enforce experiencing symptoms similar to Lhermitte's sign. In dentistry, there have been studies which found Lhermitte's sign associated with nitrous oxide abusers (believed to be tied back to depleting vitamin B12).
In the case of multiple sclerosis, an experienced neurologist should be consulted if demyelinating disorders are suspected
No active intervention is required by healthcare providers beyond an explanation and reassurance; the syndrome usually resolves spontaneously over a period of months to a year, under rare cases it can be treated with neuropathic pain medication if refractory and recurrent.
Multiple sclerosis is a chronic disease which can sometimes follow an unpredictable trajectory. It is best to enhance patient outcomes by integrating a team-based approach, including nurses, physician assistants, and physicians should be used in managing patients who live with multiple sclerosis. Consider speaking to the patient about implementing new physicians and services to their overall care, such a palliative medicine, as such resources can offer a new range of support to the family and the patient. [Level V]
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