Introduction Fibromyalgia (FM) is defined as a “syndrome of central sensitisation with dysfunction of the neuronal circuits involved in the perception, transmission and processing of nociceptive afferents, with pain predominantly expressed in the musculoskeletal system”1. It is an incurable syndrome of unknown origin with signs and symptoms often similar and overlapping with those of other syndromes. This condition, unfortunately with high frequency, delays its diagnosis. The pathogenetic mechanism underlying the clinical picture is alteration of the nociceptive system.Several hypotheses have been proposed concerning the pathogenesis of FM and the management of FM patients requires a multidisciplinary approach.Accumulating evidence suggests that hyperbaric oxygen therapy (HBOT) is a non-invasive modality with lasting efficacy to treat FM2. HBOT is defined by the Undersea and Hyperbaric Medical Society (UHMS) as a treatment in which a patient intermittently breathes 100% oxygen while the treatment chamber is pressurised to above sea level pressure (1 atmosphere absolute, 1 ATA = 760 mmHg)3. HBOT is able to induce many interesting effects on plasma oxygen concentration. Based on Henry’s Law, increased pressure will cause more gas to go into solution, and therefore,more oxygen will be transported in the plasma. As a result a lot of oxygen becomes available for the microcirculation, resulting in significant improvement of all metabolic parameters, which have also been shown in several works to influence neurological functions4. We report about a case of woman affected by FM and treated with HBOT as adjuvant, experimental and non conventional therapy.Case PresentationIn January 2021 a 54-year-old Caucasian woman with a negative medical past history reported pain in her left arm 24 hours after receiving the first dose of the Pfizer SARS-Covid 19 vaccine. Localised pain in the injection zone (the triceps muscle of the left arm) was accompanied by the onset of high fever (40°C), intense headache with vomiting and abdominal pain. After 48 hours there was defervescence with return to normo-thermia but progressive appearance of fatigue. Subsequently patient report a relief of pain in the left arm with progressive development of constant, severe and persistent pain in occipital and back neck area, low back and legs with a marked sense of heaviness in the lower limbs. The patient also complained of progressive difficulty in walking, for which the use of nordic walking sticks was necessary. Furthermore, she reported stiffening of the facial muscles with pain defined as intense, mental fogginess, severe short-term memory involvement and progressive depression, symptoms that had undoubtedly caused a significant impairment in her quality of life.The patient underwent routine blood sample tests (blood count, ESR, PCR, protein electrophoresis, AST, ALT, gamma GT), as well as more specific immuno-enzymatic tests (serum kappa and lambda chains, IgG, IgA, IgM, anti-nuclear antibodies, ENA, ANA with subclasses). The exams showed no values outside the standard range. Only a slight increase in ESR and a reduction in 25-OH-vitamin D levels was shown. The patient also performed total body CTs, spine MRI, femoral and lumbar bone densitometry from which no structural morphological alteration was highlighted except for an initial picture of osteoporosis. The exclusion of inflammatory disease, although some rheumatic diseases could coexist, suggested a possible diagnosis of FM and thus rheumatologists have sought its diagnostic criteria5. The persistence of pain was well over 3 months (the patient reported the onset of symptoms about 18 months ago).The following questionnaires were administered: Widespread Pain Index (WPI), Symptom Severity Scale (SSS), Revised Fibromyalgia Impact Questionnaire (FIQR), Pittsburgh Sleep Quality Index (PHQI), Generalized Anxiety Disorder Screener (GAD-7), Functional Assessment of Chronic Illness Therapy Fatigue (FACIT-F). The scores reported from each individual questionnaires carried out before the start of HBOT were as follows: WPI=18, SSS=10, FIQR=96, PHQI=17, GAD-7=14, FACIT=12.Thus she started a multidisciplinary therapeutic course based on antalgic therapy, physiotherapy, psychological support, relaxation techniques and healthy nutrition.Among the various therapies, HBOT was indicated as an experimental non-conventional treatment. The patient underwent forty sessions of HBOT at 2.4 ATA (absolute atmospheres), total oxygen time 60 minutes per session, once a day, five times a week, performed at the multi-place chamber (Sistemi Iperbarici Integrati-Camera Iperbarica Mod 2000) of the Hyperbaric Medicine Centre of ARNAS Ospedale Civico Di Cristina Benfratelli, Palermo, Italy. Therapy started in the first week of September and ended in mid-November 2022. Throughout each session, the patient showed stable vital parameters (blood pressure, heart rate, oxygen saturation and body temperature). Blood tests, markers of flogosis and haematochemical examinations carried out at mid term and at the end showed no change in values outside the normal range.The patient underwent 40 sessions of HBOT. The clinical improvement achieved was evident and affected all symptom areas reported before treatment. In particular, there was a complete recovery of mobility with the avoidance of walking sticks, an increase in muscular strength evidenced by the ability to climb several flights of stairs and to walk long distances independently. The patient reported a drastic reduction in pain symptoms evident from the moment she woke up in the morning, a significant improvement in sleep quality, previously reported as light, non-restorative and with multiple breaks. The highly debilitating sense of fatigue was reported after treatment as markedly reduced and in any case easily manageable. In addition, a significant change in cognitive abilities was reported, with disappearance of mental fogginess and recovery of short-term memory. In summary, there was a significant improvement in quality of life with the disappearance of the depression into which the patient had plunged, a condition confirmed by psychological advice at the end of the hyperbaric treatment. The scores reported by each individual test at the end of HBOT are shown below: WPI=6, SSS=3, FIQR=13, PHQI=2, GAD-7=3, FACIT=42.