Stiff-person syndrome and related disorders - diagnosis, mechanisms and therapies.

Stiff-person syndrome (SPS) is the prototypical and most common autoimmune neuronal hyperexcitability disorder. It presents with stiffness in the limbs and axial muscles, stiff gait with uncontrolled falls, and episodic painful muscle spasms triggered by anxiety, task-specific phobias and startle responses, collectively leading to disability. Increased awareness of SPS among patients and physicians has created concerns about diagnosis, misdiagnosis and treatment. This Review addresses the evolving diagnostic challenges in SPS and overlapping glutamic acid decarboxylase (GAD) antibody spectrum disorders, highlighting the growing number of overdiagnoses and focusing on the progress made in our understanding of SPS pathophysiology, antibodies against GAD and other inhibitory synaptic antigens, and the fundamentals of neuronal hyperexcitability. It considers the role of impaired GABAergic or glycinergic inhibition in the cortex and at multiple levels in the neuraxis; the underlying autoimmunity and involvement of GAD antibodies; immunopathogenic mechanisms beyond antibodies, including environmental triggers; familial and immunogenetic susceptibility; and potential T cell cytotoxicity. Finally, the mechanistic rationale for target-specific therapeutic interventions is presented along with the available therapeutic approaches, including enhancers of GABA signalling drugs and immunotherapies.

Stiff Person Syndrome and GAD Antibody-Spectrum Disorders.

OBJECTIVE: Antibodies against glutamic acid decarboxylase (GAD), originally associated with stiff person syndrome (SPS), define the GAD antibody-spectrum disorders that also include cerebellar ataxia, autoimmune epilepsy, limbic encephalitis, progressive encephalomyelitis with rigidity and myoclonus (PERM), and eye movement disorders, all of which are characterized by autoimmune neuronal excitability. This article elaborates on the diagnostic criteria for SPS and SPS spectrum disorders, highlights disease mimics and misdiagnoses, describes the electrophysiologic mechanisms and underlying autoimmunity of stiffness and spasms, and provides a step-by-step therapeutic scheme. LATEST DEVELOPMENTS: Very-high serum GAD antibody titers are diagnostic for GAD antibody-spectrum disorders and also predict the presence of GAD antibodies in the CSF, increased intrathecal synthesis, and reduced CSF γ-aminobutyric acid (GABA) levels. Low serum GAD antibody titers or the absence of antibodies generates diagnostic challenges that require careful distinction in patients with a variety of painful spasms and stiffness, including functional neurologic disorders. Antibodies against glycine receptors, first found in patients with PERM, are seen in 13% to 15% of patients with SPS, whereas amphiphysin and gephyrin antibodies, seen in 5% of patients with SPS spectrum disorders, predict a paraneoplastic association. GAD-IgG from different SPS spectrum disorders recognizes the same dominant GAD intracellular epitope and, although the pathogenicity is unclear, is an excellent diagnostic marker. The biological basis of muscle stiffness and spasms is related to autoimmune neuronal hyperexcitability caused by impaired reciprocal γ-aminobutyric acid-mediated (GABA-ergic) inhibition, which explains the therapeutic response to GABA-enhancing agents and immunotherapies. ESSENTIAL POINTS: It is essential to distinguish SPS spectrum disorders from disease mimics to avoid both overdiagnoses and misdiagnoses, considering that SPS is treatable if managed correctly from the outset to prevent disease progression. A step-by-step, combination therapy of GABA-enhancing medications along with immunotherapies ensures prolonged clinical benefits.

Neurological Causes of Chest Pain.

PURPOSE OF REVIEW: Chest pain is a very common presenting complaint among patients in the hospital, a large proportion of whom have non-cardiac chest pain (NCCP). Neurological causes of NCCP have not been previously reviewed although several causes have been identified. RECENT FINDINGS: Chest pain has been reported as a symptom of multiple neurological conditions such as migraine, epilepsy, and multiple sclerosis, with varying clinical presentations. The affected patients are often not formally diagnosed for long periods of time due to difficulties in recognizing the symptoms as part of neurological disease processes. This paper will briefly summarize well-known etiologies of chest pain and, then, review neurological causes of NCCP, providing an overview of current literature and possible pathophysiologic mechanisms.

Use of subcutaneous immunoglobulin in stiff person syndrome: Case series.

INTRODUCTION: Intravenous immunoglobulin (IVIG) has been shown to be effective for the treatment of stiff person syndrome (SPS). However, some patients might not tolerate it. We report the tolerability profile of subcutaneous immunoglobulin (SCIg) in patients with SPS who did not tolerate IVIG. To our knowledge, the use of SCIg in SPS has not been reported before in a case series. PATIENT CONCERNS: The five patients included in this case series presented with various combinations of symptoms of spasms, axial and limb stiffness, and exaggerated responses to outside stimuli. These symptoms often lead to gait and functional impairment. DIAGNOSIS: Patients were diagnosed with classic SPS as they met the clinical criteria, which require the presence of spasms, axial rigidity, and hyperexcitability. INTERVENTIONS: Subcutaneous immunoglobulin infusion. OUTCOMES: Five patients were identified that were treated with SCIg. Three tested positive for serum anti-glutamic acid decarboxylase 65 antibodies prior to any treatment. The mean age at SCIg initiation was 33 years (range: 22-47). The mean duration of SPS prior to SCIg initiation was 5.9 years (range: 2.5-7). All patients used IVIG for at least two months (up to 18 months) but switched to SCIg due to IVIG side effects. Duration of SCIg use ranged from 4 months to 6 years (mean, 19.2 months). Upon switching to SCIg, the SPS symptoms remained stable. SCIg was well-tolerated in most as only one patient discontinued SCIg due to side effects. CONCLUSION: This case series highlights that SCIg could be a treatment option for patients with SPS, especially when IVIG is not feasible. Injection site reactions might be a limiting factor in some patients treated with SCIg. Prospective controlled studies are needed to confirm SCIg treatment durability and efficacy.

The effect of clinical pilates-based physiotherapy program for a Stiff Person Syndrome patient: a case report.

The aim of the present report is to show the effects of a clinical pilates-based physiotherapy training program on physical functioning, including balance ability, flexibility, muscle strength, and pain severity in a case with Stiff Person Syndrome (SPS). A 43-year-old female with a 3-year history of SPS participated in the study. Clinical pilates training exercises were performed two times per week during 8 weeks. Static balance was evaluated with One Leg Stance Test and Tandem Stance Test, dynamic balance with Functional Reach Test and lower extremity strength and endurance measured by 30-s Chair Stand Test. Berg Balance Scale was used to assess balance and fall risk, and Timed Up and Go Test was used to assess functional mobility. Pain evaluation was done by Pain Quality Assessment Scale. Flexibility was measured with tape measure, range of motion with goniometer and muscle strength with manual muscle test. Among the balance and functional mobility tests, progress has been observed in all tests, except for 30-s Chair Stand test. Improvements have been observed in patient's range of motion, flexibility, strength, pain and balance parameters. Clinical pilates-based physiotherapy program had positive effects on many physical and functional parameters of the patient and can be used as a safe exercise method in physiotherapy rehabilitation of SPS.

Defining the Expanding Clinical Spectrum of Pediatric-Onset Stiff Person Syndrome.

BACKGROUND: We aimed to characterize the spectrum of clinical features and examination findings in pediatric-onset stiff person syndrome. METHODS: Medical records were reviewed for all patients treated for stiff person syndrome with symptom onset in childhood at a tertiary medical center between March 2001 and February 2019. RESULTS: Of the 15 individuals who met inclusion criteria, 11 (73%) were female and 13 (87%) were Caucasian. Median age at symptom onset was 14.8 years (range 8.4 to 16.9), and median latency from symptom onset to diagnosis was 6.2 years (range 0.4 to 15.0). Nine individuals (60%) were not diagnosed until adulthood. The most common presenting features were painful spasms (n = 12, 80%), hyper-reflexia (n = 11, 73%), axial rigidity (n = =9, 60%), lower extremity rigidity or spasticity (n = 8, 53%), gait abnormalities (n = 6, 40%), and hyperlordosis (n = 6, 40%). Other noted features included anxiety (n = 5, 33%), dysautonomia (n = 3, 20%), and cranial neuropathies (n = 3, 20%). Personal (n = 9, 60%) and family history (n = 9, 60%) of autoimmune conditions was common. Serum antiglutamate decarboxylase 65 antibodies were found in 13 individuals (87%). Nearly all individuals received immunotherapy (n = 14, 93%), symptomatic medications (n = 15, 100%), and nonpharmacologic therapies (n = 14, 93%). However, most had persistent physical limitations, particularly impaired walking (n = 7, 47%) and inability to carry out previous activities (n = 14, 93%). CONCLUSIONS: There is a wide spectrum of typical and less common features seen in individuals with pediatric-onset stiff person syndrome. Despite symptom onset in childhood, diagnosis is often delayed until adulthood, at which point disability accrual is frequently seen. Early recognition is vital to address symptoms and may potentially limit future disability.

Autologous Hematopoietic Stem Cell Transplantation for Stiff-Person Spectrum Disorder: A Clinical Trial.

OBJECTIVE: To test the hypothesis that autologous nonmyeloablative hematopoietic stem cell transplantation (HSCT) is safe and shows efficacy in the treatment of stiff-person spectrum disorder (SPSD). METHODS: Twenty-three participants were treated in a prospective open-label cohort study of safety and efficacy. After stem cell mobilization with cyclophosphamide (2 g/m2) and filgrastim (5-10 µg/kg/d), participants were treated with cyclophosphamide (200 mg/kg) divided as 50 mg/kg IV on day -5 to day -2; rabbit anti-thymocyte globulin (thymoglobulin) given intravenously at 0.5 mg/kg on day -5, 1 mg/kg on days -4 and -3, and 1.5 mg/kg on days -2, and -1 (total dose 5.5 mg/kg); and rituximab 500 mg IV on days -6 and +1. Unselected peripheral blood stem cells were infused on day 0. Safety was assessed by survival and National Cancer Institute common toxicity criteria for adverse events during HSCT. Outcome was assessed by ≥50% decrease or discontinuation of antispasmodic drugs and by quality of life instruments. RESULTS: There was no treatment-related mortality. One participant died 1 year after transplantation of disease progression. Of the 74% of participants who responded, 47% have stayed in remission for a mean of 3.5 years; 26% did not respond. Compared to nonresponders, responders were more likely to have pretransplantation intermittent muscle spasms (16 of 17 vs 0 of 6), normal reflexes (12 of 17 vs 0 of 6), and positive CSF anti-glutamic acid decarboxylase serology (12 of 14 vs 2 of 6). Compared to responders, nonresponders were more likely to have lead pipe rigidity (4 of 6 vs 0 of 17) and EMG-documented simultaneous contraction of agonist/antagonist limb muscles (4 of 6 vs 1 of 17). Pre-HSCT use of prescription serotonin selective receptor inhibitor (SSRI) or serotonin and norepinephrine reuptake inhibitor (SNRI) was more common in those who relapsed or never responded (9 of 12) compared to those responders who never relapsed (0 of 11). CONCLUSION: In this cohort, HSCT was safe, but the beneficial effect of HSCT was variable and confined predominately to participants with episodic spasms and normal tendon reflexes without simultaneous cocontraction of limb agonist/antagonist muscles who were not taking SSRI or SNRI antidepressants. CLASSIFICATION OF EVIDENCE: This study provides Class IV evidence that, for a subset of people with SPSD, autologous nonmyeloablative HSCT improves outcomes. CLINICALTRIALSGOV IDENTIFIER: NCT02282514.

The case of a 30-year-old man with subacute gait instability, weakness, and muscle spasms.

A 30-year-old healthy man presents after a fall with diffuse weakness and pain and recent onset left eye ptosis in the setting of three weeks of progressive bilateral leg weakness, gait instability, and difficulty washing his hair due to upper extremity weakness. He had also developed stiffness in his neck and shoulders and uncontrollable muscle spasms. Exam was notable for ptosis, fatiguing weakness in extremities, increased tone, hyperreflexia with clonus, and spastic gait. A mediastinal mass was found on chest CT (Fig. 1), and biopsy confirmed an invasive thymoma with positive nodes. Serum testing was positive for antibodies to acetylcholine receptors as well as glutamic acid decarboxylase.

Neurologic syndromes related to anti-GAD65: Clinical and serologic response to treatment.

OBJECTIVE: Antibodies against glutamic acid decarboxylase 65 (anti-GAD65) are associated with a number of neurologic syndromes. However, their pathogenic role is controversial. Our objective was to describe clinical and paraclinical characteristics of anti-GAD65 patients and analyze their response to immunotherapy. METHODS: Retrospectively, we studied patients (n = 56) with positive anti-GAD65 and any neurologic symptom. We tested serum and CSF with ELISA, immunohistochemistry, and cell-based assay. Accordingly, we set a cutoff value of 10,000 IU/mL in serum by ELISA to group patients into high-concentration (n = 36) and low-concentration (n = 20) groups. We compared clinical and immunologic features and analyzed response to immunotherapy. RESULTS: Classical anti-GAD65-associated syndromes were seen in 34/36 patients with high concentration (94%): stiff-person syndrome (7), cerebellar ataxia (3), chronic epilepsy (9), limbic encephalitis (9), or an overlap of 2 or more of the former (6). Patients with low concentrations had a broad, heterogeneous symptom spectrum. Immunotherapy was effective in 19/27 treated patients (70%), although none of them completely recovered. Antibody concentration reduction occurred in 15/17 patients with available pre- and post-treatment samples (median reduction 69%; range 27%-99%), of which 14 improved clinically. The 2 patients with unchanged concentrations showed no clinical improvement. No differences in treatment responses were observed between specific syndromes. CONCLUSION: Most patients with high anti-GAD65 concentrations (>10,000 IU/mL) showed some improvement after immunotherapy, unfortunately without complete recovery. Serum antibody concentrations' course might be useful to monitor response. In patients with low anti-GAD65 concentrations, especially in those without typical clinical phenotypes, diagnostic alternatives are more likely.

Improvement of stiff-person syndrome symptoms in pregnancy: Case series and literature review.

OBJECTIVE: To describe 2 cases from a single academic institution of improvement in stiff-person syndrome (SPS) symptoms during pregnancy and to review the clinical outcomes of SPS in 6 additional pregnancies described in the literature. METHODS: Evaluation of clinical symptoms and treatment changes of disease state during pregnancy. RESULTS: Seven patients with 9 pregnancies are described in women with a diagnosis of SPS. Six of 7 (86%) women were positive for glutamic acid decarboxylase (GAD65) antibody. In 5 of 9 (56%) pregnancies, symptomatic medications (antispasmodics) were significantly reduced with stabilization or improvement in symptoms through pregnancy. Nine live, healthy pregnancies resulted. All 7 (100%) women experienced worsening of symptoms after the birth of their children, and symptomatic therapies were resumed and/or increased. CONCLUSIONS: The immune pathogenesis of SPS continues to be explored. Immunomodulatory shifts during pregnancy may influence changes of clinical SPS symptoms and provide insight into the unique pathogenesis of SPS. Some women with SPS may be able to reduce symptomatic medications related to clinical improvement during pregnancy. Women with SPS may safely carry pregnancies to term, delivering healthy and unaffected babies.

Retinal pathology occurs in stiff-person syndrome.

OBJECTIVE: To evaluate whether structural and functional changes occur in the afferent visual system of patients with stiff-person syndrome (SPS) and whether these changes correlate with disease burden, given the high concentration of γ-aminobutyric acid receptors, which are generally thought to be involved in SPS pathogenesis, in the retina. METHODS: In this single-center, cross-sectional study, patients with SPS and healthy controls (HCs) underwent optical coherence tomography (OCT), with a subset undergoing high- and low-contrast visual acuity (VA) assessments. Burden of disease was assessed via the number of body regions affected. Individuals with uncontrolled hypertension or comorbid neurologic or ophthalmologic disorders were excluded. Statistical analyses were performed using mixed-effects linear regression models. RESULTS: Thirty-five patients with SPS and 40 age- and sex-matched HCs underwent OCT. A subset of 23 patients with SPS and 28 HCs underwent VA assessments. Relative to HCs, patients with SPS had lower ganglion cell + inner plexiform layer (GCIPL) thicknesses (SPS: 74.36 µm [SD 5.7]; HCs: 76.33 µm [SD 4.2]; p = 0.005), inner nuclear layer thicknesses (SPS: 44.37 µm [SD 2.7]; HCs: 45.18 µm [SD 2.2]; p = 0.042), and 100% (SPS: 53 [SD 9.6]; HCs: 57.5 [SD 6.1]; p = 0.005), 2.5% (SPS: 24.35 [SD 10.1]; HCs: 30.16 [SD 7.7]; p = 0.006), and 1.25% contrast (SPS: 16.41 [SD 10.6]; HCs: 20.84 [SD 8.6]; p = 0.034) letter acuity scores. GCIPL thicknesses correlated with the number of body regions affected in SPS (decrease of 1.25 µm [95% confidence interval, -2.2 to -0.3 µm; p = 0.008] per additional body region affected). CONCLUSIONS: Retinal neuronal pathology can occur in SPS. OCT may have utility as a biomarker of disease burden in SPS.

Rituximab improves not only back stiffness but also "stiff eyes" in stiff person syndrome: Implications for immune-mediated treatment.

OBJECTIVE: Stiff person syndrome (SPS) is usually characterized by truncal muscle rigidity and episodic painful spasms, but it sometimes appears with ocular symptoms called "stiff eyes". We recorded saccade movements in an SPS patient manifesting with "stiff eyes" conditions with slow saccade velocity and evaluated the effect of immunotherapy including rituximab on saccade parameters. METHODS: We repeatedly conducted saccade eye recordings using video-based eye tracking system on a 42-year-old male SPS patient with slow saccade. The velocity and onset latency of visual guided saccades (VGS) were measured at each recording. Because VGS velocity is affected by saccade amplitude, estimated peak velocity (Vmax) was also calculated by taking the relationship between the velocity and the amplitude of saccade into account. RESULTS: The mean VGS velocity improved significantly after two courses of rituximab administration compared with its lowest value. The estimated Vmax decreased as the clinical manifestations worsened, but it increased after rituximab administration. Other neurological symptoms in this patient such as muscle rigidity and gait instability also improved after the treatment. CONCLUSION: Slow saccade in a "stiff eyes" patient improved after rituximab administration. Our study also indicated that the saccade eye recording is useful for evaluating the clinical condition of SPS when it is complicated with ocular symptoms.

Stiff limb syndrome with lower limb myoclonus: A case report.

RATIONALE: stiff limb syndrome (SLS) is a variant of stiff-man syndrome, primarily affecting a specific limb. Its diagnosis has always been challenging due to the lack of a specific confirmation test. We present a rare case of a patient with lower limb myoclonus and rigidity. PATIENT CONCERNS: A 53-year-old male presented with a sudden onset of progressive left lower extremity myoclonus and muscle rigidity for 3 days. He rapidly showed signs of right lower limb involvement with severe joint stiffness and inability to walk. DIAGNOSIS: The symptoms nature, physical examination, careful elimination of differential diagnosis suggested a diagnosis of stiff limb syndrome. INTERVENTIONS: Intravenous infusion of gamma globulin 0.4 mg/kg coupled with baclofen and clonazepam were given after admission. He also received an injection of botulinum toxin A to relieve his muscle stiffness. OUTCOMES: The patients' condition improved after the initial treatment with complete disappearance of muscle twitching. Further improvements were seen later on after the local administration of botulinum toxin A. LESSONS: Stiff limb syndrome shares the same complex symptoms with many other conditions. Its diagnosis relies heavily on clinical presentations and on ruling out other conditions. However, unusual symptoms such as myoclonus can occur in few cases and together with the rarity of the condition, the prevalence of misdiagnosis is high. Therefore, being aware and recognizing the signs and symptoms is crucial for proper management. Additionally, EMG is a very important test if the present condition is suspected. However, a negative EMG result or a negative anti-glutamic acid decarboxylase antibody test should not exclude SLS diagnosis.

Clinical Heterogeneity in Patients with Glutamate Decarboxylase Antibody.

OBJECTIVE: To explore the diversity and clinical features of anti-glutamate decarboxylase (GAD) antibody-associated neurological diseases. METHODS: Clinical data of a series of 5 patients positive for anti-GAD antibodies were retrospectively analyzed. RESULTS: All 5 patients were female, with a median age of 41.5 years (range 19-60 years). Their neurological symptoms included stiff-person syndrome (SPS), encephalitis, myelitis, cramp, visual loss, and paresthesia. Three patients (60%) were diagnosed with tumors, 2 cases of thymic tumor and 1 of breast cancer. On immunohistochemistry for tumor pathology, expression of GAD65 was found only in 1 patient. Four patients (80%) had abnormal brain MRI findings. All patients received immunotherapy and improved significantly after treatment, but 4 (80%) then experienced a relapse. CONCLUSIONS: Neurological manifestations in anti-GAD-positive patients are diverse and include SPS, encephalitis, myelitis, cramp, visual loss, and paresthesia.

Dyspnea in Patients with Stiff-Person Syndrome.

BACKGROUND: Stiff-person syndrome (SPS) is a rare autoimmune disorder that leads to progressively worsening stiffness and spasm of thoracic and proximal-limb musculature. Dyspnea has been reported but not analyzed in patients with SPS. MATERIALS AND METHODS: For this prospective study, 17 patients were recruited from a university-based neurology clinic. History and exam were performed, demographic information collected and available imaging reviewed. Dyspnea was assessed using vertical visual analog scales (VAS), the University of California San Diego Shortness of Breath Questionnaire (UCSD-SOBQ) and dyspnea "descriptors". Standardized assessments of SPS severity were performed by an experienced neurologist. Forced vital capacity (FVC) spirometric analysis was performed on all patients. RESULTS: Fifteen of 17 patients complained of dyspnea, including dyspnea at rest, with exertion, and disturbing sleep. A restrictive pattern was the most common abnormality noted on spirometry. FVC (r = -0.67; P < 0.01) and forced expiratory volume in 1-second (FEV1) (r = -0.76; P < 0.01) percent predicted correlated with dyspnea measured by VAS over the preceding 2 weeks. Pulmonary function did not correlate with UCSB-SOBQ or standardized measures of SPS severity. CONCLUSIONS: Dyspnea in SPS is common and occurs at rest with exertion and disturbs sleep. The finding of restrictive physiology and correlation between pulmonary function variables and dyspnea support the hypothesis that thoracic cage constriction by rigidity and/or spasm of the muscles of the trunk causes or contributes to the sensation of dyspnea. The possibility of diaphragmatic involvement requires further study.

Is Stiff Person Syndrome Benefited by Physical Therapy Intervention? Summary of Case Reports.

It is interesting to be aware that there is no Randomized Clinical Trials (RCT) research article except a few case-study reports which have been reported about the physical therapy (PT) intervention for stiff person syndrome (SPS). This study was designed to determine the benefits of PT in cases with SPS through analysis of case reports, thereby to raise awareness among physical therapist about the most beneficial PT interventions for SPS. We executed acomputer-based search with a diagnosis of SPS who underwent PT and articles published only in English. We selected case-study reports because of nonavailability of RCT articles to review the complaints, deformities, contractures, precipitating factors, interventions, outcomes, results, disability, and benefits of PT management among SPS. We concluded that PT training is substantiated to be a necessary and beneficial intervention in rehabilitation of patients with SPS.

Gravity-Independent Upbeat Nystagmus in Syndrome of Anti-GAD Antibodies.

An autoimmune disorder of the central nervous system, stiff person syndrome, frequently presents with increased titers of 65KD anti-glutamic acid decarboxylase (anti-GAD) antibodies. The clinical phenomenology of this syndrome includes stiffness, ataxia, vertigo due to horizontal gaze-evoked and downbeat vertical nystagmus, and dysmetria of saccades and reaching movements. Here, we describe a novel phenomenology of syndrome of anti-GAD antibody, non-position-dependent upbeat nystagmus and superimposed horizontal gaze-evoked nystagmus. Lack of gravity dependence of primary position upbeat nystagmus, intense nystagmus on up-gaze, relatively stable gaze on downward orientation, and the exponentially decaying waveform suggests neural integrator dysfunction. The titer of anti-GAD in our patient (30 U/ml) was consistent with a variant called "low-titer anti-GAD syndrome". In addition of presenting as an unusual manifestation of a rare neurological syndrome, this case presents a neurochemical correlate of upbeat nystagmus in GABA-mediated control system involving horizontal and vertical neural integrators. Furthermore, the variant of "low-titer anti-GAD syndrome" suggests that GABAergic system may be affected at lower level or antibodies, and/or the epitopes of antibody in those with full-blown clinical syndrome, but low titers of anti-GAD may be different.

Reduced display of conformational epitopes in the N-terminal truncated GAD65 isoform: relevance for people with stiff person syndrome or DQ8/8-positive Type 1 diabetes mellitus.

AIMS: To investigate whether the N-terminal truncated glutamic acid decarboxylase 65 (GAD65) isoform is as well recognized by people with stiff person syndrome as it is by people with Type 1 diabetes, and whether conformational GAD65 antibody epitopes are displayed properly by the isoform. METHODS: GAD65 antibody-positive healthy individuals (n=13), people with stiff-person syndrome (n=15) and children with new-onset Type 1 diabetes (n=654) were analysed to determine binding to full-length GAD65 and the N-terminal truncated GAD65 isoform in each of these settings. GAD65 autoantibody epitope specificity was correlated with binding ratios of full-length GAD65/N-terminal truncated GAD65. RESULTS: The N-terminal truncated GAD65 isoform was significantly less recognized in GAD65Ab-positive people with stiff-person syndrome (P=0.002) and in healthy individuals (P=0.0001) than in people with Type 1 diabetes. Moreover, at least two specific conformational GAD65Ab epitopes were not, or were only partially, presented by the N-terminal truncated GAD65 isoform compared to full-length GAD65. Finally, an N-terminal conformational GAD65Ab epitope was significantly less recognized in DQ8/8 positive individuals with Type 1 diabetes (P=0.02). CONCLUSIONS: In people with stiff person syndrome preferred binding to the full-length GAD65 isoform over the N-terminal truncated molecule was observed. This binding characteristic is probably attributable to reduced presentation of two conformational epitopes by the N-terminal truncated molecule. These findings support the notion of disease-specific GAD65Ab epitope specificities and emphasize the need to evaluate the applicability of novel assays for different medical conditions.