A clinical and genetic study of the skeletal muscle channelopathies

The skeletal muscle channelopathies are a group of inherited muscle diseases characterised by the abnormal functioning of voltage-gated ion channels expressed in skeletal muscle. They manifest as the non-dystrophic myotonias and the periodic paralyses. This thesis increases the current understanding of the clinical and genetic basis of this group of diseases. It identifies the overall prevalence in England as 1.12/100,000 and determines the individual minimum prevalence of each disease, which has not previously been documented. It presents a detailed phenotype study of periodic paralysis (PP), paramyotonia congenita (PMC) and sodium channel myotonia (SCM), which is the first comparative study of these diseases. In the process it uncovers the marked similarity between PMC and SCM and suggests that these may be a spectrum of one disease, rather than two distinct diseases as traditionally thought. It provides the first systematic study of pregnancy and anaesthetics in a large number of channelopathy patients, identifying a marked increase in severity of symptoms during pregnancy that has not previously been documented. To widen the spectrum of genetic diagnosis and techniques in this group of diseases, this thesis describes the first cases of large scale rearrangements in CLCN1 causing myotonia congenita. It demonstrates how, using whole exome sequencing, the genetic diagnosis rate can be improved and illustrates two cases that may be explained by variations in RYR1 and another case in which a genetic diagnosis of Liddle’s syndrome may underlie secondary PP. This suggests that RYR1 variations may account for some unconfirmed cases and others may be explained by genetic causes of secondary PP. Finally this thesis presents convincing evidence of the efficacy of mexiletine in non-dystrophic myotonia in a double-blind placebo-controlled trial. It demonstrates improvement of the primary outcome measure of patient-reported stiffness and the majority of secondary outcome measures assessed

Muscle MRI reveals distinct abnormalities in genetically proven non-dystrophic myotonias.

We assessed the presence, frequency and pattern of MRI abnormalities in non-dystrophic myotonia patients. We reviewed T1-weighted and STIR (short-tau-inversion-recovery) 3T MRI sequences of lower limb muscles at thigh and calf level in 21 patients with genetically confirmed non-dystrophic myotonia: 11 with CLCN1 mutations and 10 with SCN4A mutations, and 19 healthy volunteers. The MRI examinations of all patients showed hyperintensity within muscles on either T1-weighted or STIR images. Mild extensive or marked T1-weighted changes were noted in 10/21 patients and no volunteers. Muscles in the thigh were equally likely to be affected but in the calf there was sparing of tibialis posterior. Oedema was common in calf musculature especially in the medial gastrocnemius with STIR hyperintensity observed in 18/21 patients. In 10/11 CLCN1 patients this included a previously unreported "central stripe", also present in 3/10 SCN4A patients but no volunteers. Degree of fatty infiltration correlated with age (rho=0.46, p<0.05). Muscle MRI is frequently abnormal in non-dystrophic myotonia providing evidence of fatty infiltration and/or oedema. The pattern is distinct from other myotonic disorders; in particular the "central stripe" has not been reported in other conditions. Correlations with clinical parameters suggest a potential role for MRI as a biomarker

Prevalence study of genetically defined skeletal muscle channelopathies in England.

To obtain minimum point prevalence rates for the skeletal muscle channelopathies and to evaluate the frequency distribution of mutations associated with these disorders

Skeletal muscle channelopathies: a guide to diagnosis and management.

Skeletal muscle channelopathies are a group of rare episodic genetic disorders comprising the periodic paralyses and the non-dystrophic myotonias. They may cause significant morbidity, limit vocational opportunities, be socially embarrassing, and sometimes are associated with sudden cardiac death. The diagnosis is often hampered by symptoms that patients may find difficult to describe, a normal examination in the absence of symptoms, and the need to interpret numerous tests that may be normal or abnormal. However, the symptoms respond very well to holistic management and pharmacological treatment, with great benefit to quality of life. Here, we review when to suspect a muscle channelopathy, how to investigate a possible case and the options for therapy once a diagnosis is made

Novel mutations in human and mouse SCN4A implicate AMPK in myotonia and periodic paralysis

Mutations in the skeletal muscle channel (SCN4A), encoding the Nav1.4 voltage-gated sodium channel, are causative of a variety of muscle channelopathies, including non-dystrophic myotonias and periodic paralysis. The effects of many of these mutations on channel function have been characterized both in vitro and in vivo. However, little is known about the consequences of SCN4A mutations downstream from their impact on the electrophysiology of the Nav1.4 channel. Here we report the discovery of a novel SCN4A mutation (c.1762A>G; p.I588V) in a patient with myotonia and periodic paralysis, located within the S1 segment of the second domain of the Nav1.4 channel. Using N-ethyl-N-nitrosourea mutagenesis, we generated and characterized a mouse model (named draggen), carrying the equivalent point mutation (c.1744A>G; p.I582V) to that found in the patient with periodic paralysis and myotonia. Draggen mice have myotonia and suffer from intermittent hind-limb immobility attacks. In-depth characterization of draggen mice uncovered novel systemic metabolic abnormalities in Scn4a mouse models and provided novel insights into disease mechanisms. We discovered metabolic alterations leading to lean mice, as well as abnormal AMP-activated protein kinase activation, which were associated with the immobility attacks and may provide a novel potential therapeutic target

Atypical periodic paralysis and myalgia: A novel RYR1 phenotype.

OBJECTIVE: To characterize the phenotype of patients with symptoms of periodic paralysis (PP) and ryanodine receptor (RYR1) gene mutations. METHODS: Cases with a possible diagnosis of PP but additional clinicopathologic findings previously associated with RYR1-related disorders were referred for a tertiary neuromuscular clinical assessment in which they underwent detailed clinical evaluation, including neurophysiologic assessment, muscle biopsy, and muscle MRI. Genetic analysis with next-generation sequencing and/or targeted Sanger sequencing was performed. RESULTS: Three cases with episodic muscle paralysis or weakness and additional findings compatible with a RYR1-related myopathy were identified. The McManis test, used in the diagnosis of PP, was positive in 2 of 3 cases. Genetic analysis of known PP genes was negative. RYR1 analysis confirmed likely pathogenic variants in all 3 cases. CONCLUSIONS: RYR1 mutations can cause late-onset atypical PP both with and without associated myopathy. Myalgia and cramps are prominent features. The McManis test may be a useful diagnostic tool to indicate RYR1-associated PP. We propose that clinicopathologic features suggestive of RYR1-related disorders should be sought in genetically undefined PP cases and that RYR1 gene testing be considered in those in whom mutations in SCN4A, CACNA1S, and KCNJ2 have already been excluded

Comparison of different settings for yellow subthreshold laser treatment in diabetic macular edema

Abstract Background To assess the safety and efficacy of two subthreshold parameters (5 and 15% duty cycle (DC)) compared to standard ETDRS (early treatment of diabetic retinopathy study) continuous wave (CW) laser. Methods In this prospective randomized study, 30 eyes from 20 patients with non-center involving macular edema were randomized into 3 different groups: 5% DC, 15% DC and CW navigated modified ETDRS laser treatment. Titration in subthreshold groups was performed with 30% of the threshold power, decided with microsecond pulses. CW laser was titrated to a barely visible burn. All patients underwent microperimetry, thickness measurements and visual acuity examinations at baseline, 6 weeks and 12 weeks post treatment. Results At three months follow up, retinal sensitivity was significantly reduced in the CW group by − 2.2 dB whereas in both subthreshold groups, retinal sensitivity increased by 2.4 dB for 5% and 1.9 dB for 15% DC with no significant difference. Retinal volume (mm3) decreased in both subthreshold groups by 0.08 ± 0.3 and 0.12 ± 0.11 in 5 and 15% DC group respectively. Whereas the CW group showed volume increase of 0.55 ± 0.92 (p = 0.02 and 0.01 for 5 and 15% DC groups). Visual acuity remained stable in all 3 groups (− 0.7 letter in 5% DC; 2.11 letters in 15% DC and 0.88 in CW with no significant difference). Conclusion Subthreshold microsecond laser was shown to be safe and effective with both 5 and 15% DC as compared to conventional photocoagulation with ETDRS parameters. The 15% DC setting trended to achieve better anatomical, visual and functional outcomes. Trial registration Retrospectively registered (NCT03571659, 06/26/2018)