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Toxic neuropathies result from exogenous substances damaging the peripheral nerves. There are numerous causes, including prescribed and recreational drugs, heavy metals, industrial agents and biological toxins. Timely recognition of these neuropathies gives better outcomes, as they usually improve or stabilise once the toxin is removed. Most toxic neuropathies are axonal, length-dependent and sensory predominant, although some have significant motor involvement or can present acutely or subacutely. Here, we outline our clinical approach and discuss the major causes of toxic neuropathy, while emphasising the clinical and neurophysiological features and the neuropathy phenotype. We also include an update on newer medications that can cause neuropathy, including immune checkpoint inhibitors and BRAF/MEK inhibitors.

In this update, we review the recent discovery of autosomal recessive variants in sorbitol dehydrogenase as one of the commonest and potentially treatable causes of hereditary motor neuropathy and CMT2. We also report on recent therapeutic advances in hereditary neuropathy including the use of lipid nanoparticle sequestered antisense oligonucleotides in CMT1A and lipid nanoparticle delivered CRISPR-Cas9 gene editing in ATTR amyloidosis.

Myelin is essential for rapid nerve impulse propagation and axon protection. Accordingly, defects in myelination or myelin maintenance lead to secondary axonal damage and subsequent degeneration. Studies utilizing genetic (CNPase-, MAG-, and PLP-null mice) and naturally occurring neuropathy models suggest that myelinating glia also support axons independently from myelin. Myelin protein zero (MPZ or P0), which is expressed only by Schwann cells, is critical for myelin formation and maintenance in the peripheral nervous system. Many mutations in MPZ are associated with demyelinating neuropathies (Charcot-Marie-Tooth disease type 1B [CMT1B]). Surprisingly, the substitution of threonine by methionine at position 124 of P0 (P0T124M) causes axonal neuropathy (CMT2J) with little to no myelin damage. This disease provides an excellent paradigm to understand how myelinating glia support axons independently from myelin. To study this, we generated targeted knock-in P0T124M mutant mice, a genetically authentic model of T124M-CMT2J neuropathy. Similar to patients, these mice develop axonopathy between 2 and 12 months of age, characterized by impaired motor performance, normal nerve conduction velocities but reduced compound motor action potential amplitudes, and axonal damage with only minor compact myelin modifications. Mechanistically, we detected metabolic changes that could lead to axonal degeneration, and prominent alterations in non-compact myelin domains such as paranodes, Schmidt-Lanterman incisures, and gap junctions, implicated in Schwann cell-axon communication and axonal metabolic support. Finally, we document perturbed mitochondrial size and distribution along P0T124M axons suggesting altered axonal transport. Our data suggest that Schwann cells in P0T124M mutant mice cannot provide axons with sufficient trophic support, leading to reduced ATP biosynthesis and axonopathy. In conclusion, the P0T124M mouse model faithfully reproduces the human neuropathy and represents a unique tool for identifying the molecular basis for glial support of axons.

Traditionally, neurophysiology is the primary diagnostic and prognostic biomarker in peripheral neuropathy clinical practice; however, it may lack responsiveness in the context of slowly progressive neuropathies and where there is significant axonal damage. The development of ultrasensitive platforms for measuring serum proteins at the lower limit of detection of traditional ELISA techniques has transformed the field of blood biomarkers of peripheral neuropathy. A variety of blood biomarkers have been identified from inflammatory cytokines and apokines in diabetic neuropathy through to neuron-specific proteins such as neurofilament light chain, Schwann cell-specific proteins such as TMPRSS5 and microRNAs in other acquired and hereditary neuropathies. In this article, we review blood biomarkers of disease activity for the common subtypes of peripheral neuropathy including inflammatory demyelinating neuropathies, vasculitic neuropathy, diabetic neuropathy, chemotherapy-induced neuropathy and Charcot-Marie-Tooth disease and related disorders including TTR amyloidosis.

BackgroundA 22-year-old woman was initially assessed in the context of a family history of distal wasting and weakness, typically with upper limb onset, then lower limb involvement. When assessed as a child her phenotype was recognised to be atypical compared to her sister, but she was thought to be affected. Ultimately the family was diagnosed with distal hereditary motor neuropathy due to a GARS mutation. However, this patient did not carry the mutation, prompting further evaluation.CaseAt age 11 the left foot was smaller with some toe clawing and cavus deformity. Left sided wasting of tibialis anterior and the posterior compartment was seen, ankle dorsiflexors were grade 3.ResultsNerve conduction studies and EMG were normal in the right leg and arms. Chronic denervation was identified in L4, L5 and S1. MRI of lumbar spine was normal. However, 2 areas of marked thickening and enhancement were seen in the left sciatic nerve in the upper thigh and a further lesion in the mid- thigh. Fascicular preservation of the nerve was felt to be strongly suggestive of a perineurioma.DiscussionThis case highlights the importance of careful phenotyping when validating novel variants in potentially causative genes.c.doherty@ucl.ac.uk

ImportanceDisease modifying treatments in Guillain-Barré syndrome (GBS) are not tailored to the different pathological subtypes. Strategies to prospectively identify sub-sets of patients with specific disease mecha- nisms and inform the use of non-standard therapies may lead to improved outcomes. We present a case series of seven patients with IgG1 subclass pan-neurofascin antibodies.ResultsAll seven patients presented with a severe, acute or sub-acute onset, predominantly motor, autoimmune neuropathy. In all but one case the neuropathy was severe enough to warrant mechanical ventilation and intensive care support. Cranial nerve deficits were universally present and autonomic dysfunction was common. Although clinically resembling GBS, the response to standard GBS therapies was poor. Four patients received the B-cell depleting therapy rituximab and showed significant and sustained functional improvement within weeks of treatment. The three remaining patients who did not receive rituximab died.Conclusions and RelevanceThese observations should lead to a re-evaluation of the benefit of nodal/paranodal antibody testing in neuropathies with acute onset, GBS-like presentations. We believe that such testing should form part of the routine diagnostic work up of such patients, and that the identifica- tion of a distinct, serologically-defined disease sub-type should prompt consideration of more targeted immunotherapy. The apparent benefit of rituximab reported here should be evaluated in a well-conduc- ted clinical trial, the design of which must consider the potentially grave outcome in pan-neurofascin antibody-positive patients receiving only standard therapies.simon.rinaldi@nhs.net

BackgroundA problem frequently faced by neurologists is the interpretation of genetic variants of unknown significance(VUS). Unlike in many muscle diseases, specific patterns of muscle involvement are not typically demonstrated on muscle MRI of inherited neuropathy patients. Imaging muscle involvement can be of assistance when assessing VUS.Case ExamplesTwo unrelated individuals with axonal CMT were identified to have a novel p.Lys554Glu DNM2 mutation. Neuromuscular MRI was consistent with the clinical findings and similar to literature descriptions, supporting the pathogenicity of this variant.Spinal Muscular Atrophy Lower Extremity Dominant (SMA-LED) due to DYNC1H1 mutations can be associ- ated with sparing and relative hypertrophy of the adductor longus and semitendinosus muscles at the thigh level, with diffuse involvement at the calf level, sparing the anterior-medial muscles1 on MRI. These features were demonstrated in the imaging of a proband with SMA-LED has two mutations in DYNC1H1, one of which is novel. The novel p.His75Pro mutation was also found in his mother who has normal clinical examination and neuromuscular MRI, suggesting that the previously described p.Arg251His mutation2 was the cause of the neuromuscular phenotype in the proband.DiscussionMRI has a role supporting the diagnosis of inherited neuropathies, and in evaluating VUS.Scoto et al. Novel mutations expand the clinical spectrum of DYNC1H1-associated SMA. Neurology.2015Feb17;84:668–679Antoniadi et al. Application of targeted multi-gene panel testing for the diagnosis of inherited peripheral neuropathy provides high diagnostic yield.BMCMedGenet.2015Sep21;16:84.c.doherty@ucl.ac.uk

IntroductionSORD is a newly described autosomal recessive gene that is emerging as the commonest cause of autosomal recessive CMT (Charcot-Marie-Tooth disease) and hereditary motor neuropathy (HMN).MethodsWe present nine cases of SORD-related neuropathy.ResultsNine unrelated patients were found to have biallelic mutations in SORD. Eight were male (89%) and median age of assessment was 34 years (range 18–55 years). Median age of symptom onset was 15 years (range 12–18 years) and all patients initially presented with difficulty walking and/or problems involving distal lower limbs. Only one patient (11%) had sensory symptoms. 5/9 (56%) had distal upper limb weakness and all had distal lower limb weakness. Sensory examination was abnormal to at least one modality in 7/9 cases (78%). The median CMT Examination and CMT Neuropathy Scores were 6 (range 2–12) and 7.5 (range 3–14) respectively. Neurophysiology showed conduction velocities in the axonal or intermediate ranges in all cases. Clinical diagnosis was distal HMN in 5/9 (56%), CMT2 in 2/9 (22%) and CMT intermediate in 2/9 (22%).ConclusionsThe SORD phenotype is typically a slowly progressive, length-dependent motor neuropathy of onset during the second decade. Sensory symptoms are rare and sensory signs minimal.chris.record@ucl.ac.uk