An easily overlooked disease in the early stages: acute intermittent porphyria

BACKGROUND

Acute intermittent porphyria (AIP) is an inherited metabolic disorder that can affect the central, peripheral, and autonomic nervous systems. Therefore, its clinical presentation is diverse and may include abdominal pain, as well as neurological and psychiatric symptoms. Abdominal pain, though a common initial symptom, is often overlooked or misdiagnosed due to its lack of specificity. But early diagnosis and treatment are crucial, as untreated symptoms can progressively worsen.

CASE PRESENTATION

This report describes a 26-year-old male who was admitted due to seizures and PRES changes on brain magnetic resonance imaging (MRI) for over 30 days, along with a 20-day history of sudden proximal weakness in both upper limbs. Additionally, he experienced recurrent vomiting and excessive sweating. Five months before admission, he was diagnosed with a urinary tract infection due to severe abdominal pain and tea-colored urine, and the symptoms resolved after treatment. Multiple examinations before and after admission consistently revealed hypertension, tachycardia, and hyponatremia. Electromyography (EMG) suggested axonal damage to the motor nerves of both upper limbs. During hospitalization, the patient's upper limb weakness progressively worsened, and around 12 days after admission, he began experiencing recurrent episodes of abdominal pain and limb pain. Then he was diagnosed with AIP based on the detection of positive PBG in urin and the identification of a c.445C > T (R149X) mutation in the hydroxymethylbilane synthase (HMBS) gene.

CONCLUSIONS

This case unveils that AIP is a disease that can be easily overlooked in its early stages. When a patient presents with central, peripheral, or autonomic nervous system symptoms and common causes are ruled out, AIP should be considered as a potential diagnosis. Additionally, unexplained symptoms such as abdominal pain, changes in urine color, hyponatremia should also raise suspicion. Timely screening through biochemical testing, including measurement of ALA, PBG and porphyrins in a random urine sample, is recommended. Timely administration of intravenous hemin and avoidance of precipitating factors can lead to a better prognosis.

The relationship among H2S, neuroinflammation and MMP-9 in BBB injury following ischemic stroke

Blood-brain barrier (BBB) is located at the interface between the central nervous system (CNS) and the circulatory system, which maintains the microenvironmental homeostasis of the CNS. BBB damage is a result of CNS diseases, including ischemic stroke, and is a cause of CNS deterioration. Cerebral ischemia unleashes a profound inflammatory response to remove the damaged tissue in the CNS and prepare the brain for repair. However, the excessive neuroinflammation following stroke onset is associated with BBB breakdown, resulting in neuronal injury and worse neurological outcomes. Additionally, matrix metalloproteinases (MMPs) are likewise responsible for the BBB injury and participate in the pathological processes of neuroinflammation following ischemic stroke. Hydrogen sulfide (H2S) is one of gaseous signaling and freely diffusing molecules. Low concentration of H2S yields the neuroprotection against BBB damage following stroke. This review discussed the current knowledge about the detrimental roles of neuroinflammation and MMPs in BBB injury following ischemic stroke. Specifically, we provided an updated overview of H2S in protecting against BBB injury following ischemic stroke via anti-neuroinflammation and inhibiting MMP-9.

Axonal excitability as an early biomarker of nerve involvement in hereditary transthyretin amyloidosis

OBJECTIVES

The treatment of hereditary transthyretin amyloidosis polyneuropathy (ATTRv-PN) has been revolutionised by genetic therapies, with dramatic improvements in patient outcomes. Whilst the optimal timing of treatment initiation remains unknown, early treatment is desirable. Consequently, the aim of the study was to develop biomarkers of early nerve dysfunction in ATTRv-PN.

METHODS

Ulnar motor and sensory axonal excitability studies were prospectively undertaken on 22 patients with pathogenic hereditary transthyretin amyloid (ATTRv) gene variants, 12 with large fibre neuropathy (LF+) and 10 without (LF-), with results compared to age- and sex-matched healthy controls.

RESULTS

In motor axons we identified a continuum of change from healthy controls, to LF- and LF+ ATTRv with progressive reduction in hyperpolarising threshold electrotonus (TEh40(10-20 ms): p = 0.04, TEh40(20-40 ms): p = 0.01 and TEh40(90-10 ms): p = 0.01), suggestive of membrane depolarisation. In sensory axons lower levels of subexcitability were observed on single (SubEx) and double pulse (SubEx2) recovery cycle testing in LF+ (SubEx: p = 0.015, SubEx2: p = 0.015, RC(2-1): p = 0.04) suggesting reduced nodal slow potassium conductance, which promotes sensory hyperexcitability, paraesthesia and pain. There were no differences in sensory or motor excitability parameters when comparing different ATTRv variants.

CONCLUSIONS

These progressive changes seen across the disease spectrum in ATTRv-PN suggest that axonal excitability has utility to identify early and progressive nerve dysfunction in ATTRv, regardless of genotype.

SIGNIFICANCE

Axonal excitability is a promising early biomarker of nerve dysfunction in ATTRv-PN.

Acute Intermittent Porphyria: A Review and Rehabilitation Perspective

Acute intermittent porphyria (AIP) is an uncommon metabolic disease that impacts multiple organs and can manifest in many ways. It is often misdiagnosed due to its nonspecific symptoms. Neurovisceral signs and symptoms should alert physicians to consider AIP in the differential after excluding more common causes. Identifying the underlying cause is critical in preventing acute attacks, and trigger avoidance is the optimal approach to managing AIP. Medications that are contraindicated should be reviewed thoroughly. Prompt intravenous hematin administration is the primary treatment for acute attacks, and additional pharmacological therapies may be necessary to treat concurrent symptoms. A severe neurological manifestation of AIP is flaccid paralysis or severe motor weakness, which can develop into total quadriplegia and respiratory insufficiency. A comprehensive rehabilitation program is an integral aspect of the treatment plan. Since the incidence of this disease is low, functional prognosis is not well-known. As a result, it is challenging to determine the most appropriate structure, intensity, and duration of rehabilitation therapy. By extending the treatment plan, individuals with tetraplegia due to AIP can continue to make functional gains years after the onset of weakness. Understanding the disease's functional prognosis will aid in coordinating resources and improving healthcare expenditures.

Neurological Manifestations of Acute Porphyrias

PURPOSE OF REVIEW

Porphyrias constitute a group of rare metabolic disorders that result in a deficiency of the heme biosynthetic pathway and lead to the accumulation of metabolic intermediaries. Patients with porphyria can experience recurrent neurovisceral attacks which are characterized by neuropathic abdominal pain and acute gastrointestinal symptoms, including nausea, vomiting, and constipation. Depending on the type of porphyria, patients can present with cutaneous manifestations, such as severe skin photosensitivity, chronic hemolysis, or evidence of neurologic dysfunction, including alterations in consciousness, neurovascular involvement, seizures, transient sensor-motor symptoms, polyneuropathy, and behavioral abnormalities.

RECENT FINDINGS

More recently, cases of posterior reversible encephalopathy syndrome, cerebral vasoconstriction, and acute flaccid paralysis have also been described. While the exact pathogenic mechanisms linking the accumulation of abnormal heme biosynthetic intermediaries to neurologic manifestations have not been completely elucidated, it has been proposed that these manifestations are more common than previously thought and can result in permanent neurologic injury. This article reviews the basic principles of heme synthesis as well as the pathogenic mechanism of disease, presentation, and treatment of acute hepatic porphyrias with emphasis on those with neurologic manifestations.

Neurology of the acute hepatic porphyrias

Porphyrias are a set of rare inherited metabolic disorders, each of them representing a defect in one of the eight enzymes in the haem biosynthetic pathway resulting in the accumulation of organic compounds called porphyrins. Acute hepatic porphyrias (AHP) are those in which the enzyme deficiency occurs in the liver, of which acute intermittent porphyria is by far the most common subtype. Neurology of the AHP is still challenging in practice, and patients rarely receive the correct diagnosis early in the disease course. For AHP, which primarily affects the central and peripheral nervous system, the cause of symptoms seems to be the increased production of neurotoxic precursors, in particular delta-aminolaevulinic acid and porphobilinogen. Neurological complications usually result from severe episodes of acute attacks. The neurologic hallmark of porphyrias is an acute predominantly motor axonal neuropathy resembling a Guillain-Barré syndrome that generally occurs after the onset of other clinical features such as abdominal pain and central nervous system manifestations. Neuropsychiatric syndromes, seizures, encephalopathy, and cerebrovascular disorders are among the possible central nervous system presentations. Therapeutic approach to AHP is divided into management and prophylaxis of an acute attack, including long standing options such as intravenous hematin and new therapeutic agents such as givosiran.

Porphyric neuropathy

Acute hepatic porphyrias are inherited metabolic disorders that may present with polyneuropathy, which if not diagnosed early can lead to quadriparesis, respiratory weakness, and death. Porphyric neuropathy is an acute to subacute motor predominant axonal neuropathy with a predilection for the upper extremities and usually preceded by a predominantly parasympathetic autonomic neuropathy. The rapid progression and associated dysautonomia mimic Guillain-Barré syndrome but are distinguished by the absence of cerebrospinal fluid albuminocytologic dissociation, progression beyond 4 wk, and associated abdominal pain. Spot urine test to assess the porphyrin precursors delta-aminolevulinic acid and porphobilinogen can provide a timely diagnosis during an acute attack. Timely treatment with intravenous heme, carbohydrate loading, and avoidance of porphyrinogenic medications can prevent further neurological morbidity and mortality.

Unusual Pain Disorders - What Can Be Learned from Them?

Pain is common in many different disorders and leads to a significant reduction in quality of life in the affected patients. Current treatment options are limited and often result in insufficient pain relief, partly due to the incomplete understanding of the underlying pathophysiological mechanisms. The identification of these pathomechanisms is therefore a central object of current research. There are also a number of rare pain diseases, that are generally little known and often undiagnosed, but whose correct diagnosis and examination can help to improve the management of pain disorders in general. In some of these unusual pain disorders like sodium-channelopathies or sensory modulation disorder the underlying pathophysiological mechanisms have only recently been unravelled. These mechanisms might serve as pharmacological targets that may also play a role in subgroups of other, more common pain diseases. In other unusual pain disorders, the identification of pathomechanisms has already led to the development of new drugs. A completely new therapeutic approach, the gene silencing, can even stop progression in hereditary transthyretin amyloidosis and porphyria, ie in pain diseases that would otherwise be rapidly fatal if left untreated. Thus, pain therapists and researchers should be aware of these rare and unusual pain disorders as they offer the unique opportunity to study mechanisms, identify new druggable targets and finally because early diagnosis might save many patient lives.

Crosstalk between Inflammation and the BBB in Stroke

The blood-brain barrier (BBB), which is located at the interface between the central nervous system (CNS) and the circulatory system, is instrumental in establishing and maintaining the microenvironmental homeostasis of the CNS. BBB disruption following stroke promotes inflammation by enabling leukocytes, T cells and other immune cells to migrate via both the paracellular and transcellular routes across the BBB and to infiltrate the CNS parenchyma. Leukocytes promote the removal of necrotic tissues and neuronal recovery, but they also aggravate BBB injury and exacerbate stroke outcomes, especially after late reperfusion. Moreover, the swelling of astrocyte endfeet is thought to contribute to the ‘no-reflow’ phenomenon observed after cerebral ischemia, that is, blood flow cannot return to capillaries after recanalization of large blood vessels. Pericyte recruitment and subsequent coverage of endothelial cells (ECs) alleviate BBB disruption, which causes the transmigration of inflammatory cells across the BBB to be a dynamic process. Furthermore, interneurons and perivascular microglia also make contacts with ECs, astrocytes and pericytes to establish the neurovascular unit. BBB-derived factors after cerebral ischemia triggered microglial activation. During the later stage of injury, microglia remain associated with brain ECs and contribute to repair mechanisms, including postinjury angiogenesis, by acquiring a protective phenotype, which possibly occurs through the release of microglia-derived soluble factors. Taken together, we reviewed dynamic and bidirectional crosstalk between inflammation and the BBB during stroke and revealed targeted interventions based on the crosstalk between inflammation and the BBB, which will provide novel insights for developing new therapeutic strategies.

Porphyric Neuropathy: Pathophysiology, Diagnosis, and Updated Management

PURPOSE OF REVIEW

To review the peripheral neurological complications of the acute hepatic porphyrias, as well as the latest advances in their pathophysiology and management.

RECENT FINDINGS

The diagnosis of porphyric neuropathy remains challenging as varying neuropathic patterns are encountered depending on disease stage, including a non-length-dependent distribution pattern. The major pathophysiologic mechanism is δ-aminolevulinic acid (ALA)-induced neurotoxicity. The less restrictive blood-nerve barrier in the autonomic ganglia and myenteric plexus may explain the frequency of dysautonomic manifestations. Recently, a prophylactic small interfering RNA (siRNA)-based therapy that reduces hepatic ALA Synthase-1 mRNA was approved for patients with recurrent neuro-visceral attacks. Neurologists should appreciate the varying patterns of porphyric neuropathy. As with most toxin-induced axonopathies, long-term outcomes depend on early diagnosis and treatment. While the short-term clinical and biochemical benefits of siRNA-based therapy are known, its long-term effects on motor recovery, chronic pain, and dysautonomic manifestations are yet to be determined.

Paediatric porphyria and human hemin: a treatment challenge in a lower middle income country

Here, we report a case of a 15-year-old girl who presented to the emergency department with symptoms of abdominal pain, nausea, vomiting and seizures. She was diagnosed with acute intermittent porphyria. Treatment was started by removing all porphogenic drugs, providing high glucose intake (oral and intravenous), which initially resulted in good clinical outcomes. However, she deteriorated again and also developed neurological manifestation (paraplegia) for which she required mechanical ventilation because of acute respiratory failure. This time she was initiated on human hemin for four consecutive days. After 2 days of therapy, her porphobilinogen levels decreased to 50% of the initial raised value. Increased lactic acid and blood urea nitrogen were the two side effects observed after the treatment, with no apparent signs of acute kidney injury. To the best of our knowledge, in paediatric population, this is the first reported case of treatment of acute intermittent porphyria with human hemin in Pakistan.

HCN channel-mediated neuromodulation can control action potential velocity and fidelity in central axons

Hyperpolarization-activated cyclic-nucleotide-gated (HCN) channels control electrical rhythmicity and excitability in the heart and brain, but the function of HCN channels at the subcellular level in axons remains poorly understood. Here, we show that the action potential conduction velocity in both myelinated and unmyelinated central axons can be bidirectionally modulated by a HCN channel blocker, cyclic adenosine monophosphate (cAMP), and neuromodulators. Recordings from mouse cerebellar mossy fiber boutons show that HCN channels ensure reliable high-frequency firing and are strongly modulated by cAMP (EC50 40 µM; estimated endogenous cAMP concentration 13 µM). In addition, immunogold-electron microscopy revealed HCN2 as the dominating subunit in cerebellar mossy fibers. Computational modeling indicated that HCN2 channels control conduction velocity primarily by altering the resting membrane potential and are associated with significant metabolic costs. These results suggest that the cAMP-HCN pathway provides neuromodulators with an opportunity to finely tune energy consumption and temporal delays across axons in the brain.

Measurement of axonal excitability: Consensus guidelines

Measurement of axonal excitability provides an in vivo indication of the properties of the nerve membrane and of the ion channels expressed on these axons. Axonal excitability techniques have been utilised to investigate the pathophysiological mechanisms underlying neurological diseases. This document presents guidelines derived for such studies, based on a consensus of international experts, and highlights the potential difficulties when interpreting abnormalities in diseased axons. The present manuscript provides a state-of-the-art review of the findings of axonal excitability studies and their interpretation, in addition to suggesting guidelines for the optimal performance of excitability studies.

Acute Intermittent Porphyria: A Report of 3 Cases with Neuropathy

The porphyrias are metabolic disorders due to a defect in the heme biosynthetic pathway. Patients have diverse clinical presentations with neuropathy being frequent in acute intermittent porphyria (AIP). Associated symptoms are abdominal pain and seizures. Three patients presenting with neuropathy were later diagnosed with AIP on the basis of clinical features, erythrocyte porphobilinogen deaminase activity, neuropathic patterns, and nerve conduction studies. Testing for the HMBS genetic mutation confirmed the diagnosis of AIP in 1 patient. The findings from this case series confirm that porphyric neuropathy in AIP is a predominantly motor neuropathy with differing neuropathic presentations ranging from focal motor neuropathy to quadriplegia and respiratory failure.

Ih contributes to increased motoneuron excitability in restless legs syndrome

KEY POINTS

Restless legs patients complain about sensory and motor symptoms leading to sleep disturbances. Symptoms include painful sensations, an urge to move and involuntary leg movements. The responsible mechanisms of restless legs syndrome are still not known, although current studies indicate an increased neuronal network excitability. Reflex studies indicate the involvement of spinal structures. Peripheral mechanisms have not been investigated so far. In the present study, we provide evidence of increased hyperpolarization-activated cyclic nucleotide-gated (HCN) channel-mediated inward rectification in motor axons. The excitability of sensory axons was not changed. We conclude that, in restless legs syndrome, an increased HCN current in motoneurons may play a pathophysiological role, such that these channels could represent a valuable target for pharmaceutical intervention.

ABSTRACT

Restless legs syndrome is a sensorimotor network disorder. So far, the responsible pathophysiological mechanisms are poorly understood. In the present study, we provide evidence that the excitability of peripheral motoneurons contributes to the pathophysiology of restless legs syndrome. In vivo excitability studies on motor and sensory axons of the median nerve were performed on patients with idiopathic restless legs syndrome (iRLS) who were not currently on treatment. The iRLS patients had greater accommodation in motor but not sensory axons to long-lasting hyperpolarization compared to age-matched healthy subjects, indicating greater inward rectification in iRLS. The most reasonable explanation is that hyperpolarization-activated cyclic nucleotide-gated (HCN) channels open at less hyperpolarized membrane potentials, a view supported by mathematical modelling. The half-activation potential for HCN channels (Bq) was the single best parameter that accounted for the difference between normal controls and iRLS data. A 6 mV depolarization of Bq reduced the discrepancy between the normal control model and the iRLS data by 92.1%. Taken together, our results suggest an increase in the excitability of motor units in iRLS that could enhance the likelihood of leg movements. The abnormal axonal properties are consistent with other findings indicating that the peripheral system is part of the network involved in iRLS.

Hyperpolarization-activated cyclic-nucleotide-gated channels potentially modulate axonal excitability at different thresholds

Hyperpolarization-activated cyclic-nucleotide-gated (HCN) channels mediate differences in sensory and motor axonal excitability at different thresholds in animal models. Importantly, HCN channels are responsible for voltage-gated inward rectifying (I_h) currents activated during hyperpolarization. The I_h currents exert a crucial role in determining the resting membrane potential and have been implicated in a variety of neurological disorders, including neuropathic pain. In humans, differences in biophysical properties of motor and sensory axons at different thresholds remain to be elucidated and could provide crucial pathophysiological insights in peripheral neurological diseases. Consequently, the aim of this study was to characterize sensory and motor axonal function at different threshold. Median nerve motor and sensory axonal excitability studies were undertaken in 15 healthy subjects (45 studies in total). Tracking targets were set to 20, 40, and 60% of maximum for sensory and motor axons. Hyperpolarizing threshold electrotonus (TEh) at 90-100 ms was significantly increased in lower threshold sensory axons times (F = 11.195, P < 0.001). In motor axons, the hyperpolarizing current/threshold (I/V) gradient was significantly increased in lower threshold axons (F = 3.191, P < 0.05). The minimum I/V gradient was increased in lower threshold motor and sensory axons. In conclusion, variation in the kinetics of HCN isoforms could account for the findings in motor and sensory axons. Importantly, assessing the function of HCN channels in sensory and motor axons of different thresholds may provide insights into the pathophysiological processes underlying peripheral neurological diseases in humans, particularly focusing on the role of HCN channels with the potential of identifying novel treatment targets.NEW & NOTEWORTHY Hyperpolarization-activated cyclic-nucleotide-gated (HCN) channels, which underlie inward rectifying currents (I_h), appear to mediate differences in sensory and motor axonal properties. Inward rectifying currents are increased in lower threshold motor and sensory axons, although different HCN channel isoforms appear to underlie these changes. While faster activating HCN channels seem to underlie I_h changes in sensory axons, slower activating HCN isoforms appear to be mediating the differences in I_h conductances in motor axons of different thresholds. The differences in HCN gating properties could explain the predilection for dysfunction of sensory and motor axons in specific neurological diseases.

Cortical function and corticomotoneuronal adaptation in monomelic amyotrophy

OBJECTIVE

To evaluate corticomotoneuronal integrity in monomelic amyotrophy using threshold tracking transcranial magnetic stimulation (TT-TMS).

METHODS

Cortical excitability studies were prospectively performed in 8 monomelic amyotrophy patients and compared to 21 early-onset amyotrophic lateral sclerosis (ALS) patients and 40 healthy controls. Motor evoked potentials responses were recorded over abductor pollicis brevis.

RESULTS

Maximal motor evoked potential (MEP/CMAP ratio) was significantly increased in monomelic amyotrophy compared with controls (monomelic amyotrophy 51.2±12.4%; control 22.7±2.1%, p=0.04). Averaged short-interval intracortical inhibition (SICI, ISI 1-7ms) in monomelic amyotrophy patients was similar to controls (monomelic amyotrophy 9.6±2.1%; control 10.0±0.9%, p=0.98). However, it was significantly reduced in early-onset ALS in comparison with monomelic amyotrophy patients (monomelic amyotrophy 9.6±2.1%; ALS 2.3±1.7%, p<0.001). Averaged SICI is a good parameter (area under the curve 0.79, p=0.02) to discriminate between monomelic amyotrophy and early-onset ALS patients.

CONCLUSIONS

TT-TMS technique has identified normal cortical function in monomelic amyotrophy, a feature that distinguishes it from early-onset ALS. The greater corticomotoneuronal projections to spinal motoneurons may represent central nervous system adaptive change in monomelic amyotrophy.

SIGNIFICANCE

Corticomotoneuronal dysfunction does not drive the lower motor neurone loss presented in monomelic amyotrophy.

Burning pain: axonal dysfunction in erythromelalgia

To gain insights into erythromelalgia disease pathophysiology, this study elucidated changes in peripheral axonal excitability and influences of temperature and mexiletine on axonal function.

Erythromelalgia (EM) is a rare neurovascular disorder characterized by intermittent severe burning pain, erythema, and warmth in the extremities on heat stimuli. To investigate the underlying pathophysiology, peripheral axonal excitability studies were performed and changes with heating and therapy explored. Multiple excitability indices (stimulus–response curve, strength–duration time constant (SDTC), threshold electrotonus, and recovery cycle) were investigated in 23 (9 EMSCN9A+ and 14 EMSCN9A−) genetically characterized patients with EM stimulating median motor and sensory axons at the wrist. At rest, patients with EM showed a higher threshold and rheobase (P < 0.001) compared with controls. Threshold electrotonus and current–voltage relationships demonstrated greater changes of thresholds in both depolarizing and hyperpolarizing preconditioning electrotonus in both EM cohorts compared with controls in sensory axons (P < 0.005). When average temperature was raised from 31.5°C to 36.3°C in EMSCN9A+ patients, excitability changes showed depolarization, specifically SDTC significantly increased, in contrast to the effects of temperature previously established in healthy subjects (P < 0.05). With treatment, 4 EMSCN9A+ patients (4/9) reported improvement with mexiletine, associated with reduction in SDTC in motor and sensory axons. This is the first study of primary EM using threshold tracking techniques to demonstrate alterations in peripheral axonal membrane function. Taken together, these changes may be attributed to systemic neurovascular abnormalities in EM, with chronic postischaemic resting membrane potential hyperpolarization due to Na⁺/K⁺ pump overactivity. With heating, a trigger of acute symptoms, axonal depolarization developed, corresponding to acute axonal ischaemia. This study has provided novel insights into EM pathophysiology.

In vivo evidence of reduced nodal and paranodal conductances in type 1 diabetes

OBJECTIVES

Diabetic neuropathy is a debilitating complication of diabetes. Animal models of type 1 diabetes (T1DM) suggest that functional and structural changes, specifically axo-glial dysjunction, may contribute to neuropathy development. The present study sought to examine and characterise early sensory axonal function in T1DM patients in the absence of clinical neuropathy.

METHODS

Thirty patients with T1DM (15M:15F) without neuropathy underwent median nerve sensory and motor axonal excitability studies to examine axonal function. A verified mathematical model of human motor and sensory axons was used to elucidate the underlying causes of observed alterations.

RESULTS

Compared to controls (NC), T1DM patients demonstrated significant axonal excitability abnormalities in sensory and motor axons. These included marked reductions in sensory and motor subexcitability during the recovery cycle (T1DM 7.9 ± 0.4:10.4 ± 0.6%, NC 10.4 ± 0.7:15.4 ± 1.2%, P<0.01) and during hyperpolarizing threshold electrotonus at 10-20 ms (T1DM -75.5 ± 0.8:-69.7 ± 0.8%, NC -78.4 ± 1:-72.7 ± 0.9%, P<0.01). Mathematical modelling demonstrated that these changes were due to reduced nodal Na(+) currents, nodal/paranodal K(+) conductances and Na(+)/K(+) pump dysfunction, consistent with axo-glial dysjunction as outlined in animal models of T1DM.

CONCLUSIONS

The study provided support for the occurrence of early changes in nodal and paranodal conductances in patients with T1DM.

SIGNIFICANCE

These data indicate that axonal excitability techniques may detect early changes in diabetic patients, providing a window of opportunity for prophylactic intervention in T1DM.

Clinical presentation and electrophysiological findings of porphyric neuropathies: a follow-up study

INTRODUCTION

A case series of acute intermittent porphyria (AIP) is described that focuses on the clinical course of the disease with regard to neurological manifestations of the peripheral nervous system.

METHODS

Eight patients were diagnosed with AIP on the basis of characteristic clinical findings, erythrocyte porphobilinogendeaminase activity, neuropathic patterns, serial changes in nerve conduction studies (NCS), and temporal relationship of central nervous system involvement.

RESULTS

Six patients diagnosed with AIP<2 months after symptom onset had neuropathy that was predominantly upper extremity, motor, and proximal. NCS recovery rates were slower in the lower than the upper limbs. Two patients diagnosed >2 months after symptom onset had distal sensorimotor polyneuropathy.

CONCLUSIONS

The findings from this case series suggest that the peripheral nerves may be differentially and selectively involved in different diagnostic stages of porphyric neuropathy.

Potassium and the excitability properties of normal human motor axons in vivo

Hyperkalemia is an important cause of membrane depolarization in renal failure. A recent theoretical model of axonal excitability explains the effects of potassium on threshold electrotonus, but predicts changes in superexcitability in the opposite direction to those observed. To resolve this contradiction we assessed the relationship between serum potassium and motor axon excitability properties in 38 volunteers with normal potassium levels. Most threshold electrotonus measures were strongly correlated with potassium, and superexcitability decreased at higher potassium levels (P = 0.016), contrary to the existing model. Improved modelling of potassium effects was achieved by making the potassium currents obey the constant-field theory, and by making the potassium permeabilities proportional to external potassium, as has been observed in vitro. This new model also accounted well for the changes in superexcitability and other excitability measures previously reported in renal failure. These results demonstrate the importance of taking potassium levels into account when assessing axonal membrane dysfunction by excitability testing, and provide evidence that potassium currents are activated by external potassium in vivo.

Quantitative proteomics reveals diverse roles of miR-148a from gastric cancer progression to neurological development

MicroRNAs (miRNAs) are noncoding RNAs that control gene expression either by degradation of mRNAs or inhibition of protein translation. miR-148a has been reported to have the impacts on tumor progression. Here, a quantitative proteomics combined with stable isotope labeling was applied to identify the global profile of miR-148a-regulated downstream proteins. The data have been deposited to the ProteomeXchange with identifier PXD000190. A total of 2938 proteins were quantified, and 55 proteins were considered to be regulated by miR-148a. We found that not only proteins associated with cancer progression but also molecules involved in neural development were regulated by miR-148a. This study is the first to identify the function of miR-148a in neural development by using a proteomic approach. Analysis of a public clinical database also showed that the patients with neural diseases could display abnormal expression of miR-148a. Moreover, silencing of miR-148a led to the abnormal morphology and decreased expression of neuron-related markers in the developing brain of zebrafish. These results provided important insight into the regulation of neurological development elicited by miR-148a.

Evidence for a causal relationship between hyperkalaemia and axonal dysfunction in end-stage kidney disease

OBJECTIVE

Potassium (K(+)) has been implicated as a factor in the development of uraemic neuropathy. This study was undertaken to investigate whether hyperkalaemia plays a causal role in axonal dysfunction in end-stage kidney disease (ESKD).

METHODS

Median motor nerve excitability studies were undertaken in four haemodialysis patients during a modified dialysis session. The serum K(+) level was "clamped" (fixed) for the first 3h of dialysis, whilst allowing all other solutes to be removed, this was followed by dialysis against low dialysate K(+) for a further 4 h. Blood chemistry and nerve excitability studies were undertaken prior to, during and following dialysis. Results were compared to results from the same patients during routine dialysis sessions.

RESULTS

All patients demonstrated significant nerve excitability abnormalities reflective of nerve membrane depolarization in pre-dialysis recordings (p<0.01). After the 3 h clamp period, serum K(+) remained elevated (5.0 mmol/L) and nerve excitability remained highly abnormal, despite the significant clearance of other uraemic toxins. In contrast, studies undertaken during routine dialysis sessions demonstrated significant improvement in both serum K(+) and nerve function after 3 h.

CONCLUSIONS

The current study has established a causal relationship between serum K(+) and axonal membrane depolarization in haemodialysis patients.

SIGNIFICANCE

From a clinical perspective, strict K(+) control may help improve nerve function in ESKD.

Excitability and the safety margin in human axons during hyperthermia

Abstract  Hyperthermia challenges the nervous system's ability to transmit action potentials faithfully. Neuromuscular diseases, particularly those involving demyelination have an impaired safety margin for action potential generation and propagation, and symptoms are commonly accentuated by increases in temperature. The aim of this study was to examine the mechanisms responsible for reduced excitability during hyperthermia. Additionally, we sought to determine if motor and sensory axons differ in their propensity for conduction block during hyperthermia. Recordings of axonal excitability were performed at normal temperatures and during focal hyperthermia for motor and sensory axons in six healthy subjects. There were clear changes in excitability during hyperthermia, with reduced superexcitability following an action potential, faster accommodation to long-lasting depolarization and reduced accommodation to hyperpolarization. A verified model of human motor and sensory axons was used to clarify the effects of hyperthermia. The hyperthermia-induced changes in excitability could be accounted for by increasing the modelled temperature by 6°C (and adjusting the maximum conductances and activation kinetics according to their Q10 values; producing a 2 mV hyperpolarization of resting membrane potential), further hyperpolarizing the voltage dependence of Ih (motor, 11 mV; sensory, 7 mV) and adding a small depolarizing current at the internode (motor, 20 pA; sensory, 30 pA). The modelling suggested that slow K(+) channels play a significant role in reducing axonal excitability during hyperthermia. The further hyperpolarization of the activation of Ih would limit its ability to counter the hyperpolarization produced by activity, thereby allowing conduction block to occur during hyperthermia.

Effects of hemodiafiltration and high flux hemodialysis on nerve excitability in end-stage kidney disease

OBJECTIVES

Peripheral neuropathy is the most common neurological complication in end-stage kidney disease. While high flux hemodialysis (HFHD) and hemodiafiltration (HDF) have become the preferred options for extracorporeal dialysis therapy, the effects of these treatments on nerve excitability have not yet been examined.

METHODS

An observational proof-of-concept study of nerve excitability and neuropathy was undertaken in an incident dialysis population (n = 17) receiving either HFHD or HDF. Nerve excitability techniques were utilised to assess nerve ion channel function and membrane potential, in conjunction with clinical assessment and standard nerve conduction studies. A mathematical model of axonal excitability was used to investigate the underlying basis of the observed changes. Nerve excitability was recorded from the median nerve, before, during and after a single dialysis session and correlated with corresponding biochemical markers. Differences in nerve excitability were compared to normal controls with longitudinal follow-up over an 18 month period.

RESULTS

Nerve excitability was performed in patient cohorts treated with either HFHD (n = 9) or online HDF (n = 8), with similar neuropathy status. Nerve excitability measures in HDF-treated patients were significantly closer to normal values compared to HFHD patients obtained over the course of a dialysis session (p<0.05). Longitudinal studies revealed stability of nerve excitability findings, and thus maintenance of improved nerve function in the HDF group.

CONCLUSIONS

This study has provided evidence that nerve excitability in HDF-treated patients is significantly closer to normal values prior to dialysis, across a single dialysis session and at longitudinal follow-up. These findings offer promise for the management of neuropathy in ESKD and should be confirmed in randomised trials.

Porphyric neuropathy

Porphyric neuropathy often poses a diagnostic dilemma; it is typically associated with the hepatic porphyrias, characterized by acute life-threatening attacks of neurovisceral symptoms that mimic a range of acute medical and psychiatric conditions. The development of acute neurovisceral attacks is responsive to environmental factors, including drugs, hormones, and diet. This chapter reviews the clinical manifestations, genetics, pathophysiology, and mechanisms of neurotoxicity of the acute hepatic porphyrias. While the etiology of the neurological manifestations in the acute porphyrias remains undefined, the main hypotheses include toxicity of porphyrin precursors and deficiency of heme synthesis. These hypotheses will be discussed with reference to novel experimental models of porphyric neuropathy.

Evolution of peripheral nerve function in humans: novel insights from motor nerve excitability

While substantial alterations in myelination and axonal growth have been described during maturation, their interactions with the configuration and activity of axonal membrane ion channels to achieve impulse conduction have not been fully elucidated. The present study utilized axonal excitability techniques to compare the changes in nerve function across healthy infants, children, adolescents and adults. Multiple excitability indices (stimulus-response curve, strength-duration time constant, threshold electrotonus, current-threshold relationship and recovery cycle) combined with conventional neurophysiological measures were investigated in 57 subjects (22 males, 35 females; age range 0.46-24 years), stimulating the median motor nerve at the wrist. Maturational changes in conduction velocity were paralleled by significant alterations in multiple excitability parameters, similarly reaching steady values in adolescence. Maturation was accompanied by reductions in threshold (P < 0.005) and rheobase (P = 0.001); depolarizing and hyperpolarizing electrotonus progressively reduced (P < 0.001), or 'fanned-in'; resting current-threshold slope increased (P < 0.0001); accommodation to depolarizing currents prolonged (P < 0.0001); while greater threshold changes in refractoriness (P = 0.001) and subexcitability (P < 0.01) emerged. Taken together, the present findings suggest that passive membrane conductances and the activity of K(+) conductances decrease with formation of the axo-glial junction and myelination. In turn, these functional alterations serve to enhance the efficiency and speed of impulse conduction concurrent with the acquisition of motor skills during childhood, and provide unique insight into the evolution of postnatal human peripheral nerve function. Significantly, these findings bring the dynamics of axonal development to the clinical domain and serve to further illuminate pathophysiological mechanisms that occur during development.

Neurophysiological approach to disorders of peripheral nerve

Disorders of the peripheral nerve system (PNS) are heterogeneous and may involve motor fibers, sensory fibers, small myelinated and unmyelinated fibers and autonomic nerve fibers, with variable anatomical distribution (single nerves, several different nerves, symmetrical affection of all nerves, plexus, or root lesions). Furthermore pathological processes may result in either demyelination, axonal degeneration or both. In order to reach an exact diagnosis of any neuropathy electrophysiological studies are crucial to obtain information about these variables. Conventional electrophysiological methods including nerve conduction studies and electromyography used in the study of patients suspected of having a neuropathy and the significance of the findings are discussed in detail and more novel and experimental methods are mentioned. Diagnostic considerations are based on a flow chart classifying neuropathies into eight categories based on mode of onset, distribution, and electrophysiological findings, and the electrophysiological characteristics in each type of neuropathy are discussed.

Different mechanisms underlying changes in excitability of peripheral nerve sensory and motor axons in multiple sclerosis

INTRODUCTION

Subtle involvement of peripheral nerves may occur in multiple sclerosis. Motor excitability studies have suggested upregulation of slow K+ currents, probably secondary to altered motoneuron properties resulting from the central lesion. This study concentrates on sensory axons.

METHODS

Excitability of median nerve axons at the wrist was studied in 26 patients.

RESULTS

Sensory recordings were possible in 22 patients, and reduced superexcitability was the sole abnormality. There was no evidence for changes in membrane potential or demyelination. The decrease was significant in patients taking immunomodulatory therapy. These findings could be reproduced in a computer model by changing the gating of fast K+ channels. Motor axon findings were consistent with previously reported increased slow K+ current.

CONCLUSIONS

The sensory findings differ from motor findings. They can be explained by a humoral factor, possibly cytokines, which can penetrate the paranode and have been documented to alter the gating of K+ channels.

Clarifying distal axonal properties of the median nerve

INTRODUCTION

Although length-dependent axonal excitability changes have been reported in the median nerve, the mechanisms underlying these changes remain to be further clarified.

METHODS

Axonal excitability studies were performed on median nerve at the palm and wrist in 20 healthy controls, with responses recorded over the abductor pollicis brevis.

RESULTS

The strength-duration time constant was significantly shorter (palm: 0.35 ± 0.01 ms; wrist: 0.48 ± 0.03 ms; P < 0.001), whereas rheobase was significantly increased (palm: 2.90 ± 1.12 mA; wrist: 2.09 ± 1.11 mA; P < 0.05) at the palm. In addition, there was a significant increase in depolarizing threshold electrotonus at 90-100 ms (P < 0.001) and a reduction in S2 accommodation (P < 0.001) and late subexcitability (P < 0.001) at the palm. The changes in excitability were independent of factors influencing median nerve cross-sectional area.

CONCLUSIONS

The present study reveals significant length dependent changes in median nerve excitability which may reflect differences in intrinsic membrane properties.

The voltage dependence of I(h) in human myelinated axons

HCN channels are responsible for I(h), a voltage-gated inwardly rectifying current activated by hyperpolarization. This current appears to be more active in human sensory axons than motor and may play a role in the determination of threshold. Differences in I(h) are likely to be responsible for the high variability in accommodation to hyperpolarization seen in different subjects. The aim of this study was to characterise this current in human axons, both motor and sensory. Recordings of multiple axonal excitability properties were performed in 10 subjects, with a focus on the changes in threshold evoked by longer and stronger hyperpolarizing currents than normally studied. The findings confirm that accommodation to hyperpolarization is greater in sensory than motor axons in all subjects, but the variability between subjects was greater than the modality difference. An existing model of motor axons was modified to take into account the behaviour seen with longer and stronger hyperpolarization, and a mathematical model of human sensory axons was developed based on the data collected. The differences in behaviour of sensory and motor axons and the differences between different subjects are best explained by modulation of the voltage dependence, along with a modest increase of expression of the underlying conductance of I(h). Accommodation to hyperpolarization for the mean sensory data is fitted well with a value of -94.2 mV for the mid-point of activation (V(0.5)) of I(h) as compared to -107.3 mV for the mean motor data. The variation in response to hyperpolarization between subjects is accounted for by varying this parameter for each modality (sensory: -89.2 to -104.2 mV; motor -87.3 to -127.3 mV). These voltage differences are within the range that has been described for physiological modulation of I(h) function. The presence of slowly activated I(h) isoforms on both motor and sensory axons was suggested by modelling a large internodal leak current and a masking of the Na(+)/K(+)-ATPase pump activity by a tonic depolarization. In addition to an increased activation of I(h), the modelling suggests that in sensory axons the nodal slow K(+) conductance is reduced, with consequent depolarization of resting membrane potential, and action potential of shorter duration.

Neurological complications of acute intermittent porphyria

BACKGROUND

Acute intermittent porphyria (AIP) is an inherited disorder of heme biosynthesis, the clinical manifestations of which are incompletely understood. In this report, we describe 12 cases of AIP, focusing on the neurological manifestations.

METHODS

Twelve patients were diagnosed with AIP on the basis of characteristic clinical findings, erythrocyte porphobilinogen deaminase (PBGD) activity, and molecular genetics. Central and peripheral nervous system manifestations were noted, and electrophysiological and radiological studies performed. Potential precipitating factors were recorded.

RESULTS

Eleven PBGD gene mutations were identified in 12 patients. Nine patients experienced neurological symptoms involving the central nervous system (consciousness disturbance, n = 8; convulsion/seizure, n = 4; behavior change, n = 1), while 7 patients experienced peripheral neuropathies (motor paresis, n = 7; impairment of bulbar or respiratory function, n = 4). The electrophysiological and electroencephalographic findings were consistent with the neurological symptoms of AIP. Urinary PBG and δ-aminolevulinic acid levels were elevated in all patients. PBGD enzyme activity levels were below normal in all patients. Eight patients had documented exposure to porphyrogenic agents.

CONCLUSIONS

Our detailed description of a relatively large number of cases of AIP may help clinicians to recognize this often difficult-to-diagnose disorder.

Purple pigments: the pathophysiology of acute porphyric neuropathy

The porphyrias are inherited metabolic disorders arising from disturbance in the haem biosynthesis pathway. The neuropathy associated with acute intermittent porphyria (AIP) occurs due to mutation involving the enzyme porphobilinogen deaminase (PBGD) and is characterised by motor-predominant features. Definitive diagnosis often encompasses a combination of biochemical, enzyme analysis and genetic testing, with clinical neurophysiological findings of a predominantly motor axonal neuropathy. Symptomatic and supportive treatment are the mainstays during an acute attack. If administered early, intravenous haemin may prevent progression of neuropathy. While the pathophysiology of AIP neuropathy remains unclear, axonal dysfunction appears intrinsically linked to the effects of neural energy deficits acquired through haem deficiency coupled to the neurotoxic effects of porphyrin precursors. The present review will provide an overview of AIP neuropathy, including discussion of recent advances in understanding developed through neurophysiological approaches that have further delineated the pathophysiology of axonal degeneration.

Neuropathic pain in a patient with porphyria. Case report

BACKGROUND AND OBJECTIVES

Porphyrias represent a group of inherited or acquired disorders that involve enzymes that participate in heme synthesis. Acute manifestations affect the nervous system resulting in abdominal pain, vomiting, acute neuropathy, seizures, and mental disorders. The physiopathogeny results from the toxic effects of porphyrin precursors, and it can be triggered by drugs used routinely in medical practice, severe carbohydrate restriction, and metabolic stress. The objective of this report was to present the case of a late onset porphyria evolving to chronic pain.

CASE REPORT

This is a 27 years old female who was admitted 5 months prior to her clinic appointment with severe abdominal pain without diagnosis. An exploratory laparotomy was indicated, which failed to demonstrate a cause for her pain. The patient was exposed to surgical trauma and drugs that can trigger porphyria, such as ketoprophen, metoclopramide, and antibiotics, and she evolved with significant hyponatremia, increased liver enzymes, seizures, and loss of movements that led to be admitted to the Intensive Care Unit. After the diagnosis of porphyria was made, the patient remained with pain in the lower limbs, being referred to the Pain Department of Santa Casa de Misericórdia de São Paulo. Treatment with amitriptyline, gabapentin, opioid, and simple analgesics was instituted; however, the patient continued to present recurring episodes of porphyria and, feeling insecure with the conduction of the case, she stopped the treatment.

CONCLUSIONS

Porphyria is one of the rare groups of enzymatic disorders that remain unknown by the great majority of health professionals. Upon recognizing this lack of knowledge about the disease, the level of stress and insecurity of the patient increases hindering adhesion to and continuity of the treatment.