Ethambutol toxicity exacerbating the phenotype of CMT2A2

Maculopapillary Bundle Degeneration in Optic Neuropathies

PURPOSE OF REVIEW

Degeneration of the maculopapillary bundle (MPB) is a prominent feature in a spectrum of optic neuropathies. MPB-selective degeneration is seen in specific conditions, such as nutritional and toxic optic neuropathies, Leber hereditary optic neuropathy (LHON), and dominant optic atrophy (DOA). Despite their distinct etiologies and clinical presentations, which encompass variations in age of incidence and monocular or binocular onset, these disorders share a core molecular mechanism: compromised mitochondrial homeostasis. This disruption is characterized by dysfunctions in mitochondrial metabolism, biogenesis, and protein synthesis. This article provides a comprehensive understanding of the MPB's role in optic neuropathies, emphasizing the importance of mitochondrial mechanisms in the pathogenesis of these conditions.

RECENT FINDINGS

Optical coherence tomography studies have characterized the retinal nerve fiber layer changes accompanying mitochondrial-affiliated optic neuropathies. Selective thinning of the temporal optic nerve head is preceded by thickening in early stages of these disorders which correlates with reductions in macular ganglion cell layer thinning and vascular atrophy. A recently proposed mechanism underpinning the selective atrophy of the MPB involves the positive feedback of reactive oxygen species generation as a common consequence of mitochondrial dysfunction. Additionally, new research has revealed that the MPB can undergo degeneration in the early stages of glaucoma, challenging the historically held belief that this area was not involved in this common optic neuropathy. A variety of anatomical risk factors influence the propensity of glaucomatous MPB degeneration, and cases present distinct patterns of ganglion cell degeneration that are distinct from those observed in mitochondria-associated diseases. This review synthesizes clinical and molecular research on primary MPB disorders, highlighting the commonalities and differences in their pathogenesis.

KEY POINTS (BOX)

1. Temporal degeneration of optic nerve fibers accompanied by cecocentral scotoma is a hallmark of maculopapillary bundle (MPB) degeneration. 2. Mechanisms of MPB degeneration commonly implicate mitochondrial dysfunction. 3. Recent research challenges the traditional belief that the MPB is uninvolved in glaucoma by showing degeneration in the early stages of this common optic neuropathy, yet with features distinct from other MPB-selective neuropathies. 4. Reactive oxygen species generation is a mechanism linking mitochondrial mechanisms of MPB-selective optic neuropathies, but in-vivo and in-vitro studies are needed to validate this hypothesis.

Potential underlying mechanisms of ethambutol induced optic neuropathy: Evidence from in vitro to clinical studies

Ethambutol is an antibiotic widely used for treatment of Mycobacterium species. Although it is safe to use in patients, the ocular toxic impact, including optic neuropathy and retinopathy, can be observed in patients using ethambutol. After discontinuation of the drug, the ocular toxic effects can be reversible in some patients, but some are not. Ethambutol-induced optic neuropathy has been recognized for more than six decades and the prevalence of optic neuropathy from a standard dose of ethambutol has been reported as 0.7-1.29%. Several factors associated with ethambutol-induced optic neuropathy include dosage/duration of drug, the medical conditions of patients such as renal and hepatic dysfunction and preexisting mitochondrial mutations. Currently, there is no specific treatment and prevention of ethambutol-induced optic neuropathy. In addition, the potential underlying mechanisms of ethambutol-induced optic neuropathy is still unclear. Therefore, this review aimed to summarize and discuss evidence from clinical, in vitro, and in vivo studies in order to explore the potential pathophysiology of ethambutol-induced optic neuropathy. Any contradictory findings are also included and discussed. The insights gained from the review will facilitate the discovery of novel approaches for prevention and treatment of optic neuropathy-induced by ethambutol.

Genetic polymorphism related to ethambutol outcomes and susceptibility to toxicity

The World Health Organization (WHO) stated that ensuring access to effective and optimal treatment is a key component to eradicate tuberculosis (TB) through the End TB Strategy. Personalized medicine that depends on the genetic profile of an individual is one way to optimize treatment. It is necessary because of diverse drug responses related to the variation in human DNA, such as single-nucleotide polymorphisms (SNPs). Ethambutol (EMB) is a drug widely used as the treatment for Mycobacterium Tuberculosis (Mtb) and/non-tuberculous mycobacteria and has become a potential supplementary agent for a treatment regimen of multidrug-resistant (MDR) and extensively drug-resistant (XDR) TB. In human genetic polymorphism studies of anti-tuberculosis, the majority focus on rifampicin or isoniazid, which discuss polymorphisms related to their toxicity. Whereas there are few studies on EMB, the incidence of EMB toxicity is lower than that of other first-line anti-TB drugs. To facilitate personalized medicine practice, this article summarizes the genetic polymorphisms associated with alterations in the pharmacokinetic profile, resistance incidence, and susceptibility to EMB toxicity. This study includes 131 total human studies from 17 articles, but only eight studies that held in the low-middle income country (LMIC), while the rest is research conducted in developed countries with high incomes. Personalized medicine practices are highly recommended to maintain and obtain the optimal therapeutic effect of EMB.

The Genetics of Ethambutol-Induced Optic Neuropathy: A Narrative Review

Tuberculosis (TB) is a global health problem with the major brunt of disease occurring in developing countries. The cornerstone of treatment of TB is anti-tubercular therapy (ATT), which includes rifampicin, isoniazid, pyrazinamide and ethambutol. Because of emerging drug resistance, treatment failures, defaulters and increasing incidence of disseminated and extrapulmonary TB, the guidelines have been modified in some countries. Ethambutol is prescribed for longer times (in some cases >8 months) and hence the incidence of ethambutol-induced optic neuropathy (EtON) is expected to rise. The fundamental question which needs explanation is why only a small subset of patients on ethambutol are prone to develop loss of vision. This review focuses on available genetic studies which provide evidence that mitochondria are the likely substrates involved in the final pathway of reactive oxidative damage of the papillo-macular bundle. Genetic analysis of mitochondrial mutations encoding genes involved in oxidative phosphorylation pathways may help in isolating the subset of patients who are genetically susceptible. If the groups having high risk of developing EtON are recognised then prolonged duration of ethambutol treatment can be avoided in these susceptible individuals. A better understanding of the pathophysiology will also pave the way for the development of management strategies in this condition.

Sibling Ethambutol Optic Chiasmopathy

Ethambutol is utilised in the treatment of Mycobacterium avium and Mycobacterium tuberculosis infection. The authors report two siblings who developed the adverse effect of ethambutol-induced optic chiasmopathy, with recovery following cessation of ethambutol. Discussion explores potential genetic predisposition to development of this condition and its resolution. Ethambutol optic neuropathy (EON), Leber's hereditary optic neuropathy (LHON), and other optic neuropathies of mitochondrial origin share a common pathophysiology. Consequently, the authors postulate treatments utilised in LHON, including vitamin B supplementation and idebenone, may have benefit in EON. This article presents concepts for further research, suggesting a potential genetic susceptibility to EON and its treatment.

Mitochondrial fusion, fission, and mitochondrial toxicity

Mitochondrial dynamics are regulated by two sets of opposed processes: mitochondrial fusion and fission, and mitochondrial biogenesis and degradation (including mitophagy), as well as processes such as intracellular transport. These processes maintain mitochondrial homeostasis, regulate mitochondrial form, volume and function, and are increasingly understood to be critical components of the cellular stress response. Mitochondrial dynamics vary based on developmental stage and age, cell type, environmental factors, and genetic background. Indeed, many mitochondrial homeostasis genes are human disease genes. Emerging evidence indicates that deficiencies in these genes often sensitize to environmental exposures, yet can also be protective under certain circumstances. Inhibition of mitochondrial dynamics also affects elimination of irreparable mitochondrial DNA (mtDNA) damage and transmission of mtDNA mutations. We briefly review the basic biology of mitodynamic processes with a focus on mitochondrial fusion and fission, discuss what is known and unknown regarding how these processes respond to chemical and other stressors, and review the literature on interactions between mitochondrial toxicity and genetic variation in mitochondrial fusion and fission genes. Finally, we suggest areas for future research, including elucidating the full range of mitodynamic responses from low to high-level exposures, and from acute to chronic exposures; detailed examination of the physiological consequences of mitodynamic alterations in different cell types; mechanism-based testing of mitotoxicant interactions with interindividual variability in mitodynamics processes; and incorporating other environmental variables that affect mitochondria, such as diet and exercise.

Vocal cord paralysis in Charcot-Marie-Tooth type 4b1 disease associated with a novel mutation in the myotubularin-related protein 2 gene: A case report and review of the literature

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Vocal cord paralysis is a relevant symptom of Charcot–Marie–Tooth type 4B1.

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Patients harboring MTMR2 mutations should be investigated for laryngeal function.

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A new mutation in the MTMR2 gene is described.

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The frequency of vocal cord paralysis in early-onset CMT subtypes is explored.

Charcot–Marie–Tooth type 4B1 (CMT4B1) is an autosomal recessive motor and sensory demyelinating neuropathy characterized by the association of early-onset neurological symptoms and typical histological findings. The natural history and the clinical variability of the disease are still poorly known, thus further clarification of the different phenotypes is needed. We report on the case of a Pakistani girl born to consanguineous parents harboring a novel mutation in the MTMR2 gene. When aged 18 months, reduced limb tone, muscle wasting associated with proximal and distal weakness prevalent in lower limbs, absence of tendon reflexes, hoarseness and inspiratory stridor were detected. Vocal cord palsy was diagnosed shortly after. We suggest that laryngeal involvement might be a relevant and initial feature of early-onset CMT4B1 neuropathy. Thus, affected patients should undergo early laryngological evaluation in order to prompt an appropriate management.

Macular thickness as a predictor of loss of visual sensitivity in ethambutol-induced optic neuropathy

Ethambutol is a common cause of drug-related optic neuropathy. Prediction of the onset of ethambutol-induced optic neuropathy and consequent drug withdrawal may be an effective method to stop visual loss. Previous studies have shown that structural injury to the optic nerve occurred earlier than the damage to visual function. Therefore, we decided to detect structural biomarkers marking visual field loss in early stage ethambutol-induced optic neuropathy. The thickness of peripapillary retinal nerve fiber layer, macular thickness and visual sensitivity loss would be observed in 11 ethambutol-induced optic neuropathy patients (22 eyes) using optical coherence tomography. Twenty-four healthy age- and sex-matched participants (48 eyes) were used as controls. Results demonstrated that the temporal peripapillary retinal nerve fiber layer thickness and average macular thickness were thinner in patients with ethambutol-induced optic neuropathy compared with healthy controls. The average macular thickness was strongly positively correlated with central visual sensitivity loss (r² =0.878, P=0.000). These findings suggest that optical coherence tomography can be used to efficiently screen patients. Macular thickness loss could be a potential factor for predicting the onset of ethambutol-induced optic neuropathy.

The genetics of Charcot-Marie-Tooth disease: current trends and future implications for diagnosis and management

Charcot–Marie–Tooth (CMT) disease is the most common hereditary polyneuropathy and is classically associated with an insidious onset of distal predominant motor and sensory loss, muscle wasting, and pes cavus. Other forms of hereditary neuropathy, including sensory predominant or motor predominant forms, are sometimes included in the general classification of CMT, but for the purpose of this review, we will focus primarily on the forms associated with both sensory and motor deficits. CMT has a great deal of genetic heterogeneity, leading to diagnostic considerations that are still rapidly evolving for this disorder. Clinical features, inheritance pattern, gene mutation frequencies, and electrodiagnostic features all are helpful in formulating targeted testing algorithms in practical clinical settings, but these still have shortcomings. Next-generation sequencing (NGS), combined with multigene testing panels, is increasing the sensitivity and efficiency of genetic testing and is quickly overtaking targeted testing strategies. Currently, multigene panel testing and NGS can be considered first-line in many circumstances, although obtaining initial targeted testing for the PMP22 duplication in CMT patients with demyelinating conduction velocities is still a reasonable strategy. As technology improves and cost continues to fall, targeted testing will be completely replaced by multigene NGS panels that can detect the full spectrum of CMT mutations. Nevertheless, clinical acumen is still necessary given the variants of uncertain significance encountered with NGS. Despite the current limitations, the genetic diagnosis of CMT is critical for accurate prognostication, genetic counseling, and in the future, specific targeted therapies. Although whole exome and whole genome sequencing strategies have the power to further elucidate the genetics of CMT, continued technological advances are needed.

Ethambutol-related impaired visual function in childrens less than 5 years of age treated for a mycobacterial infection: diagnosis and evolution

BACKGROUND

The effects of ethambutol (EMB) on vision are particularly difficult to detect in children less than 5 years of age because of a lack of complaints and objective clinical signs. The aim of this study was to assess the frequency of visual abnormalities and the utility of visual-evoked potentials (VEPs) recordings in monitoring the visual function of children less than 5 years of age who were exposed to EMB during anti-mycobacterial treatment.

METHODS

We performed a retrospective study in Robert-Debré University Hospital, Paris, France, including all children less than 5 years of age, who were treated with EMB for a mycobacterial infection from January 2002 to December 2012.

RESULTS

Fourteen patients were enrolled, including 12 treated for Mycobacterium tuberculosis infection. The sex ratio was 1:1. The median age was 1.65 years (0.3 to 4.7). Five patients had subarachnoid involvement. The median EMB dose was 22 mg/kg/day (15 to 27). Only 11 patients were monitored using VEPs. Three children (27.3%) developed a visual impairment secondary to EMB, with delays of 4, 7 and 36 weeks. One of the 3 patients developed an impairment of the retrochiasmatic visual pathways, and 2 other patients developed classical retrobulbar optic neuritis. In all cases, the discontinuation of EMB resulted in a normalization of these findings.

CONCLUSION

Alterations in visual function related to the use of EMB are not uncommon in young children and are most likely underestimated. Systematic close monitoring using VEPs recordings is needed in young children treated with EMB.

Toxic optic neuropathies

PURPOSE OF REVIEW

Many causes of toxic optic neuropathy have been described to date and novel causes of toxicity are continuously being added to the current literature. The pathophysiological basis for the toxicity or a direct causal relationship is yet to be determined for many of these agents. This review highlights the reports made over the last year about the commonly reported agents, with emphasis on the mechanisms of toxicity.

RECENT FINDINGS

Mitochondria of retinal ganglion cells and papillomacular bundle in particular could be the common target of many causes of toxic optic neuropathy, if not all. Agents or their metabolites responsible for the toxicity seem to interfere with the oxidative phosphorylation in mitochondria, causing a buildup of reactive oxidation species, energy depletion, oxidative stress, and activation of apoptosis.

SUMMARY

Further data are still necessary to understand how some of the usual suspects cause damage to the optic nerve or whether they indeed cause damage or not. A basic algorithm, as proposed, could be a useful addition to discriminate the novel causes of toxic optic neuropathy.

VIDEO ABSTRACT

See the Supplemental Digital Content 1 (http://links.lww.com/COOP/A11).