Those diseases understood as congenital in origin could either be specific to the ocular organ system (LHON, DOA) or syndromic (MELAS, Multiple Sclerosis). It is estimated that these inherited optic neuropathies in the aggregate affect 1 in 10,000

Of the acquired category, disease falls into further etiological distinction as arising from toxic (drugs or chemicals) or nutritional/metabolic (vitamin deficiency/diabetes) insult. It is worth mentioning that under-nutrition and toxic insult can occur simultaneously, so a third category may be understood as having a combined or mixed etiology. We will refer to this as Toxic/Nutritional Optic Neuropathy, whereby nutritional deficiencies and toxic/metabolic insults are the simultaneous culprits of visual loss associated with damage and disruption of the RGC and optic nerve mitochondria.

Toxic optic neuropathy refers to the ingestion of a toxin or an adverse drug reaction that results in vision loss from optic nerve damage. Patients may report either a sudden loss of vision in both eyes, in the setting of an acute intoxication, or an insidious asymmetric loss of vision from an adverse drug reaction. The most important aspect of treatment is recognition and drug withdrawal.

Among the many causes of TON, the top 10 toxins include:

- Medications

- Ethambutol, rifampin, isoniazid, streptomycin (tuberculosis treatment)

- Linezolid (taken for bacterial infections, including pneumonia)

- Chloramphenicol (taken for serious infections not helped by other antibiotics)

- Isoretinoin (taken for severe acne that fails to respond to other treatments)

- Ciclosporin (widely used immunosuppressant)

- Acute Toxins

- Methanol (component of some moonshine, and some cleaning products)

- Ethylene glycol (present in anti-freeze and hydraulic brake fluid)

Metabolic disorders may also cause this version of disease. Systemic problems such as diabetes mellitus, kidney failure, and thyroid disease can cause optic neuropathy, which is likely through buildup of toxic substances within the body. In most cases, the cause of the toxic neuropathy impairs the tissue’s vascular supply or metabolism. It remains unknown as to why certain agents are toxic to the optic nerve while others are not and why particularly the papillomacular bundle gets affected.

Idebenone is a short-chain benzoquinone that interacts with the mitochondrial electron transport chain to enhance cellular respiration. When used in individuals with LHON, it is believed to allow electrons to bypass the dysfunctional complex I. Successful treatment using idebenone was initially reported in a small number of patients.

Two large-scale studies have demonstrated the benefits of idebenone. The Rescue of Hereditary Optic Disease Outpatient Study (RHODOS) evaluated the effects of idebenone in 85 patients with LHON who had lost vision within the prior five years. In this study, the group taking idebenone 900 mg per day for 24 weeks showed a slight improvement in visual acuity compared to the placebo group, though this difference was not statistically significant. Importantly, however, patients taking idebenone were protected from further vision loss, whereas the placebo group had a steady decline in visual acuity. Further, individuals taking idebenone demonstrated preservation of color vision and persistence of the effects of idebenone 30 months after discontinuing therapy. A retrospective analysis of 103 LHON patients by Carelli et al. builds upon these results. This study highlighted that 44 subjects who were treated with idebenone within one year of onset of vision loss had better outcomes, and, further, that these improvements with idebenone persisted for years.

Idebenone, combined with avoidance of smoke and limitation of alcohol intake, is the preferred standard treatment protocol for patients affected by LHON. Idebenone doses are prescribed to be taken spaced out throughout the day, rather than all at one time. For example, to achieve a dose of 900 mg per day, patients take 300 mg three times daily with meals. Idebenone is fat soluble, and may be taken with a moderate amount of dietary fat in each meal to promote absorption. It is recommended that patients on idebenone also take vitamin C 500 mg daily to keep idebenone in its reduced form, as it is most active in this state.

Treatment of toxic and nutritional optic neuropathy is dictated by the cause of the disorder.

- Toxic optic neuropathy is treated by identification and removal of the offending agent. Depending upon the individual affected, the nature of the agent, total exposure prior to removal, and degree of vision loss at the time of diagnosis, the prognosis is variable.

- Nutritional optic neuropathy is treated with improved nutrition. A well-balanced diet with plenty of protein and green leafy vegetables, vitamin supplementation (thiamine, vitamin B, folic acid, multivitamins), and reduction of smoking and/or drinking are the mainstay of treatment. Again, prognosis is variable and dependent upon the affected individual, treatment compliance, and degree of vision loss at diagnosis.

In both toxic and nutritional neuropathy, vision generally recovers to normal over several days to weeks, though it may take months for full restoration and there is always the risk of permanent vision loss. Visual acuity usually recovers before color vision.

Currently there is no effective therapy for dominant optic atrophy, and consequently, these patients are simply monitored for changes in vision by their eye-care professional. Children of patients should be screened regularly for visual changes related to dominant optic atrophy. Research is underway to further characterize the disease so that therapies may be developed.

Currently, human clinical trials are underway at GenSight Biologics (ClinicalTrials.gov # NCT02064569) and the University of Miami (ClinicalTrials.gov # NCT02161380) to examine the safety and efficacy of mitochondrial gene therapy in LHON. In these trials, participants affected by LHON with the G11778A mutation will have a virus expressing the functional version of ND4 – the gene mutated in this variant of LHON – injected into one eye. A sham injection will be administered to the other eye for comparison. It is hypothesized that introduction of the viral vector may be able to rescue the function of the mutant gene. Preliminary results have demonstrated tolerability of the injections in a small number of subjects.

Stealth BioTherapeutics is presently investigating the potential use of elamipretide (MTP-131), a mitochondrial protective agent, as a therapy for LHON. Elamipretide helps stabilize cardiolipin – an important component of mitochondrial inner membranes – and has been shown to reduce damaging reactive oxygen species in animal models. Clinical trials in LHON patients are planned for the future.

In industrialized nations, toxic and nutritional optic neuropathy is relatively uncommon and is primarily associated with specific medications, occupational exposures, or tobacco and alcohol abuse. However, in developing nations, nutritional optic neuropathy is much more common, especially in regions afflicted by famine. Both genders and all races are equally affected, and all ages are susceptible.

The severity and prognosis vary with the type of mutation involved.

The most recognized cause of a toxic optic neuropathy is methanol intoxication. This can be a life-threatening event that normally accidentally occurs when the victim mistook, or substituted, methanol for ethyl alcohol. Blindness can occur with drinking as little as an ounce of methanol, but this can be counteracted by concurrent drinking of ethyl alcohol. The patient initially has nausea and vomiting, followed by respiratory distress, headache, and visual loss 18–48 hours after consumption. Without treatment, patients can go blind, and their pupils will dilate and stop reacting to light.

- Ethylene glycol, a component of automobile antifreeze, is a poison that is toxic to the whole body including the optic nerve. Consumption can be fatal, or recovery can occur with permanent neurologic and ophthalmologic deficits. While visual loss is not very common, increased intracranial pressure can cause bilateral optic disc swelling from cerebral edema. A clue to the cause of intoxication is the presence of oxalate crystals in the urine. Like methanol intoxication, treatment is ethanol consumption.

- Ethambutol, a drug commonly used to treat tuberculosis, is notorious for causing toxic optic neuropathy. Patients with vision loss from ethambutol toxicity lose vision in both eyes equally. This initially presents with problems with colors (dyschromatopsia) and can leave central visual deficits. If vision loss occurs while using ethambutol, it would be best to discontinue this medication under a doctor’s supervision. Vision can improve slowly after discontinuing ethambutol but rarely returns to baseline.

- Amiodarone is an antiarrhythmic medication commonly used for abnormal heart rhythms (atrial or ventricular tachyarrythmias). Most patients on this medication get corneal epithelial deposits, but this medication has also been controversially associated with NAION. Patients on amiodarone with new visual symptoms should be evaluated by an ophthalmologist.

- Tobacco exposure, most commonly through pipe and cigar smoking, can cause an optic neuropathy. Middle-aged or elderly men are often affected and present with painless, slowly progressive, color distortion and visual loss in both eyes. The mechanism is unclear, but this has been reported to be more common in individuals who are already suffering from malnutrition.

NARP syndrome is not curable. Symptomatic relief is targeted. Antioxidants play a role in improving the oxidative phosphorylation that is otherwise impaired.

Dominant optic atrophy, or dominant optic atrophy, Kjer's type, is an autosomally inherited disease that affects the optic nerves, causing reduced visual acuity and blindness beginning in childhood. This condition is due to mitochondrial dysfunction mediating the death of optic nerve fibers. Dominant optic atrophy was first described clinically by Batten in 1896 and named Kjer’s optic neuropathy in 1959 after Danish ophthalmologist Poul Kjer, who studied 19 families with the disease. Although dominant optic atrophy is the most common autosomally inherited optic neuropathy (i.e., disease of the optic nerves) aside from glaucoma, it is often misdiagnosed.

Once NAION happens, it was thought that there was no accepted treatment to reverse the damage. However, a recent uncontrolled retrospective large study has shown that if patients are treated with large doses of corticosteroid therapy during the early stages of NAION, in eyes with initial visual acuity of 20/70 or worse, seen within 2 weeks of onset, there was visual acuity improvement in 70% in the treated group compared to 41% in the untreated group (odds ratio of improvement: 3.39; 95% CI:1.62, 7.11; p ¼ 0.001). That study and a natural history study on NAION (Ophthalmology 2008;115: 298–305.) showed that visual acuity can improve up to 6 months and not after that. To minimize the risk of further visual loss in the fellow eye or the same eye, it is essential to reduce the risk factors. Common sense dictates trying to control the cardiovascular risk factors for many reasons, including protection from this happening to the second eye. Sudden vision loss should lead to an ophthalmological consultation. If NAION is suspected, then ideally a neuro-ophthalmologist's consultation should be obtained.

A recent Cochrane Review sought to determine the extent of safety and efficacy of optic nerve decompression surgery for NAION, compared to other treatments, or no treatment. The one study included in the review found no improvements in visual acuity among patients who underwent surgery for NAION, and adverse events (pain, double vision) experienced by participants who underwent surgery.

There is much research currently underway looking at ways to protect the nerve (neuroprotection) or even regenerate new fibers within the optic nerve. So far there is no evidence in human studies that the so-called neuroprotectors have any beneficial effect in NAION.

However, there is a new current clinical trial for the treatment of NAION in the United States with plans to include sites in India, Israel, Germany and Australia (see NORDICclinicaltrials.com and https://clinicaltrials.gov/). This trial will test the use of a synthetic siRNA that blocks caspase 2, an important enzyme in the apoptosis cycle.

In addition to such research, patents have been applied for by Pfizer, The University of Southern California, Otsuka Pharmaceutical and other individual inventors for innovations related to the treatment of anterior ischemic optic neuropathy.

In most MS-associated optic neuritis, visual function spontaneously improves over 2–3 months, and there is evidence that corticosteroid treatment does not affect the long term outcome. However, for optic neuritis that is not MS-associated (or atypical optic neuritis) the evidence is less clear and therefore the threshold for treatment with intravenous corticosteroids is lower. Intravenous corticosteroids also reduce the risk of developing MS in the following two years in patients with MRI lesions; but this effect disappears by the third year of follow up.

Paradoxically, oral administration of corticosteroids in this situation may lead to more recurrent attacks than in non-treated patients (though oral steroids are generally prescribed after the intravenous course, to wean the patient off the medication). This effect of corticosteroids seems to be limited to optic neuritis and has not been observed in other diseases treated with corticosteroids.

A Cochrane Systematic Review studied the effect of corticosteroids for treating people with acute optic neuritis. Specific corticosteroids studied included intravenous and oral methylprednisone, and oral prednisone. The authors conclude that current evidence does not show a benefit of either intravenous or oral corticosteroids for rate of recovery of vision (in terms of visual acuity, contrast sensitivity, or visual fields)..

There is no known direct treatment. Current treatment efforts focus on managing the complications of Wolfram syndrome, such as diabetes mellitus and diabetes insipidus.

Mitochondria play a central role in maintaining the life cycle of retinal ganglion cells because of their high energy dependence. Mitochondria are made within the central somata of the retinal ganglion cell, transported down axons, and distributed where they are needed. Genetic mutations in mitochondrial DNA, vitamin depletion, alcohol and tobacco abuse, and use of certain drugs can cause derangements in efficient transport of mitochondria, which can cause a primary or secondary optic neuropathy.

There is currently no defined treatment to ameliorate the muscle weakness of CPEO. Treatments used to treat other pathologies causing ophthalmoplegia has not been shown to be effective.

Experimental treatment with tetracycline has been used to improve ocular motility in one patient. Coenzyme Q has also been used to treat this condition. However, most neuro-ophthalmologists do not ascribe to any treatment.

Ptosis associated with CPEO may be corrected with surgery to raise the lids, however due to weakness of the orbicularis oculi muscles, care must be taken not to raise the lids in excess causing an inability to close the lids. This results in an exposure keratopathy. Therefore, rarely should lid surgery be performed and only by a neuro-ophthalmologist familiar with the disease.

The most common strabismus finding is large angle exotropia which can be treated by maximal bilateral eye surgery, but due to the progressive nature of the disease, strabismus may recur. Those that have diplopia as a result of asymmetric ophthalmoplegia may be corrected with prisms or with surgery to create a better alignment of the eyes.

Patients with optic disc drusen should be monitored periodically for ophthalmoscopy, Snellen acuity, contrast sensitivity, color vision, intraocular pressure and threshold visual fields. For those with visual field defects optical coherence tomography has been recommended for follow up of nerve fiber layer thickness. Associated conditions such as angioid streaks and retinitis pigmentosa should be screened for. Both the severity of optic disc drusen and the degree of intraocular pressure elevation have been associated with visual field loss. There is no widely accepted treatment for ODD, although some clinicians will prescribe eye drops designed to decrease the intra-ocular pressure and theoretically relieve mechanical stress on fibers of the optic disc. Rarely choroidal neovascular membranes may develop adjacent to the optic disc threatening bleeding and retinal scarring. Laser treatment or photodynamic therapy or other evolving therapies may prevent this complication.

While radiation or chemotherapy may be helpful, treatment is often not necessary. Optical gliomas often cannot be surgically resected. If no visual symptoms wait 6 months and then in 6 months only treat if there are symptoms (visual loss, eye pain), otherwise do not treat.

Although no cure currently exists, there is hope in treatment for this class of hereditary diseases with the use of an embryonic mitochondrial transplant.

AON is a rare disease and the natural history of the disease process is not well defined. Unlike typical optic neuritis, there is no association with multiple sclerosis, but the visual prognosis for AON is worse than typical optic neuritis. Thus AON patients have different treatment, and often receive chronic immunosuppression. No formal recommendation can be made regarding the best therapeutic approach. However, the available evidence to date supports treatment with corticosteroids and other immunosuppressive agents.

Early diagnosis and prompt treatment with systemic corticosteroids may restore some visual function but the patient may remain steroid dependent; vision often worsens when corticosteroids are tapered. As such, long-term steroid-sparing immunosuppressive agents may be required to limit the side-effects of steroids and minimize the risk of worsening vision.

Currently there is no curative treatment for KSS. Because it is a rare condition, there are only case reports of treatments with very little data to support their effectiveness. Several promising discoveries have been reported which may support the discovery of new treatments with further research. Satellite cells are responsible for muscle fiber regeneration. It has been noted that mutant mtDNA is rare or undetectable in satellite cells cultured from patients with KSS. Shoubridge et al. (1997) asked the question whether wildtype mtDNA could be restored to muscle tissue by encouraging muscle regeneration. In the forementioned study, regenerating muscle fibers were sampled at the original biopsy site, and it was found that they were essentially homoplasmic for wildtype mtDNA. Perhaps with future techniques of promoting muscle cell regeneration and satellite cell proliferation, functional status in KSS patients could be greatly improved.

One study described a patient with KSS who had reduced serum levels of coenzyme Q10. Administration of 60–120 mg of Coenzyme Q10 for 3 months resulted in normalization of lactate and pyruvate levels, improvement of previously diagnosed first degree AV block, and improvement of ocular movements.

A screening ECG is recommended in all patients presenting with CPEO. In KSS, implantation of pacemaker is advised following the development of significant conduction disease, even in asymptomatic patients.

Screening for endocrinologic disorders should be performed, including measuring serum glucose levels, thyroid function tests, calcium and magnesium levels, and serum electrolyte levels. Hyperaldosteronism is seen in 3% of KSS patients.

The first symptom is typically diabetes mellitus, which is usually diagnosed around the age of 6. The next symptom to appear is often optic atrophy, the wasting of optic nerves, around the age of 11. The first signs of this are loss of colour vision and peripheral vision. The condition worsens over time, and people with optic atrophy are usually blind within 8 years of the first symptoms. Life expectancy of people suffering from this syndrome is about 30 years.

Optic neuritis typically affects young adults ranging from 18–45 years of age, with a mean age of 30–35 years. There is a strong female predominance. The annual incidence is approximately 5/100,000, with a prevalence estimated to be 115/100,000.

Succinic acid has been used successfully to treat MELAS syndrome, and also Leighs disease. Patients are managed according to what areas of the body are affected at a particular time. Enzymes, amino acids, antioxidants and vitamins have been used.

Also the following supplements may help:

- CoQ10 has been helpful for some MELAS patients. Nicotinamide has been used because complex l accepts electrons from NADH and ultimately transfers electrons to CoQ10.

- Riboflavin has been reported to improve the function of a patient with complex l deficiency and the 3250T-C mutation.

- The administration of L-arginine during the acute and interictal periods may represent a potential new therapy for this syndrome to reduce brain damage due to impairment of vasodilation in intracerebral arteries due to nitric oxide depletion.

- There is also a case report where succinate was successfully used to treat uncontrolled convulsions in MELAS patients, although this treatment modality is yet to be thoroughly investigated or widely recommended.

ONSM does not improve without treatment. In many cases, there is gradual progression until vision is lost in the affected eye. However, this takes at least several months to occur, and a minority of patients remain stable for a number of years.