PARTICIPANT INCLUSION CRITERIA To be eligible for study participation, an individual must meet all of the following criteria: 1. Ability to understand, and willingness and ability to read and sign, the informed consent form. 2. Ability to understand and follow instructions and study procedures. 3. Willingness to comply with all study procedures and be available for the duration of the study. 4. Ability to take oral medication, and are willing to adhere to study medication regimen. 5. Age 18 years or older. 6. Diagnosed with HZO in one eye based on both of these criteria: 1. History of characteristic unilateral vesicular rash in the dermatomal distribution of cranial nerve V1. 2. Medical record documentation of an episode of active dendriform epithelial keratitis, stromal keratitis, endothelial keratitis, and/or iritis due to HZO within the preceding year. This episode of active anterior segment ocular disease may be due to HZO of recent onset (within the preceding 6 months); or chronic HZO (with onset six or more months ago); may be new, worsening, or recurrent disease after a period of inactivity; and may occur after medication was reduced. i. Study participants with chronic HZO must be on a stable treatment regimen and off antivirals for at least 30 days before enrollment. Study participants with chronic HZO who do not meet this criterion may be rescreened, if they are able to meet this criterion within 3 months after the study visit. (This is not a requirement for study participants with recent onset HZO, who may be enrolled at any time, preferably after completing recommended acute antiviral treatment, if prescribed, is completed). 7. For females with reproductive potential, willingness to use highly effective contraception (e.g., hormonal contraception, barrier contraception, intrauterine device, or abstinence). PARTICIPANT EXCLUSION CRITERIA An individual who meets any of the following criteria will be excluded from participation in this study: 1. History of immunocompromised status as defined by current CDC contraindications for the vaccine against zoster (44). 1. Study participants who are diagnosed with leukemia, lymphomas or other malignant neoplasms affecting bone marrow or lymphatic system, unless leukemia in remission and off chemotherapy for at least 3 months. 2. Study participants who are diagnosed with Acquired Immune Deficiency Syndrome (AIDS) or presents with other clinical manifestations of Human Immunodeficiency virus (HIV) including CD4 count of ≤ 200 cells/ml. 3. Study participants on immunosuppressive therapy including: i. High-dose corticosteroids (greater than equivalent of prednisone 20 mg/day within 1 month) ii. Chemotherapy, other than low dose used for treatment of immune-mediated diseases within 3 months iii. Study participants receiving recombinant human immune mediators and immune modulators, especially antitumor necrosis agents, within 1 month prior to enrollment d. Study participants with unspecified cellular immunodeficiency. e. Study participants with history of hematopoietic stem cell transplantation. 2. Medical history of a systemic disease and thought likely to meet one of the exclusion criteria listed in exclusion criterion #1 during the 18-month study period. 3. Renal insufficiency: 1. Requires dialysis or has history of renal transplant or 2. eGFR less than 45, determined within 30 days preceding enrollment. 4. Allergy or adverse reaction to valacyclovir or acyclovir. 5. History of vaccination against zoster within one month prior to enrollment. Study participants who meet this exclusion criterion may be rescreened. 6. Keratoplasty or keratorefractive surgery of the involved eye with zoster. 7. On systemic antivirals with activity against herpes within the past 30 days, including acyclovir, valacyclovir, or famciclovir, for any reason except for treatment of acute HZO, including investigational drug trial. 8. History of another condition that may require treatment with one of these three antivirals listed above in exclusion criterion #7, during the course of the study; study participants who require chronic suppressive antiviral treatment with these medications will be excluded. 9. Sexually active women who are pregnant, nursing, or in their reproductive years who do not agree to use contraception during the 1-year treatment period. 10. Incarceration 11. Any condition or circumstance that in the opinion of the study investigator, would place the study participant in increased risk or affect his/her full compliance or completion of the study. 12. Participation in a clinical study testing a drug, biologic, device or other intervention within the last 30 days from enrollment visit. Study participants who meet this criterion may be rescreened.

• Inborn or out born infants of either gender or any race with best obstetrical estimate of gestation <28 weeks (27 6/7 weeks and younger). Gestational age will be determined by best obstetrical estimate using the hierarchy of best obstetrical estimate using early ultrasound dating, maternal menstrual dating confirmed by examination, or neonatal gestational age assessment by physical examination.
• Alive at 12 hours.
• Age in hours up to 72 hours, although we will seek enrollment as early as feasible after consent and 12 hours.
• Informed consent signed and dated by parent and/or guardian, which includes likelihood of completing follow-up ophthalmic examinations as an outpatient, and long-term follow-up. Exclusion Criteria
• Major congenital malformations
• Congenital malformations of the eye identified prior to randomization.
• Overt evidence of intrauterine congenital infections ("TORCH") or life threatening impairment of renal, hepatic, or cardiac function (considered moribund).

Who have experienced a progressive deterioration in their vision, such thot they can no longer achieve adequate functional vision on a daily basis with their contact lenses or spectacles; Who are 21 years of age or older; Who have clear central corneas; Who have a corneal thickness of 450 microns or greater at the proposed incision site; Who have corneal transplantation as the only remaining option to improve their functional vision. Who have abnormally thin corneas or who have a corneal thickness of 449 microns or less at the proposed incision site; Patients with collagen vascular, autoimmune or immunodeficiency disease; Pregnant or nursing patients; Presence of ocular conditions, such as recurrent corneal erosion syndrome or corneal dystrophy, that my predispose the patient to future complications; Patients who are taking on or more of following medications: isotretinoin (Accutane); amiodarone HCL (Cordarone).

• 2 to 21 years of age
• Have a visually significant cataract or need IOL replacement surgery
• Compromised capsular bag prohibiting implantation of standard posterior IOL
• Subject or parent/guardian must be able to comply with visit schedule and study requirements
• Subject's legal representative must be able to sign the Informed Consent
• Under 2 years of age
• Unable to meet Postoperative evaluation requirements
• No useful vision or vision potential in fellow eye
• Mentally retarded patients
• History of corneal disease
• Abnormality of the iris or ocular structure
• ACD less than 3.2 mm
• Uncontrolled glaucoma
• IOP > 25 mmHg
• Chronic or recurrent uveitis
• Preexisting macular pathology that may complicate the ability to assess the benefit of this lens
• Retinal detachment or family history
• Retinal disease that may limit visual potential
• Optic nerve disease that may limit visual potential
• Diabetes mellitus
• Pregnant, lactating or plan to become pregnant

• Male or female:
• 2 years of age at 5 pediatric CMS study sites
• 10 years of age at other study sites.
• Confirmed physician diagnosis of LEMS, CMS or downbeat nystagmus.
• Completion of anti-cancer treatment at least 3 months (90 days) before treatment.
• Negative urine pregnancy test for females of childbearing potential at Screening.
• If sexually active and of childbearing potential, willing to use 2 acceptable methods of contraception from screening visit until 3 months after the last dose of investigational product. No adequate clinical data on exposed pregnancies are available for amifampridine. No nonclinical safety data are available regarding the effects of amifampridine on reproductive function. Amifampridine phosphate should not be used during pregnancy. It is unknown whether amifampridine is excreted in human breast milk. The excretion of amifampridine in milk has not been studied in animals. Amifampridine phosphate should not be used during breastfeeding.
• Any subject currently participating in studies LMS-002, LMS-002EXT, or CMS 001 is immediately eligible for enrollment into study EAP-001, as long as inclusion/exclusion criteria are still met.
• Willing and able to provide written informed consent after the nature of the study has been explained and before the start of any research-related procedures.
• History of epilepsy.
• CMS subtypes including slow-channel syndrome, LRP4 deficiency, and acetylcholinesterase deficiency.
• Known active brain metastasis. Patients with treated brain metastasis (radiotherapy and/or surgery) who have completed treatment for their brain metastasis >90 days before Screening, are neurologically stable (neurological symptoms grade <1), are on a stable dose of corticosteroids and have no evidence of new disease on magnetic resonance imaging (MRI) are eligible, provided they meet the other inclusion/exclusion criteria.
• Current use of dalfampridine (Ampyra®; 4-aminopyridine), and any form of 3,4 DAP other than the investigational product provided, such as amifampridine base and does not agree to discontinue use for the duration of the study.
• Use of guanidine hydrochloride within 7 days of starting amifampridine phosphate treatment.
• History of drug allergy to any pyridine-containing substances or any amifampridine phosphate excipients (i.e. microcrystalline cellulose, colloidal silicon dioxide or calcium stearate).
• Use of any other investigational product (other than 3,4 DAP or amifampridine phosphate) or investigational medical device within 30 days before starting treatment or requirement for any investigational agent before completion of all scheduled study assessments.
• An electrocardiogram (ECG) within 6 months before starting treatment that shows clinically significant abnormality(ies), in the opinion of the patient's personal physician.
• History of additional risk factors for torsade de pointes (e.g. history of surviving a near drowning due to loss of consciousness, family history of congenital QT syndrome, long QT syndrome, family history of unexplained early sudden death, or heart failure).
• Breastfeeding or pregnant or planning to become pregnant (self or partner). Male patients with breastfeeding partners are not excluded from the study.
• History of severe renal impairment or evidence of severe renal impairment at time of Screening on laboratory tests, specifically a creatinine clearance <30 mL/min (within 30 days) as calculated using the Cockcroft Gault formula.
• History at time of Screening of laboratory tests (within 30 days) indicating hepatic impairment: o In patients without liver metastases from cancer, alanine aminotransferase (ALT), aspartate aminotransferase (AST), and/or total bilirubin >1.5 × upper limit of normal (ULN).
• Any condition that, in the view of the Principal Investigator, places the patient at high risk of poor treatment compliance or of not completing the study.