Monofixation syndrome (MFS) (also: microtropia or microstrabismus) is an eye condition defined by less-than-perfect binocular vision. It is defined by a small angle deviation with suppression of the deviated eye and the presence of binocular peripheral fusion. That is, MFS implies peripheral fusion without central fusion. Aside the manifest small-angle deviation ("tropia"), subjects with MFS often also have a large-angle latent deviation (phoria). Their stereoacuity is often in the range of 3000 to 70 arcsecond, and a small central suppression scotoma of 2 to 5 deg.A rare condition, MFS is estimated to affect only 1% of the general population. There are three distinguishable forms of this condition:  primary constant, primary decompensating and consecutive MFS. It is believed that primary MFS is a result of a primary sensorial defect, predisposing to anomalous retinal correspondence.Secondary MFS is a frequent outcome of surgical treatment of congenital esotropia. A study of 1981 showed MFS to result in the vast majority of cases if surgical alignment is reached before the age of 24 months and only in a minority of cases if it is reached later.MFS was first described by Marshall Parks. References


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Spinocerebellar ataxia type 13 (SCA13) is a rare autosomal dominant disorder, which, like other types of SCA, is characterized by dysarthria, nystagmus, and ataxia of gait, stance and the limbs due to cerebellar dysfunction. Patients with SCA13 also tend to present with epilepsy, an inability to run, and increased reflexes. This cerebellar dysfunction is permanent and progressive. SCA13 is caused by mutations in KCNC3, a gene encoding a voltage-gated potassium channel KV3.3. There are two known mutations in this gene causative for SCA13. Unlike many other types of SCA, these are not polyglutamine expansions but, rather, point mutations resulting in channels with no current or altered kinetics. Signs and symptoms
SCA13 is typified by early onset, mildly progressive cerebellar ataxia with accompanying dysarthria, mental retardation, and nystagmus. Symptoms and age of onset can vary slightly according to the causative mutation. Pathophysiology
Mutations in KCNC3 are responsible for SCA13. This gene is expressed heavily in Purkinje cells, as is the case for some other SCA subtypes, where it is believed to play an important role in facilitating high-frequency action potential firing. There are two known mutations in this gene associated with SCA13. The first mutation, R420H, is located in the voltage-sensing S4 segment of the channel. As this mutation neutralizes a site important for voltage sensing, it is not surprising that it results in non-conducting channels. Neurons expressing such channels are unable to follow high-frequency input with adequate fidelity.The second SCA13 associated mutation, F448L, results in functional channels that have altered kinetics.