Hydrocephalus is an active distension of the ventricular system of the brain resulting from inadequate passage of CSF from its point of production within the cerebral ventricles to its point of absorption into the systemic circulation.

A seizure is an intermittent abnormality of nervous system physiology characterized by a transient occurrence of signs and/or symptoms due to abnormal excessive or synchronous neuronal activity in the brain.

Severe mental retardation is defined as an intelligence quotient (IQ) in the range of 20-34.

The corpus callosum is the largest fiber tract in the central nervous system and the major interhemispheric fiber bundle in the brain. Formation of the corpus callosum begins as early as 6 weeks' gestation, with the first fibers crossing the midline at 11 to 12 weeks' gestation, and completion of the basic shape by age 18 to 20 weeks (Schell-Apacik et al., 2008). Agenesis of the corpus callosum (ACC) is one of the most frequent malformations in brain with a reported incidence ranging between 0.5 and 70 in 10,000 births. ACC is a clinically and genetically heterogeneous condition, which can be observed either as an isolated condition or as a manifestation in the context of a congenital syndrome (see MOLECULAR GENETICS and Dobyns, 1996). Also see mirror movements-1 and/or agenesis of the corpus callosum (MRMV1; 157600). Schell-Apacik et al. (2008) noted that there is confusion in the literature regarding radiologic terminology concerning partial absence of the corpus callosum, where various designations have been used, including hypogenesis, hypoplasia, partial agenesis, or dysgenesis.

Cerebellar hypoplasia is a descriptive term implying a cerebellum with a reduced volume, but a normal shape and is stable over time.

A form of lissencephaly characterized by an uneven cortical surface with a so called 'cobblestone' appearace. There are no distinguishable cortical layers.

Underdevelopment of the brainstem.

Profound mental retardation is defined as an intelligence quotient (IQ) below 20.

Cerebellar dysplasia (abnormal growth or development) is defined by abnormal cerebellar foliation, white matter arborization, and gray-white matter junction. Cerebellar dysplasia is a neuroimaging finding that describes abnormalities of both the cerebellar cortex and white matter and is associated with variable neurodevelopmental outcome. Dysplasia may globally involve the cerebellum or affect only one cerebellar hemisphere. In addition, cerebellar dysplasia may be associated with cortical/subcortical cysts.

An increase in size of the ventricular system of the brain.

Congenital absence of a part of the vermis of cerebellum.

A neural tube defect characterized by sac-like protrusions of the brain and the membranes that cover it through openings in the skull.

Pachygyria is a malformation of cortical development with abnormally wide gyri with sulci 1,5-3 cm apart and abnormally thick cortex measuring more than 5 mm (radiological definition). See also neuropathological definitions for 2-, 3-, and 4-layered lissencephaly.

The term dystrophy means abnormal growth. However, muscular dystrophy is used to describe primary myopathies with a genetic basis and a progressive course characterized by progressive skeletal muscle weakness and wasting, defects in muscle proteins, and histological features of muscle fiber degeneration (necrosis) and regeneration. If possible, it is preferred to use other HPO terms to describe the precise phenotypic abnormalities.

The presence of an abnormal lateral curvature of the spine.

One or more flexion contractures (a bent joint that cannot be straightened actively or passively) that are present at birth.

A severe degree of muscular hypotonia characterized by markedly reduced muscle tone.

An abnormally high degree of muscle fiber size variation. This phenotypic feature can be observed upon muscle biopsy.

Reduced ability to move the vertebral column with a resulting limitation of neck and trunk flexion.

Abnormal increase in muscle size and mass not due to training.

Head circumference below 2 standard deviations below the mean for age and gender.

An elevation of the level of the enzyme creatine kinase (also known as creatine phosphokinase (CK; EC 2.7.3.2) in the blood. CK levels can be elevated in a number of clinical disorders such as myocardial infarction, rhabdomyolysis, and muscular dystrophy.

A gap or groove in the upper lip. This is a congenital defect resulting from nonfusion of tissues of the lip during embryonal development.

Increased length and width of the tongue.

Cleft palate is a developmental defect of the palate resulting from a failure of fusion of the palatine processes and manifesting as a separation of the roof of the mouth (soft and hard palate).

Glaucoma refers loss of retinal ganglion cells in a characteristic pattern of optic neuropathy usually associated with increased intraocular pressure.

An abnormality of refraction characterized by the ability to see objects in the distance clearly, while objects nearby appear blurry.

Microphthalmia is an eye abnormality that arises before birth. In this condition, one or both eyeballs are abnormally small. In some affected individuals, the eyeball may appear to be completely missing; however, even in these cases some remaining eye tissue is generally present. Such severe microphthalmia should be distinguished from another condition called anophthalmia, in which no eyeball forms at all. However, the terms anophthalmia and severe microphthalmia are often used interchangeably. Microphthalmia may or may not result in significant vision loss.\n\nPeople with microphthalmia may also have a condition called coloboma. Colobomas are missing pieces of tissue in structures that form the eye. They may appear as notches or gaps in the colored part of the eye called the iris; the retina, which is the specialized light-sensitive tissue that lines the back of the eye; the blood vessel layer under the retina called the choroid; or in the optic nerves, which carry information from the eyes to the brain. Colobomas may be present in one or both eyes and, depending on their size and location, can affect a person's vision.\n\nPeople with microphthalmia may also have other eye abnormalities, including clouding of the lens of the eye (cataract) and a narrowed opening of the eye (narrowed palpebral fissure). Additionally, affected individuals may have an abnormality called microcornea, in which the clear front covering of the eye (cornea) is small and abnormally curved.\n\nBetween one-third and one-half of affected individuals have microphthalmia as part of a syndrome that affects other organs and tissues in the body. These forms of the condition are described as syndromic. When microphthalmia occurs by itself, it is described as nonsyndromic or isolated.

Nearsightedness, also known as myopia, is an eye condition that causes blurry distance vision. People who are nearsighted have more trouble seeing things that are far away (such as when driving) than things that are close up (such as when reading or using a computer). If it is not treated with corrective lenses or surgery, nearsightedness can lead to squinting, eyestrain, headaches, and significant visual impairment.\n\nNearsightedness usually begins in childhood or adolescence. It tends to worsen with age until adulthood, when it may stop getting worse (stabilize). In some people, nearsightedness improves in later adulthood.\n\nFor normal vision, light passes through the clear cornea at the front of the eye and is focused by the lens onto the surface of the retina, which is the lining of the back of the eye that contains light-sensing cells. People who are nearsighted typically have eyeballs that are too long from front to back. As a result, light entering the eye is focused too far forward, in front of the retina instead of on its surface. It is this change that causes distant objects to appear blurry. The longer the eyeball is, the farther forward light rays will be focused and the more severely nearsighted a person will be.\n\nNearsightedness is measured by how powerful a lens must be to correct it. The standard unit of lens power is called a diopter. Negative (minus) powered lenses are used to correct nearsightedness. The more severe a person's nearsightedness, the larger the number of diopters required for correction. In an individual with nearsightedness, one eye may be more nearsighted than the other.\n\nEye doctors often refer to nearsightedness less than -5 or -6 diopters as "common myopia." Nearsightedness of -6 diopters or more is commonly called "high myopia." This distinction is important because high myopia increases a person's risk of developing other eye problems that can lead to permanent vision loss or blindness. These problems include tearing and detachment of the retina, clouding of the lens (cataract), and an eye disease called glaucoma that is usually related to increased pressure within the eye. The risk of these other eye problems increases with the severity of the nearsightedness. The term "pathological myopia" is used to describe cases in which high myopia leads to tissue damage within the eye.

A cataract is an opacity or clouding that develops in the crystalline lens of the eye or in its capsule.

The presence of remnants of a fetal membrane that persist as strands of tissue crossing the pupil.

Anterior segment dysgeneses (ASGD or ASMD) are a heterogeneous group of developmental disorders affecting the anterior segment of the eye, including the cornea, iris, lens, trabecular meshwork, and Schlemm canal. The clinical features of ASGD include iris hypoplasia, an enlarged or reduced corneal diameter, corneal vascularization and opacity, posterior embryotoxon, corectopia, polycoria, an abnormal iridocorneal angle, ectopia lentis, and anterior synechiae between the iris and posterior corneal surface (summary by Cheong et al., 2016). Anterior segment dysgenesis is sometimes divided into subtypes including aniridia (see 106210), Axenfeld and Rieger anomalies, iridogoniodysgenesis, Peters anomaly, and posterior embryotoxon (Gould and John, 2002). Patients with ASGD5 have been reported with the Peters anomaly, Axenfeld anomaly, and Rieger anomaly subtypes. Peters anomaly consists of a central corneal leukoma, absence of the posterior corneal stroma and Descemet membrane, and a variable degree of iris and lenticular attachments to the central aspect of the posterior cornea (Peters, 1906). It occurs as an isolated ocular abnormality or in association with other ocular defects. In Axenfeld anomaly, strands of iris tissue attach to the Schwalbe line; in Rieger anomaly, in addition to the attachment of iris tissue to the Schwalbe line, there is clinically evident iris stromal atrophy with hole or pseudo-hole formation and corectopia (summary by Smith and Traboulsi, 2012).

Diffusely large eye (with megalocornea) associated with glaucoma.