Glossary

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Leber Hereditary Optic Neuropathy(also: LHON)

A maternally inherited genetic condition that causes sudden or rapid loss of central vision, typically affecting young adults between the ages of 15 and 35. LHON results from mutations in mitochondrial DNA that damage the optic nerve, leading to significant bilateral central…

Leukodystrophy(also: Leukodystrophies)

Leukodystrophy is a group of rare, progressive, metabolic, genetic diseases that affect the brain, spinal cord, and peripheral nerves by damaging the white matter (myelin sheath) that insulates nerve fibers. Symptoms can include difficulty with movement and coordination, speech…

Limb Difference(also: Limb Deficiency, Upper-Limb Difference)

Limb difference is a term used to describe the absence or malformation of one or more limbs or parts of limbs. It can be congenital (present from birth) or acquired through amputation due to injury, disease, or surgery. The term "limb difference" is increasingly preferred over…

Low-Functioning Autism(also: LFA, Classic Autism, Level 3 Autism)

A descriptor historically used to characterize individuals with autism spectrum disorder who have significant support needs, including limited or no verbal communication, intellectual disability, and difficulty with daily living skills. The term is increasingly considered…

Mathematics Learning Disability(also: Math Disability, Mathematical Disability, Mathematics Disorder)

A specific learning disability that affects a person's ability to understand, learn, or perform mathematical operations. Mathematics learning disabilities can manifest as difficulty with number sense, memorisation of arithmetic facts, calculation fluency, mathematical reasoning,…

Meares-Irlen Syndrome(also: Irlen Syndrome, Visual Stress Syndrome, Scotopic Sensitivity Syndrome)

Meares-Irlen Syndrome is a perceptual processing disorder that affects how the brain processes visual information, causing symptoms of visual stress including pattern glare, perceptual distortions, eye strain, headaches, and discomfort when reading. Unlike dyslexia, it is not a…

Memory Impairment(also: Memory Loss, Memory Deficit)

A reduction in the ability to encode, store, or retrieve information, ranging from mild forgetfulness associated with normal ageing to severe deficits caused by conditions such as dementia or traumatic brain injury. Memory impairment affects digital accessibility in multiple…

Micrographia(also: Small Handwriting)

A condition characterized by abnormally small, cramped handwriting that often becomes progressively smaller within a line or paragraph. Micrographia is a common early sign of Parkinson's Disease, resulting from the same motor control difficulties that cause bradykinesia. The…

Motor Disability(also: Motor Impairment, Physical Disability, Movement Disorder)

A condition that affects a person's ability to control voluntary muscle movements, impacting mobility, dexterity, coordination, or speech production. Motor disabilities can result from neurological conditions (cerebral palsy, ALS, multiple sclerosis), spinal cord injuries,…

Motor Fluctuation(also: On-Off Phenomenon, Wearing Off, On-Off Periods)

Variations in motor function experienced by people with Parkinson's Disease, typically related to medication timing. During 'on' periods when medication is working optimally, motor symptoms are controlled; during 'off' periods as medication wears off, symptoms like tremor,…

Myopathy(also: Muscle Disease)

A group of disorders affecting skeletal muscle function, characterized by muscle weakness, cramping, stiffness, or wasting. Myopathies can be inherited (muscular dystrophies) or acquired (inflammatory myopathies, metabolic myopathies). For digital accessibility, people with…

Myopia(also: Nearsightedness, Short-Sightedness)

A common refractive error in which the eye focuses light in front of the retina rather than on it, causing distant objects to appear blurred while near objects remain clear. Myopia occurs when the eyeball is too long or the cornea is too curved, and affects an estimated 30% of…

Neurodevelopmental Disorders(also: NDD, Neurodevelopmental Disabilities)

A group of conditions that originate during the developmental period and are characterized by impairments in personal, social, academic, or occupational functioning. Neurodevelopmental disorders include intellectual disability, autism spectrum disorder, ADHD, specific learning…

Neuromuscular Disease(also: Neuromuscular Disorder, NMD)

A broad category of conditions that affect the nerves controlling voluntary muscles, the muscles themselves, or the communication between nerves and muscles. Neuromuscular diseases include muscular dystrophies, motor neurone disease (ALS), spinal muscular atrophy, myasthenia…

Neuromuscular Disorders(also: Neuromuscular Diseases, NMDs)

A broad group of medical conditions that impair the functioning of muscles, either through direct pathology of the muscle tissue, the peripheral nerves that control them, or the neuromuscular junction where nerves connect to muscles. Neuromuscular disorders include amyotrophic…

Oral Motor Impairment(also: Oral-Motor Dysfunction, Oromotor Impairment)

A condition affecting the muscles and movements of the mouth, jaw, lips, and tongue that are involved in speech production, feeding, and swallowing. Oral motor impairments can result from neurological conditions such as cerebral palsy, stroke, or traumatic brain injury, and may…

Paraplegia

A condition resulting from injury to the thoracic, lumbar, or sacral segments of the spinal cord, causing partial or complete loss of motor function and sensation in the legs, pelvis, and trunk while arm and hand function is preserved. People with paraplegia can typically use…

Paresis(also: Partial Paralysis)

A partial loss of voluntary muscle movement, distinguished from full paralysis (plegia). Paresis can affect a single limb (monoparesis), one side of the body (hemiparesis), or all four limbs (tetraparesis), and may result from stroke, cerebral palsy, spinal cord injury, or…

Parkinsonian Tremor(also: Rest Tremor, Parkinson Tremor)

Parkinsonian tremor is an involuntary, rhythmic shaking that occurs primarily at rest and is associated with Parkinson's disease. It is caused by the reciprocal activation of antagonistic muscle groups and typically has a frequency of 4-6 Hz with a harmonic frequency…

Pervasive Developmental Disorder(also: PDD, PDD-NOS, Pervasive Developmental Disorder Not Otherwise Specified)

A historical diagnostic category encompassing a group of neurodevelopmental conditions characterised by delays in socialisation, communication, and restricted patterns of behaviour. Under the DSM-IV, Pervasive Developmental Disorders included autism, Asperger syndrome, childhood…

Presbyopia(also: Age-Related Farsightedness, Loss of Accommodation)

An age-related vision condition in which the eye gradually loses the ability to focus on nearby objects, typically becoming noticeable after age 40. Presbyopia affects the vast majority of older adults and is caused by the hardening of the eye's lens, reducing its flexibility.…

Proliferative Diabetic Retinopathy(also: PDR)

An advanced stage of diabetic retinopathy in which abnormal new blood vessels grow on the retina, causing bleeding, scarring, and progressive vision loss that can include blurred vision, floaters, dark spots, and partial or complete blindness. A major cause of acquired low…

Repetitive Strain Injury(also: RSI, Repetitive motion injury, Overuse injury)

A category of injuries affecting muscles, tendons, and nerves caused by repetitive movements, sustained awkward postures, or overuse of a body part. In the context of accessibility, repetitive strain injuries are a significant concern for manual wheelchair users, who experience…

Repetitive Stress Injury(also: RSI, Repetitive Strain Injury, Repetitive Motion Injury)

A group of conditions caused by repetitive movements, forceful exertions, or sustained awkward postures over extended periods, resulting in damage to muscles, tendons, nerves, or other soft tissues. In computing contexts, RSI commonly affects the hands, wrists, arms, shoulders,…

Restricted Field of View(also: Tunnel Vision, Peripheral Vision Loss, Visual Field Loss)

A condition in which the area of the visual environment that a person can see at any given moment is significantly reduced, often described as looking through a tunnel or a narrow tube. Restricted field of view can result from conditions such as glaucoma, retinitis pigmentosa,…

Restricted Interest(also: Circumscribed Interest, Perseverative Interest, Fixated Interest)

An intense, narrow focus on specific topics, objects, or activities that is characteristic of autism spectrum disorder. Restricted interests can range from common subjects like trains, maps, or specific fictional characters to highly unusual topics like electricity pylons or…

Retinopathy of Prematurity(also: ROP)

An eye condition that can occur in premature infants when abnormal blood vessels grow in the retina. In severe cases, these vessels can cause scarring, retinal detachment, and significant vision loss or blindness. ROP is one of the leading causes of childhood blindness and low…

Rigidity(also: Muscle Rigidity, Stiffness)

Increased muscle tone causing stiffness and resistance to passive movement, commonly experienced in Parkinson's Disease and other neurological conditions. Unlike spasticity (which varies with movement speed), rigidity is constant throughout the range of motion. Rigidity affects…

Schizophrenia(also: Schizophrenia Spectrum Disorders)

A chronic psychiatric condition characterized by disturbances in thought, perception, emotion, and behavior, including symptoms such as hallucinations, delusions, disorganized thinking, reduced emotional expression, and cognitive difficulties with memory, attention, and…

Scotopic Sensitivity(also: Irlen Syndrome, Visual Stress, Meares-Irlen Syndrome)

A visual-perceptual condition in which certain wavelengths of light cause discomfort, distortion, or difficulty when reading. People with scotopic sensitivity may experience text appearing to shimmer, move, or blur on the page, particularly with high-contrast black text on white…

Sensory Disability(also: Sensory Impairment)

A disability that affects one or more of the senses — most commonly vision and hearing, but also including touch, taste, and smell. Sensory disabilities encompass conditions such as blindness, low vision, deafness, hard of hearing, and deafblindness. In digital accessibility,…

Severe Speech and Motor Impairment(also: SSMI)

A condition where an individual has significant limitations in both spoken communication and physical movement, often co-occurring in conditions such as cerebral palsy, amyotrophic lateral sclerosis (ALS), or traumatic brain injury. People with SSMI typically rely on…

Severe Speech and Physical Impairments(also: SSPI)

A classification describing individuals who have significant limitations in both speech production and physical movement, often co-occurring in conditions such as cerebral palsy. People with SSPI may have little or no functional speech and limited fine motor control, which…

Severe and Multiple Disabilities(also: Profound and Multiple Learning Disabilities, PMLD, Severe Multiple Disabilities)

A condition where an individual has two or more significant disabilities occurring simultaneously, typically involving profound physical impairments combined with intellectual disability and often sensory impairments. People with severe and multiple disabilities frequently have…

Spasm(also: Muscle Spasm, Spasticity)

A sudden, involuntary contraction of a muscle or group of muscles that can cause uncontrolled movement or a temporary inability to move. Spasms are common in conditions such as cerebral palsy, spinal cord injury, and multiple sclerosis. In the context of computer accessibility,…

Spastic Cerebral Palsy(also: Spastic CP)

The most common subtype of cerebral palsy, characterised by stiff or tight muscles (hypertonia) and exaggerated reflexes, caused by damage to the motor cortex or corticospinal tracts of the developing brain. Subtypes are further classified by the limbs affected (diplegia,…

Spastic Quadriplegia(also: Spastic Tetraplegia)

The most severe subtype of spastic cerebral palsy, characterised by stiff, tight muscles (spasticity) and significantly reduced voluntary movement in all four limbs and often the trunk. People with spastic quadriplegia commonly use powered wheelchairs and may rely on switch…

Specific Language Impairment(also: SLI, Developmental Language Disorder)

A neurodevelopmental condition characterised by significant difficulties in acquiring and using language that cannot be attributed to hearing loss, intellectual disability, neurological damage, or environmental deprivation. Children with specific language impairment may have…

Speech Delay(also: Language Delay, Delayed Speech)

A condition in which a child does not develop speech and language skills at the expected rate for their age. Speech delay can affect the production of sounds (articulation), the ability to form words and sentences (expressive language), or the understanding of language…

Spina Bifida(also: Neural Tube Defect, Myelomeningocele)

A congenital birth defect in which the embryonic neural tube fails to close completely during development, resulting in an incompletely formed spinal cord. The severity ranges from mild (spina bifida occulta, often asymptomatic) to severe (myelomeningocele, where the spinal cord…

Spinal Muscular Atrophy(also: SMA)

Spinal Muscular Atrophy (SMA) is an autosomal recessive neurodegenerative disease characterized by degeneration of spinal cord motor neurons, atrophy of skeletal muscles, and generalized weakness. Affecting approximately 1 in 6,000-10,000 live births, SMA is the leading genetic…

Spinal Stenosis(also: Lumbar Stenosis, Cervical Stenosis)

A narrowing of the spaces within the spine that can put pressure on the nerves travelling through the spinal column. When spinal stenosis occurs in the cervical (neck) region, it can cause weakness, numbness, or tingling in the hands and arms, affecting fine motor control and…

Stargardt Disease(also: Stargardt Macular Dystrophy, Fundus Flavimaculatus)

An inherited eye condition that causes progressive vision loss affecting the macula, the central part of the retina responsible for sharp, detailed vision. Stargardt disease typically begins in childhood or adolescence and leads to reduced central visual acuity while peripheral…

Stuttering(also: Stammering, Speech Disfluency Disorder)

A speech disorder characterized by involuntary disruptions in the flow of speech, including repetitions of sounds, syllables, or words (e.g., "w-w-water"), prolongations of sounds (e.g., "sssssnake"), and blocks where airflow is stopped and no sound is produced. Stuttering…

Symbrachydactyly

A rare congenital condition in which a child is born with abnormally short fingers that may be webbed, misshapen, or missing. The name combines "sym" (joined), "brachy" (short), and "dactyly" (finger). Typically only one hand is affected, and the underlying bone, muscle,…

Tetraparesis(also: Quadriparesis)

Partial weakness or reduced voluntary movement affecting all four limbs, commonly caused by cerebral palsy, spinal cord injury, or neuromuscular disease. Tetraparesis sits on a spectrum with tetraplegia (complete paralysis of all four limbs) and has significant implications for…

Tetraplegia(also: Quadriplegia)

A condition resulting from injury to the cervical (neck) portion of the spinal cord, causing partial or complete loss of motor function and sensation in all four limbs as well as the trunk. The extent of impairment varies depending on the level and completeness of the injury,…

Tremor(also: Hand Tremor, Essential Tremor)

An involuntary, rhythmic shaking movement of one or more parts of the body, most commonly affecting the hands. Tremor can result from various conditions including essential tremor, Parkinson's disease, multiple sclerosis, stroke, or medication side effects, and its severity can…

Upper-Limb Impairment(also: Upper Extremity Impairment, Arm Impairment, Hand Impairment)

A physical disability affecting the arms, hands, or fingers that limits a person's ability to perform fine motor tasks such as typing, using a mouse, or operating touchscreen devices. Upper-limb impairments can result from conditions including spinal cord injury, stroke,…

Usher Syndrome(also: Usher's Syndrome)

A genetic condition that is the most common cause of combined deafness and blindness (deafblindness). It is characterized by sensorineural hearing loss present from birth and progressive vision loss from retinitis pigmentosa, which typically begins in adolescence or young…