- Nystagmus (HP:0000639): Rhythmic, involuntary oscillations of one or both eyes related to abnormality in fixation, conjugate gaze, or vestibular mechanisms. Evidence: TAS. Frequency: Very frequent (HP:0040281). (ORPHA:280219)
- Delayed speech and language development (HP:0000750): A degree of language development that is significantly below the norm for a child of a specified age. Evidence: TAS. Frequency: Very frequent (HP:0040281). (ORPHA:280219)
- Ataxia (HP:0001251): Ataxia refers to impaired coordination of voluntary muscle movement. Cerebellar ataxia refers to ataxia due to dysfunction of the cerebellum. This causes a variety of elementary neurological deficits including asynergy (lack of coordination between muscles, limbs and joints), dysmetria (lack of ability to judge distances that can lead to under- or overshoot in grasping movements), and dysdiadochokinesia (inability to perform rapid movements requiring antagonizing muscle groups to be switched on and off repeatedly). Evidence: TAS. Frequency: Very frequent (HP:0040281). (ORPHA:280219)
- Mild intellectual disability (HP:0001256): Mild intellectual disability (ID) is defined as a type of ID characterized by mildly sub-average adaptive functioning and intellectual functioning, with an intelligence quotient (IQ) the range of 50-69. Evidence: TAS. Frequency: Very frequent (HP:0040281). (ORPHA:280219)
- Global developmental delay (HP:0001263): A delay in the achievement of motor or mental milestones in the domains of development of a child, including motor skills, speech and language, cognitive skills, and social and emotional skills. This term should only be used to describe children younger than five years of age. Evidence: TAS. Frequency: Very frequent (HP:0040281). (ORPHA:280219)
- Cerebral hypomyelination (HP:0006808): Reduced amount of myelin in the nervous system resulting from defective myelinogenesis in the white matter of the central nervous system. Evidence: TAS. Frequency: Very frequent (HP:0040281). (ORPHA:280219)
- EEG with abnormally slow frequencies (HP:0011203): EEG with abnormally slow frequencies. Evidence: TAS. Frequency: Very frequent (HP:0040281). (ORPHA:280219)
- Delayed ability to walk (HP:0031936): A failure to achieve the ability to walk at an appropriate developmental stage. Most children learn to walk in a series of stages, and learn to walk short distances independently between 12 and 15 months. Evidence: TAS. Frequency: Very frequent (HP:0040281). (ORPHA:280219)
- Confluent hyperintensity of cerebral white matter on MRI (HP:0040330): Areas of brighter than expected MRI signal in the white matter of the brain whereby individual patches run together. Evidence: TAS. Frequency: Very frequent (HP:0040281). (ORPHA:280219)
- Hypotonia (HP:0001252): Hypotonia is an abnormally low muscle tone (the amount of tension or resistance to movement in a muscle). Even when relaxed, muscles have a continuous and passive partial contraction which provides some resistance to passive stretching. Hypotonia thus manifests as diminished resistance to passive stretching. Hypotonia is not the same as muscle weakness, although the two conditions can co-exist. Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:280219)
- Spasticity (HP:0001257): A motor disorder characterized by a velocity-dependent increase in tonic stretch reflexes with increased muscle tone, exaggerated (hyperexcitable) tendon reflexes. Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:280219)
- Spastic tetraparesis (HP:0001285): Spastic weakness affecting all four limbs. Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:280219)
- Gait disturbance (HP:0001288): The term gait disturbance can refer to any disruption of the ability to walk. Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:280219)
- Dystonia (HP:0001332): An abnormally increased muscular tone that causes fixed abnormal postures. There is a slow, intermittent twisting motion that leads to exaggerated turning and posture of the extremities and trunk. Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:280219)
- Abnormal speech pattern (HP:0002167): An abnormality in the sound (volume) or cadence (rate) of speech. Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:280219)
- Athetosis (HP:0002305): A slow, continuous, involuntary writhing movement that prevents maintenance of a stable posture. Athetosis involves continuous smooth movements that appear random and are not composed of recognizable sub-movements or movement fragments. In contrast to chorea, in athetosis, the same regions of the body are repeatedly involved. Athetosis may worsen with attempts at movement of posture, but athetosis can also occur at rest. Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:280219)
- Head tremor (HP:0002346): An unintentional, oscillating to-and-fro muscle movement affecting head movement. Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:280219)
- Poor head control (HP:0002421): Difficulty to maintain correct position of the head while standing or sitting. Infant head lag is observed when the head seems to flop around or lags posteriorly behind the trunk. Several articles have maintained that head lag should be absent by age 3 to 4 months. Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:280219)
- Abnormal pyramidal sign (HP:0007256): Functional neurological abnormalities related to dysfunction of the pyramidal tract. Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:280219)
- Abnormality of somatosensory evoked potentials (HP:0007377): An abnormality of somatosensory evoked potentials (SSEP), i.e., of the electrical signals of sensation going from the body to the brain in response to a defined stimulus. Recording electrodes are placed over the scalp, spine, and peripheral nerves proximal to the stimulation site. Clinical studies generally use electrical stimulation of peripheral nerves to elicit potentials. SSEP testing determines whether peripheral sensory nerves are able to transmit sensory information like pain, temperature, and touch to the brain. Abnormal SSEPs can result from dysfunction at the level of the peripheral nerve, plexus, spinal root, spinal cord, brain stem, thalamocortical projections, or primary somatosensory cortex. Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:280219)
- Axial hypotonia (HP:0008936): Muscular hypotonia (abnormally low muscle tone) affecting the musculature of the trunk. Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:280219)
- Delayed ability to sit (HP:0025336): A failure to achieve the ability to sit at an appropriate developmental stage. Most children sit with support at 6 months of age and sit steadily without support at 9 months of age. Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:280219)
- Titubation (HP:0030187): Nodding movement of the head or body. Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:280219)
- Cognitive impairment (HP:0100543): Abnormal cognition is characterized by deficits in thinking, reasoning, or remembering. Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:280219)
- Abnormality of extrapyramidal motor function (HP:0002071): A neurological condition related to lesions of the basal ganglia leading to typical abnormalities including akinesia (inability to initiate changes in activity and perform volitional movements rapidly and easily), muscular rigidity (continuous contraction of muscles with constant resistance to passive movement), chorea (widespread arrhythmic movements of a forcible, rapid, jerky, and restless nature), athetosis (inability to sustain the muscles of the fingers, toes, or other group of muscles in a fixed position), and akathisia (inability to remain motionless). Evidence: TAS. Frequency: Occasional (HP:0040283). (ORPHA:280219)
- Orofacial dyskinesia (HP:0002310). Evidence: TAS. Frequency: Occasional (HP:0040283). (ORPHA:280219)
- Dystonic gait (HP:0031954): Dystonic gait disorders frequently appear bizarre, particularly because activity increases dystonic tonus and posture. The abnormal posture of the foot in dystonic gait typically involves inversion, plantar flexion and tonic extension of the big toe. In many patients complex types of walking, such as walking backwards and running are paradoxically less impaired than walking forward and may seem completely unaffected. Sensory tricks, for instance, if the affected individual rests a hand on his or her neck, may improve or even normalize dystonic gait in some patients. Evidence: TAS. Frequency: Occasional (HP:0040283). (ORPHA:280219)
These phenotypes are associated with the disease Pelizaeus-Merzbacher disease, classic form (ORPHA:280219).