A functional motor deficit where individuals whose responses to the challenges of exercise fail to achieve levels considered normal for their age and gender.

Reduced strength and weakness of the muscles of the arms and legs.

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).

Dysarthric speech is a general description referring to a neurological speech disorder characterized by poor articulation. Depending on the involved neurological structures, dysarthria may be further classified as spastic, flaccid, ataxic, hyperkinetic and hypokinetic, or mixed.

Upturning of the big toe (and sometimes fanning of the other toes) in response to stimulation of the sole of the foot. If the Babinski sign is present it can indicate damage to the corticospinal tract.

The term gait disturbance can refer to any disruption of the ability to walk.

Reduction of neurologic reflexes such as the knee-jerk reaction.

A type of Developmental delay characterized by a delay in acquiring motor skills.

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.

A type of myopathy associated with mitochondrial disease and characterized by findings on biopsy such as ragged red muscle fibers.

A lack of strength of the proximal muscles.

Decreased strength of the neck musculature.

Facial nerve palsy is a dysfunction of cranial nerve VII (the facial nerve) that results in inability to control facial muscles on the affected side with weakness of the muscles of facial expression and eye closure. This can either be present in unilateral or bilateral form.

Reduced strength of the orbicularis oculi, the circumorbital muscle in the face that closes the eyelid.

Amyotrophy (muscular atrophy) affecting the proximal musculature.

Generalized muscular hypotonia (abnormally low muscle tone).

An abnormal appearance of muscle fibers observed on muscle biopsy. Ragged red fibers can be visualized with Gomori trichrome staining as irregular and intensely red subsarcolemmal zones, whereas the normal myofibrils are green. The margins of affect fibers appear red and ragged. The ragged-red is due to the accumulation of abnormal mitochondria below the plasma membrane of the muscle fiber, leading to the appearance of a red rim and speckled sarcoplasm.

An abnormally reduced activity of the enzyme cytochrome C oxidase in muscle tissue.

The presence of abnormal electromyographic patterns indicative of myopathy, such as small-short polyphasic motor unit potentials.

WFS1 spectrum disorder (WFS1-SD) comprises classic WFS1 spectrum disorder and nonclassic WFS1 spectrum disorder. Classic WFS1-SD, a progressive neurodegenerative disorder, is characterized by onset of diabetes mellitus and optic atrophy before age 16 years. Additional complications may include one or more of the following: variable hearing impairment / deafness, diabetes insipidus, neurologic abnormalities, neurogenic bladder, and psychiatric abnormalities. Nonclassic WFS1-SD is less common than classic WFS1-SD. Phenotypes that appear to be milder than classic WFS1-SD include: optic atrophy and hearing impairment; neonatal diabetes, profound congenital deafness, and cataracts; isolated diabetes mellitus; isolated congenital cataracts; and isolated congenital, slowly progressive, and low-frequency (<2000 Hz) sensorineural hearing loss.

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 reduction in the activity of the mitochondrial respiratory chain complex IV, which is part of the electron transport chain in mitochondria.