- 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: PCS. Frequency: 1/2. (PMID:31290619)
- Short 5th toe (HP:0011917): Underdevelopment (hypoplasia) of the fifth toe. Evidence: PCS. Frequency: 1/2. (PMID:31290619)
- Decreased body weight (HP:0004325): Abnormally low body weight. Evidence: PCS. Frequency: 2/2. (PMID:31290619)
- Hypsarrhythmia (HP:0002521): Hypsarrhythmia is abnormal interictal high amplitude waves and a background of irregular spikes. There is continuous (during wakefulness), high-amplitude (>200 Hz), generalized polymorphic slowing with no organized background and multifocal spikes demonstrated by electroencephalography (EEG). Evidence: PCS. Frequency: 1/2. (PMID:31290619)
- Short stature (HP:0004322): A height below that which is expected according to age and gender norms. Although there is no universally accepted definition of short stature, many refer to "short stature" as height more than 2 standard deviations below the mean for age and gender (or below the 3rd percentile for age and gender dependent norms). Evidence: PCS. Frequency: 3/3. (PMID:31290619;PMID:28766551)
- Cerebellar atrophy (HP:0001272): Cerebellar atrophy is defined as a cerebellum with initially normal structures, in a posterior fossa with normal size, which displays enlarged fissures (interfolial spaces) in comparison to the foliae secondary to loss of tissue. Cerebellar atrophy implies irreversible loss of tissue and result from an ongoing progressive disease until a final stage is reached or a single injury, e.g. an intoxication or infectious event. Evidence: PCS. Frequency: 2/2. (PMID:31290619)
- Generalized hypotonia (HP:0001290): Generalized muscular hypotonia (abnormally low muscle tone). Evidence: PCS. Frequency: 2/2. (PMID:31290619)
- Elevated CSF fumarate concentration (HP:0033503): An increased concentration of fumarate, an intermediate in the citric acid cycle, in the cebrebrospinal fluid. Evidence: PCS. Frequency: 1/1. (PMID:31290619)
- Short 4th toe (HP:0008093): Underdevelopment (hypoplasia) of the fourth toe. Evidence: PCS. Frequency: 1/2. (PMID:31290619)
- Hyperactive patellar reflex (HP:0007083). Evidence: PCS. Frequency: 1/2. (PMID:31290619)
- Failure to thrive (HP:0001508): Failure to thrive (FTT) refers to a child whose physical growth is substantially below the norm. Evidence: PCS. Frequency: 1/1. (PMID:28766551)
- Short 2nd toe (HP:0001885): Underdevelopment (hypoplasia) of the second toe. Evidence: PCS. Frequency: 1/2. (PMID:31290619)
- Cerebral atrophy (HP:0002059): Atrophy (wasting, decrease in size of cells or tissue) affecting the cerebrum. Evidence: PCS. Frequency: 2/2. (PMID:31290619)
- Short 3rd toe (HP:0005643): Underdevelopment (hypoplasia) of the third toe. Evidence: PCS. Frequency: 1/2. (PMID:31290619)
- Delayed skeletal maturation (HP:0002750): A decreased rate of skeletal maturation. Delayed skeletal maturation can be diagnosed on the basis of an estimation of the bone age from radiographs of specific bones in the human body. Evidence: PCS. Frequency: 1/1. (PMID:28766551)
- Developmental regression (HP:0002376): Loss of developmental skills, as manifested by loss of developmental milestones. Evidence: PCS. Frequency: 2/2. Onset: Infantile onset (HP:0003593). (PMID:31290619)
- Chromosomal breakage induced by crosslinking agents (HP:0003221): Increased amount of chromosomal breaks in cultured blood lymphocytes or other cells induced by treatment with DNA cross-linking agents such as diepoxybutane and mitomycin C. Evidence: PCS. Frequency: 1/1. (PMID:28766551)
- Increased circulating lactate concentration (HP:0002151): Abnormally increased level of blood lactate (2-hydroxypropanoic acid). Lactate is produced from pyruvate by lactate dehydrogenase during normal metabolism. The terms lactate and lactic acid are often used interchangeably but lactate (the component measured in blood) is strictly a weak base whereas lactic acid is the corresponding acid. Lactic acidosis is often used clinically to describe elevated lactate but should be reserved for cases where there is a corresponding acidosis (pH below 7.35). Evidence: PCS. Frequency: 1/2. (PMID:31290619)
- Increased CSF lactate (HP:0002490): Increased concentration of lactate in the cerebrospinal fluid. Evidence: PCS. Frequency: 1/1. (PMID:31290619)
- Epileptic spasm (HP:0011097): A sudden flexion, extension, or mixed extension-flexion of predominantly proximal and truncal muscles that is usually more sustained than a myoclonic movement but not as sustained as a tonic seizure. Limited forms may occur: Grimacing, head nodding, or subtle eye movements. Epileptic spasms frequently occur in clusters. Infantile spasms are the best known form, but spasms can occur at all ages. Evidence: PCS. Frequency: 2/2. Onset: Childhood onset (HP:0011463). (PMID:31290619)
- Prominent nasal bridge (HP:0000426): Anterior positioning of the nasal root in comparison to the usual positioning for age. Evidence: PCS. Frequency: 1/1. (PMID:28766551)
- Autosomal recessive inheritance (HP:0000007): A mode of inheritance that is observed for traits related to a gene encoded on one of the autosomes (i.e., the human chromosomes 1-22) in which a trait manifests in individuals with two pathogenic alleles, either homozygotes (two copies of the same mutant allele) or compound heterozygotes (whereby each copy of a gene has a distinct mutant allele). Evidence: PCS. (PMID:28766551)
- Decreased activity of mitochondrial complex IV (HP:0008347): A reduction in the activity of the mitochondrial respiratory chain complex IV, which is part of the electron transport chain in mitochondria. Evidence: PCS. Frequency: 1/1. (PMID:31290619)
- Short 5th finger (HP:0009237): Hypoplasia (congenital reduction in size) of the fifth finger, also known as the little finger. Evidence: PCS. Frequency: 1/1. (PMID:28766551)
- Frontal bossing (HP:0002007): Bilateral bulging of the lateral frontal bone prominences with relative sparing of the midline. Evidence: PCS. Frequency: 1/1. (PMID:28766551)
- Myoclonus (HP:0001336): Very brief, involuntary random muscular contractions occurring at rest, in response to sensory stimuli, or accompanying voluntary movements. Evidence: PCS. Frequency: 1/2. (PMID:31290619)
These phenotypes are associated with the disease mitochondrial complex IV deficiency, nuclear type 16 (OMIM:619060).