Question: What is (are) X-linked creatine deficiency ?
Answer: X-linked creatine deficiency is a rare condition that primarily affects the brain. Signs and symptoms generally develop before age 2 and may include mild to severe intellectual disability; delayed speech development, behavioral problems (i.e. autistic features, hyperactivity), and seizures. Less commonly, affected people may have distinctive facial features, heart abnormalities, and gastrointestinal disorders. X-linked creatine deficiency is caused by changes (mutations) in the SLC6A8 gene and is inherited in an X-linked manner. Treatment with high doses of creatine monohydrate, L-arginine, and L-glycine has been used to treat some of the symptoms associated with X-linked creatine deficiency with variable success.
Source: GARD

Question: What are the symptoms of X-linked creatine deficiency ?
Answer: What are the signs and symptoms of X-linked creatine deficiency? The Human Phenotype Ontology provides the following list of signs and symptoms for X-linked creatine deficiency. If the information is available, the table below includes how often the symptom is seen in people with this condition. You can use the MedlinePlus Medical Dictionary to look up the definitions for these medical terms. Signs and Symptoms Approximate number of patients (when available) Abnormality of metabolism/homeostasis 90% Cognitive impairment 90% Neurological speech impairment 90% Abnormality of movement 50% Aganglionic megacolon 50% Autism 50% Constipation 50% Decreased body weight 50% Hypertonia 50% Hypoplasia of the zygomatic bone 50% Incoordination 50% Intestinal obstruction 50% Muscular hypotonia 50% Open mouth 50% Seizures 50% Short stature 50% Cutis laxa 7.5% Joint hypermobility 7.5% Mask-like facies 7.5% Microcephaly 7.5% Ptosis 7.5% Aggressive behavior - Attention deficit hyperactivity disorder - Broad forehead - Delayed myelination - Delayed speech and language development - Dystonia - Exotropia - Failure to thrive - Feeding difficulties in infancy - Gait disturbance - Hypermetropia - Hypoplasia of midface - Hypoplasia of the corpus callosum - Ileus - Impaired social interactions - Infantile onset - Intellectual disability - Long face - Malar flattening - Mandibular prognathia - Motor delay - Myopathic facies - Narrow face - Neonatal hypotonia - Pes cavus - Poor hand-eye coordination - Spasticity - Stereotypic behavior - Tall stature - Underfolded superior helices - Vomiting - X-linked recessive inheritance - The Human Phenotype Ontology (HPO) has collected information on how often a sign or symptom occurs in a condition. Much of this information comes from Orphanet, a European rare disease database. The frequency of a sign or symptom is usually listed as a rough estimate of the percentage of patients who have that feature. The frequency may also be listed as a fraction. The first number of the fraction is how many people had the symptom, and the second number is the total number of people who were examined in one study. For example, a frequency of 25/25 means that in a study of 25 people all patients were found to have that symptom. Because these frequencies are based on a specific study, the fractions may be different if another group of patients are examined. Sometimes, no information on frequency is available. In these cases, the sign or symptom may be rare or common.
Source: GARD