Turner Syndrome

Definition And Epidemiology

Turner syndrome is the most common sex chromosome abnormality in females, characterized by complete or partial absence of the second sex chromosome.
Incidence occurs in approximately 1 in 2000 to 1 in 2500 live female births.
Accounts for approximately 10% of spontaneous first-trimester abortions, with 95-99% of 45,X conceptuses miscarrying.
Advanced maternal age is not a predisposing factor.

Loss of X-linked genes that escape X-inactivation contributes to ovarian failure, neurocognitive profiles, and visceral organ malformations.
Haploinsufficiency of the SHOX (Short Stature Homeobox) gene on the pseudoautosomal region of the X chromosome directly causes short stature and skeletal anomalies.

Karyotype Variant	Prevalence	Clinical Characteristics
Classic Monosomy (45,X)	45-50%	Typically results from paternal meiotic non-disjunction involving loss of paternal X or Y chromosome.
Mosaicism (45,X/46,XX)	15-25%	Associated with milder phenotype and higher chance of spontaneous puberty or menstruation.
Y Chromosome Material (45,X/46,XY)	10-12% of mosaics	Carries a 15-30% risk of developing gonadoblastoma or dysgerminoma in dysgenetic gonads.
Structural Abnormalities	Variable	Includes isochromosome Xq (46,X,i(Xq)), ring X chromosome, or Xp deletions.

System	Manifestations
Fetal And Neonatal	Increased nuchal translucency, cystic hygroma, non-immune hydrops fetalis, left-sided cardiac defects, congenital lymphedema of hands and feet, redundant nuchal skin.
Growth And Skeletal	Short stature, micrognathia, high arched palate, low-set ears, broad shield-like chest, widely spaced nipples, cubitus valgus, short fourth metacarpals, genu valgum, Madelung deformity, scoliosis.
Reproductive	Gonadal dysgenesis with streak ovaries, primary amenorrhea, delayed or absent pubertal development, hypergonadotropic hypogonadism.
Cardiovascular	Bicuspid aortic valve, coarctation of aorta, aortic stenosis, mitral valve prolapse.
Renal	Horseshoe kidney, duplicated collecting systems, unilateral renal agenesis, abnormal renal vasculature.
Endocrine And Metabolic	Autoimmune thyroiditis, acquired hypothyroidism, insulin resistance, type 2 diabetes mellitus, dyslipidemia.
Neurocognitive	Normal global intelligence, impaired non-verbal/visual-spatial skills, mathematics struggles, impaired executive functioning, ADHD, social anxiety.
Otorhinolaryngology	Recurrent acute otitis media, progressive sensorineural hearing loss.
Ophthalmology	Strabismus, amblyopia, ptosis, hypermetropia, red-green color blindness.

Standard karyotype requires analysis of at least 30 peripheral blood lymphocytes.
Fluorescence in situ hybridization (FISH) is recommended to detect hidden Y-chromosome mosaicism using SRY/DYZ3 probes.
Chromosomal microarray (CMA) detects small deletions or duplications not visible on standard karyotype.
Baseline screening requires transthoracic echocardiogram, renal ultrasound, thyroid stimulating hormone (TSH), free T4, tissue transglutaminase IgA, HbA1c, lipid profile, and baseline audiogram.
Luteinizing hormone (LH) and follicle stimulating hormone (FSH) confirm hypergonadotropic hypogonadism.

Recombinant human growth hormone (rhGH) initiation is recommended around 4-6 years of age when growth failure becomes evident.
Supraphysiological doses (0.045 to 0.05 mg/kg/day) are required due to relative growth hormone resistance.
Oxandrolone addition after 10 years of age is considered if extreme short stature is predicted.

Estrogen replacement therapy (ERT) initiation occurs between 11 to 12 years of age.
Regimen starts with ultra-low dose transdermal 17-beta estradiol to prevent rapid bone age advancement, titrating upwards every 6 months over 2-3 years.
Cyclic progesterone addition occurs 2 years after starting estrogen to prevent endometrial hyperplasia and induce regular withdrawal bleeds.

Lifelong surveillance requires repeat echocardiography every 5 years or MRI aorta every 5-10 years in adults.
Strict blood pressure control using beta-blockers or angiotensin receptor blockers minimizes aortic wall stress.

Prophylactic laparoscopic gonadectomy is mandated if Y-chromosome material is detected due to high malignant transformation risk.
Infertility options include oocyte donation and in vitro fertilization (IVF).
Pregnancy poses high risks, requiring pre-conception cardiovascular screening, with aortic diameter index >2.5 cm/m2 serving as an absolute contraindication.