Turner Syndrome: 45,X and Its Mosaic Variants

Introduction

Turner syndrome is the complete or partial absence of a second sex chromosome in a phenotypic female, most often written as 45,X. It is the only monosomy compatible with survival to term, and even so the great majority of 45,X conceptions end in first-trimester miscarriage; the live birth prevalence is around 1 in 2,000-2,500 girls.¹

Two features anchor the diagnosis - short stature and ovarian failure - but the syndrome is genuinely multisystem, and cardiovascular and renal anomalies matter more for long-term prognosis than either of the defining features. This article covers recognition, the associated anomalies that must be actively screened for, and the hormonal management that shapes growth, puberty and fertility.

Aetiology and karyotype

Unlike Down syndrome, Turner syndrome usually arises from loss of a chromosome rather than non-disjunction producing an extra one, and it is not associated with advanced maternal age. The missing X is paternal in origin in around 80% of cases, implying the error most often occurs in paternal meiosis or in an early post-zygotic division.

Clinical features

Presentation varies with age: some cases are picked up antenatally on scan, some in infancy with lymphoedema, and many not until investigation of short stature or delayed puberty in later childhood or adolescence.

Antenatal and neonatal

• Increased nuchal translucency or a cystic hygroma on antenatal ultrasound
• Congenital lymphoedema of the hands and feet, from impaired lymphatic drainage, often the presenting feature in a newborn
• Redundant nuchal skin in infancy, the precursor of the webbed neck seen later

Childhood and adolescence

• Short stature - the single most consistent feature, present in over 95%, with a growth trajectory that falls away from the normal centiles from early childhood and no pubertal growth spurt without treatment
• Gonadal dysgenesis - streak (non-functioning, fibrous) gonads leading to absent, delayed or incomplete puberty and primary or, less often, secondary amenorrhoea
• Webbed neck (from residual nuchal skin), a low posterior hairline, and a broad ('shield') chest with widely spaced nipples
• Cubitus valgus - an increased carrying angle of the elbows
• Short fourth metacarpals, multiple pigmented naevi, and a high-arched palate
• Recurrent otitis media, related to craniofacial shape, with a risk of conductive hearing loss, and later-onset sensorineural hearing loss in adulthood

Examination

A focused examination looks for the features that anchor the diagnosis and screens for its major associations in a single pass.

• Growth parameters - height and weight plotted against age- and sex-matched, ideally Turner-specific, centile charts
• Neck and chest - webbing of the neck, low posterior hairline, broad chest with widely spaced nipples
• Limbs - cubitus valgus, short fourth metacarpals, lymphoedema of the hands and feet in an infant
• Cardiovascular - four-limb blood pressure and pulses (radio-femoral delay in coarctation), auscultation for an ejection click or murmur of a bicuspid valve
• Pubertal staging - Tanner staging to document whether spontaneous puberty has begun
• Skin - multiple pigmented naevi, a recognised but non-specific associated feature

Cardiovascular anomalies

Cardiovascular disease is the leading determinant of premature mortality in Turner syndrome and is present in roughly a quarter to a third of patients, which is why cardiac imaging is mandatory at diagnosis regardless of symptoms.²

• Bicuspid aortic valve - the commonest cardiac anomaly, present in around 15%, predisposing to later aortic stenosis or regurgitation and to aortic dilatation
• Coarctation of the aorta - classically causing radio-femoral delay and upper-limb hypertension with weak femoral pulses
• Aortic dilatation and dissection - the most feared complication; the aorta is intrinsically abnormal in Turner syndrome independent of valve or coarctation status, and dissection can occur even without a previously identified structural lesion
• Hypertension, which is more common independent of coarctation, and compounds aortic risk

Other associated conditions

Renal

Structural renal anomalies occur in around a third, most often a horseshoe kidney, along with duplex collecting systems and other malformations. A renal ultrasound is performed at diagnosis, since these anomalies are usually asymptomatic but relevant to the interpretation of urinary tract infections and to blood pressure management.

Endocrine and metabolic

• Autoimmune hypothyroidism - substantially increased prevalence; thyroid function is checked annually
• Type 2 diabetes - increased risk, related in part to a tendency to android fat distribution and insulin resistance
• Coeliac disease - increased prevalence, similar to Down syndrome, warranting periodic screening
• Osteoporosis - from a combination of oestrogen deficiency and an intrinsic bone density effect; adequate oestrogen replacement and calcium/vitamin D optimisation reduce fracture risk

Neurodevelopmental

General intelligence is typically within the normal range, and this is an important reassurance point for families, since Turner syndrome is frequently and wrongly assumed to involve learning disability. Specific difficulties with visuospatial processing, non-verbal reasoning and social cognition are common, and can affect mathematics performance and peer relationships at school, so educational support should be targeted at these specific domains rather than assumed to be unnecessary.

Investigations

Karyotype from peripheral blood lymphocytes confirms the diagnosis and identifies the pattern (full monosomy, mosaic, or structurally abnormal X), which has implications for prognosis, particularly the likelihood of spontaneous puberty.⁴ A karyotype from a second tissue, or a higher cell count, may be requested if mosaicism is suspected clinically but not confirmed on a standard blood count.

• Echocardiogram and cardiac MRI - at diagnosis and at intervals thereafter to monitor aortic dimensions
• Renal ultrasound - at diagnosis
• Auxology - height, growth velocity plotted on Turner-specific growth charts
• Thyroid function, coeliac serology and HbA1c - at diagnosis and periodically thereafter
• Pelvic ultrasound - to assess uterine and ovarian development, particularly relevant to pubertal induction and later fertility discussions
• Hearing assessment - baseline and periodic review through childhood and adulthood
• LH, FSH and oestradiol - typically show a hypergonadotrophic hypogonadism pattern (raised gonadotrophins, low oestradiol) reflecting primary ovarian failure

Management

Care is coordinated through a multidisciplinary team - paediatric or adult endocrinology, cardiology, gynaecology and, where relevant, fertility services - with a planned transition from paediatric to adult care in adolescence, since surveillance needs (particularly cardiac) continue lifelong.

Growth

Recombinant growth hormone is started in childhood, often from around age 4-6 once growth failure is confirmed, and continued until near-final height is reached. Started early, it adds several centimetres to final adult height compared with no treatment, though final height typically remains below the general population average.³

Puberty and hormone replacement

Most girls need oestrogen replacement to induce puberty, typically starting at a low dose around age 11-12 and increasing gradually over two to three years to mimic normal pubertal development and to protect final height (starting too early or too fast can close growth plates prematurely). A progestogen is added once breakthrough bleeding occurs or after 1-2 years of oestrogen, to establish a withdrawal bleed and protect the endometrium. Oestrogen replacement is continued into adulthood, for bone and cardiovascular health as well as symptom control, in patients who do not have a contraindication.

Fertility

Spontaneous puberty and fertility occur in a minority, mainly those with mosaic karyotypes, but ovarian reserve declines rapidly even in this group, so early, honest fertility counselling is important. Most women with Turner syndrome who wish to conceive do so through donor oocyte IVF; oocyte or ovarian tissue cryopreservation in adolescence is an option for a minority identified early enough with sufficient ovarian reserve. Pregnancy itself carries increased cardiovascular risk, particularly of aortic dissection, and requires pre-pregnancy cardiac assessment and specialist joint obstetric-cardiology care.

Lifelong surveillance

As in Down syndrome, outcomes in Turner syndrome are shaped less by any single treatment than by a structured, lifelong surveillance schedule that continues well past the paediatric transition, since cardiac and metabolic risk persists - and in some respects increases - into adulthood.¹

Presentation by age

Turner syndrome is unusual in that the same condition presents in quite different ways depending on when it comes to attention, and a substantial minority are not diagnosed until adulthood. Knowing the age-specific presentations is what stops the diagnosis being missed.

Differential diagnosis

• Noonan syndrome - overlapping short stature, webbed neck and cardiac disease (typically pulmonary stenosis rather than left-sided lesions), but affects both sexes and has a normal karyotype with a known autosomal dominant genetic cause
• Constitutional delay of growth and puberty - short stature and delayed puberty in an otherwise well child, distinguished by a normal karyotype and typically a family history of similar delayed but ultimately normal development
• Other causes of primary amenorrhoea - androgen insensitivity syndrome, Mullerian agenesis and hypothalamic causes are distinguished by karyotype and hormone profile

Genetic counselling and recurrence risk

Turner syndrome is almost always a sporadic event rather than an inherited one - the loss of a sex chromosome typically occurs during gametogenesis or an early post-zygotic division, not because a parent carries a predisposing genetic change. Recurrence risk for a future pregnancy is therefore not significantly increased above the general population baseline, and this is an important and reassuring point to make explicit to parents, who often assume - by analogy with conditions like Down syndrome - that a specific recurrence risk figure must exist.

The exception is the rare structurally abnormal X chromosome arising from a parental balanced rearrangement, which is why parental karyotyping may be offered where an unusual structural variant, rather than simple monosomy or common mosaicism, is identified. As with any antenatal diagnosis of a chromosomal difference, parents should be offered a clear, unhurried, non-directive discussion of prognosis, and connection with patient support organisations such as the Turner Syndrome Support Society.

Prognosis

With modern multidisciplinary care, life expectancy is improved but on average remains somewhat below the general population, chiefly because of cardiovascular disease. Regular lifelong cardiac surveillance, blood pressure control, and attention to the associated autoimmune and metabolic conditions are what most improve long-term outcomes, rather than any single intervention. With appropriate support, most women with Turner syndrome lead full, independent lives, and the psychological impact of the diagnosis - on body image, fertility and identity - deserves as much attention in follow-up as the physical surveillance schedule.