Hemihypertrophy (Hemihyperplasia)

Hemihypertrophy, more correctly called hemihyperplasia, is congenital asymmetric overgrowth of one side, or part of one side, of the body. It is a comparative clinical diagnosis: you recognise it by comparing the two halves of the body and finding that one side (a limb, several limbs, the face or a single digit) is larger.

The name. Hyperplasia is an increase in the number of cells; hypertrophy is an increase in cell size. The underlying process is excessive cell proliferation, so "hemihyperplasia" is the preferred term, although "hemihypertrophy" remains in common clinical use. The name matters in the exam.

Two separate problems. For the orthopaedic surgeon there are two completely separate concerns, and the examiner is testing whether you remember both:

1. The orthopaedic problem - a structural limb-length discrepancy, sometimes with localised digit or foot enlargement, that tends to worsen as the child grows and needs monitoring and, often, growth-modulating surgery.
2. The oncological problem - an increased risk of embryonal tumours, principally Wilms tumour (nephroblastoma) and hepatoblastoma, which mandates a tumour-surveillance programme and referral to clinical genetics.

Isolated or syndromic. Hemihyperplasia may be isolated, with no other features, or a sign of a wider overgrowth syndrome. The two that matter most are the Beckwith-Wiedemann spectrum, driven by imprinting defects at chromosome 11p15.5, and the PIK3CA-related overgrowth spectrum (PROS).

Why one side grows more. Lateralised overgrowth almost always reflects post-zygotic (somatic) mosaicism. A growth-promoting genetic or epigenetic change arises in one cell after fertilisation, and all of its descendants form the overgrown segment. This explains why the overgrowth is segmental and asymmetric rather than affecting the whole body, and why most cases are sporadic with a low recurrence risk for siblings.

The Beckwith-Wiedemann spectrum. The single most important syndromic association is the Beckwith-Wiedemann spectrum (BWSp), an imprinting disorder of the 11p15.5 region. The region contains growth-regulating genes, including IGF2 and CDKN1C, controlled by two imprinting centres. Loss of methylation, gain of methylation, paternal uniparental disomy (UPD) or mutations disturb the balance towards overgrowth.

Why the subtype matters. The molecular subtype predicts both the risk and the type of tumour. Paternal uniparental disomy of chromosome 11 carries a higher risk of Wilms tumour and is associated with more severe lateralised overgrowth, whereas isolated loss of methylation at one centre carries a different, often lower, risk.

The PIK3CA-related overgrowth spectrum. PROS is caused by somatic activating mutations in PIK3CA, which switch on the PI3K-AKT-mTOR growth pathway in the affected tissue. It includes overlapping entities such as CLOVES syndrome, fibroadipose hyperplasia and isolated macrodactyly, and produces segmental overgrowth often combined with vascular and lipomatous lesions. Recognising it matters because it opens the door to targeted PI3K-pathway inhibition, discussed under Management.

Histology. Biopsy of the overgrown tissue shows an increase in the number of otherwise normal cells, true hyperplasia, rather than abnormal or atypical cells. That distinguishes constitutional overgrowth from a localised tumour, even though the overall predisposition to embryonal tumours is increased.

Presentation. Most children present in infancy or early childhood, when a parent or clinician notices that one limb, or one side, is bigger than the other. The orthopaedic referral is often triggered by a noticeable limb-length discrepancy, a limp or pelvic obliquity. Some are detected during a Beckwith-Wiedemann work-up after neonatal features such as macroglossia, an abdominal wall defect or hypoglycaemia.

Examination. Because hemihyperplasia is a comparative diagnosis, the examination is a systematic side-to-side comparison:

• The pattern - which segments are enlarged (whole limb, part of a limb, single digit, face), and whether it is ipsilateral or crossed
• Leg lengths - true and apparent lengths, and the block test for the functional discrepancy with its resulting pelvic obliquity and compensatory scoliosis
• Girth and digits - circumference, and digit length and width with callipers, compared with the normal side and recorded for serial monitoring
• Syndromic clues - macroglossia, an abdominal wall scar or defect, ear creases or pits (Beckwith-Wiedemann); capillary or lipomatous and vascular lesions and macrodactyly (PROS); a port-wine stain with varicosities (Klippel-Trenaunay)
• The abdomen - an organomegaly or mass; a clinical adjunct to imaging surveillance, never a substitute for it

Imaging the discrepancy. The orthopaedic work-up is the standard limb-length discrepancy pathway:

• Standing long-leg radiographs (scanogram or EOS) - quantify the discrepancy and identify the segments involved
• Left hand and wrist radiograph for bone age - essential, because growth prediction and the timing of epiphysiodesis depend on skeletal rather than chronological age
• Serial measurement - a single value is not enough; the trajectory drives decisions

The first measurement. The discrepancy at first assessment predicts the final discrepancy, so early measurement is prognostic, not just a baseline. The card below shows the limit of that: the initial value predicts the endpoint far better than it predicts how much a given child will worsen, which is why re-measurement is not optional.

Genetics. Refer to clinical genetics for molecular testing: 11p15.5 methylation and UPD studies for Beckwith-Wiedemann spectrum, and PIK3CA testing where PROS is suspected. PIK3CA is tested on overgrown tissue, not blood, since mosaic mutations may be undetectable in blood.

MRI of an enlarged segment is for planning surgery on a digit or limb, or when a vascular or lipomatous lesion is suspected.

Two tracks at once. Management runs on two tracks, and the orthopaedic surgeon is responsible for making sure both happen. Oncological safety comes first: tumour surveillance and a genetics referral, the priority that protects the child's life. Orthopaedic care of the limb-length discrepancy, and of any localised digit or foot overgrowth, follows standard deformity principles. The right model is a multidisciplinary team of paediatric orthopaedics, clinical genetics, paediatric oncology or nephrology, and the family.

From the condition.

• Embryonal tumour - Wilms tumour and hepatoblastoma, the rationale for the entire surveillance programme; a late tumour can still occur after the screening window closes
• Progressive limb-length discrepancy - untreated, it leads to a limp, pelvic obliquity, compensatory scoliosis and gait inefficiency
• Psychosocial impact - visible asymmetry and, with facial or digit involvement, appearance-related distress

From treatment.

• Epiphysiodesis errors - over- or undercorrection from mistimed surgery, angular deformity if a physis is partially arrested, and the irreversibility of the procedure
• Lengthening complications - pin-site infection, joint stiffness, premature consolidation, neurovascular injury and refracture, the reason lengthening is reserved for larger discrepancies

Why it matters. Klippel-Trenaunay, Proteus syndrome, an arteriovenous malformation and pseudo-discrepancy from contralateral shortening all present as a big leg. The classic vascular malformations and pseudo-atrophy do not carry the embryonal-tumour risk of constitutional hemihyperplasia, so telling them apart decides whether a child is committed to years of surveillance.

The anchor. Constitutional (true) hemihyperplasia is proportionate enlargement of otherwise normal-looking tissue, present from birth and hyperplastic on biopsy, and it carries the embryonal-tumour predisposition that mandates surveillance.

The mimics.

• Klippel-Trenaunay syndrome - the triad of a capillary malformation (port-wine stain), varicose or anomalous veins and limb overgrowth (usually a lower limb): a low-flow capillary-lymphatico-venous malformation. The cutaneous stain and venous varicosities give it away; the overgrowth is driven by the vascular anomaly.
• Parkes-Weber syndrome - overgrowth with a high-flow arteriovenous malformation. A bruit or thrill, warmth and the risk of high-output cardiac failure separate it from low-flow Klippel-Trenaunay.
• Proteus syndrome - a mosaic AKT1 disorder that is postnatal, progressive, distorting and disproportionate, with the near-pathognomonic cerebriform connective-tissue naevus (typically plantar). Its relentless asymmetric progression contrasts with the more static, proportionate hemihyperplasia.
• Neurofibromatosis type 1 - cafe-au-lait macules, neurofibromas and plexiform lesions can drive regional overgrowth of a limb or digit.
• Acquired overgrowth - growth after injury or inflammation, for example after a fracture or with juvenile arthritis.
• Pseudo-hemihyperplasia (contralateral atrophy) - the "long" side is normal and the other side is small (hemiplegic cerebral palsy, prior poliomyelitis, physeal growth arrest). Always confirm which side is abnormal by measuring both against normal percentiles, because a wasted contralateral limb needs neither tumour surveillance nor an overgrowth work-up.

Who is screened. Every child with hemihyperplasia, isolated or syndromic, needs an embryonal-tumour surveillance programme. Modern cancer-predisposition guidance recommends renal (Wilms) surveillance for any condition carrying a Wilms tumour risk greater than 1 percent. Isolated hemihyperplasia (a quoted Wilms risk in the order of a few percent, around 5 percent in some series) and most Beckwith-Wiedemann molecular subgroups comfortably clear that threshold, which is exactly why a child labelled "isolated", with no syndromic features, is still enrolled. The Fischer case below frames this explicitly: children with conditions conferring more than 1 percent Wilms risk are screened.

The programme. Abdominal (renal and hepatic) ultrasound every 3 months from diagnosis to about age 7-8 years, the window in which most Wilms tumours and hepatoblastomas occur. Serum alpha-fetoprotein (AFP) is added in some protocols, chiefly for Beckwith-Wiedemann spectrum, to help detect hepatoblastoma.

Why the interval is short. Wilms tumours grow quickly. A 3 to 4-monthly interval is chosen so that a tumour is detected while it is still small and low-stage, and therefore curable, before it can advance a stage between scans. Annual imaging would miss that window and let tumours present larger and at higher stage; the interval is a deliberate compromise, frequent enough to catch fast-growing tumours early but not so frequent as to be unfeasible.

Why renal and hepatic, and why the age cut-off. The two dominant embryonal tumours differ in their age of peak incidence, which sets both the imaging targets and the duration:

• Hepatoblastoma peaks earliest, largely in the first few years of life (mostly under age 3 to 4), so AFP (in selected, chiefly Beckwith-Wiedemann, protocols) and hepatic ultrasound matter most early; some protocols taper the hepatic and AFP component sooner.
• Wilms tumour peaks a little later, with roughly 95 percent occurring by age 7, which is the evidence base for continuing renal ultrasound to about age 7 to 8 and then stopping.

Alpha-fetoprotein. AFP is a genuinely awkward test in infancy because it is physiologically high and falls variably over the first months. Only markedly elevated values are reliably meaningful: the Zarate cohort below found very high AFP correlated with identifiable liver lesions, while normal-range values were uninterpretable.

Counsel about false positives before the first scan, not after one. In the largest real-world series on this page, 40% of children had an abnormal ultrasound at some point, almost all nephromegaly or a size discrepancy rather than tumour, against a 3.2% tumour detection rate. Roughly one child in two and a half will generate an abnormal scan, repeat imaging, anxiety and a frightening conversation. That is the price of a programme worth running, and families cope with it far better when it is predicted than when it arrives unannounced.

When screening ends. Counsel the family that the programme has a defined endpoint, that surveillance lowers but does not abolish the risk, and that any new abdominal symptoms warrant earlier review.

Hemihyperplasia is too rare for arthroplasty/implant registries, and there is no single global orthopaedic guideline. Practice is shaped by clinical-genetics consensus statements (which define tumour surveillance) and by standard paediatric limb-length-discrepancy principles for the orthopaedic component.

Global epidemiology

• A recognised but rare cause of paediatric limb-length discrepancy; most cases are sporadic and reflect somatic mosaicism.
• Reported embryonal-tumour risk with isolated hemihyperplasia is in the order of a few percent (Wilms tumour quoted around 5% in some series), higher in certain Beckwith-Wiedemann molecular subgroups.
• Limb-length discrepancy increases with growth and is more severe in Beckwith-Wiedemann spectrum (especially paternal chromosome 11 uniparental disomy) than in isolated overgrowth.

Side-by-side practice positions

High- versus limited-resource practice

• High-resource settings: multidisciplinary overgrowth clinics with molecular testing, scheduled ultrasound surveillance, bone-age-based growth prediction (EOS/scanogram), and access to PI3K-inhibitor pathways for selected PROS cases.
• Limited-resource settings: diagnosis remains clinical and radiographic; the priority is to still implement abdominal ultrasound tumour surveillance (the highest-value, low-cost intervention) and to manage the discrepancy with serial clinical measurement, shoe raises and timed epiphysiodesis, which do not require advanced technology.

Lifelong-into-adolescence orthopaedic follow-up and a clear transition to adult services are important everywhere, alongside ensuring the family understands when tumour surveillance ends and that new abdominal symptoms always warrant review.

The evidence underpinning hemihyperplasia care is drawn from clinical-genetics consensus work and paediatric orthopaedic cohorts (full citations are in the EvidenceCards above).

• Beckwith-Wiedemann spectrum consensus (Brioude et al, Nat Rev Endocrinol 2018) defines lateralized overgrowth as a core feature and links molecular subtype to tumour risk and surveillance - the framework for the genetics referral.
• Limb-length discrepancy evolution (Carli/De Pellegrin/Mussa et al, J Pediatr 2021) is the key orthopaedic study: discrepancy worsens with growth, the first measurement predicts the final discrepancy, and syndromic (especially paternal chromosome 11 uniparental disomy) cases are most severe.
• Tumour surveillance rationale and real-world yield (Tan & Amor, J Paediatr Child Health 2006; Zarate et al, Am J Med Genet A 2009) justify the 3-monthly abdominal ultrasound programme and show that ultrasound detects early renal/hepatic pathology while alpha-fetoprotein is hard to interpret in infancy.
• A late Wilms tumour after normal screening (Fischer/Kalish et al, Urology 2021) is a cautionary reminder that surveillance reduces but does not abolish risk.
• Timing of growth modulation (Lee et al, Bone Joint J 2013) shows the Green-Anderson growth-remaining method is the most accurate for planning epiphysiodesis, that prediction is imperfect, and that all methods tend to overcorrect.

Overall the evidence is level IV-V (consensus statements, retrospective cohorts and case reports); there are no randomised trials, reflecting the rarity of the condition.