Caudal Regression Syndrome (Sacral Agenesis)

Caudal regression syndrome (CRS) - also called sacral agenesis or caudal dysgenesis - is a spectrum of congenital malformations caused by premature termination of development of the caudal (tail) end of the spine and cord.

• Spectrum: From isolated coccygeal absence, through partial and complete sacral agenesis, to the most severe extreme of sirenomelia (fused lower limbs).
• Prevalence: Rare, around 1 in 25,000 live births; markedly higher in pregnancies complicated by diabetes.
• Sex: Roughly equal overall, with a female excess in the Currarino subgroup.
• Cognition: Usually normal intelligence - the disability is musculoskeletal, neurological (below the lesion) and sphincteric.

According to PubMed, an orthopaedic narrative review by Jasiewicz and Kacki divides CRS into three broad groups (sirenomelia, complete sacral absence, and partial sacral absence) and identifies spinal deformity, spinopelvic instability and lower-limb deformity as the dominant orthopaedic problems (DOI).

Embryology

• The caudal cell mass and secondary neurulation form the lower sacrum, coccyx and conus. An early insult (around weeks 3-7) disturbs caudal mesoderm and notochord, producing the bony and neural deficit together.
• Because the bone and the corresponding cord segments form together, the level of motor loss tends to mirror the level of bony absence.

Mechanisms (no single proven cause)

• Maternal diabetes / diabetic embryopathy: the strongest association; hyperglycaemia is teratogenic to the caudal mesoderm.
• Vascular hypoperfusion: a proposed "caudal steal" reducing blood flow to the developing caudal region (most invoked for sirenomelia).
• Genetic: the MNX1 (HLXB9) homeobox gene underlies Currarino syndrome (sacral agenesis + presacral mass + anorectal malformation) and familial sacral agenesis.

According to PubMed, Gajagowni et al. classify CRS as a recognised form of diabetic embryopathy, emphasising the link between maternal hyperglycaemia and caudal malformation (DOI). Dworschak et al. confirm MNX1 as the major causative gene in Currarino syndrome, mutated in roughly half of patients tested and most familial cases (DOI).

Sirenomelia is named repeatedly as the most severe end of the caudal spectrum, but never developed. It is worth knowing both as the conceptual extreme and because the relationship between sirenomelia and caudal regression syndrome is itself an exam point.

What it is. Sirenomelia (the "mermaid" malformation) is fusion of the lower limbs into a single midline lower extremity, with severe sacral/caudal agenesis. It sits at the extreme end of caudal dysgenesis but is distinguished from "ordinary" CRS by a near-constant cluster of lethal visceral anomalies:

The exam point is the cause of death: a baby with sirenomelia dies not because the legs are fused but because of bilateral renal agenesis → oligohydramnios → Potter sequence and pulmonary hypoplasia. Whether sirenomelia is the same disease as CRS or a distinct "vascular steal" entity is debated, but for orthopaedic exams it is best framed as the lethal extreme of the same caudal spectrum, defined by limb fusion and incompatible visceral malformations.

History

• Antenatal: maternal diabetes/glycaemic control, abnormal anomaly scan (absent distal vertebrae), oligohydramnios.
• Function: sitting balance, ability to stand/walk, ambulatory aids.
• Sphincters: urinary and faecal continence, recurrent urinary tract infection, constipation.
• Progression: any new weakness, sensory change or deterioration (red flag).

Examination

1. Posture: characteristic cross-legged "Buddha" / frog-leg sitting posture; flattened buttocks with a short natal cleft.
2. Spine: lumbosacral kyphosis, the palpable "step-off" of the deficient sacrum, scoliosis.
3. Lower limbs: muscle wasting (calf/thigh), fixed hip and knee flexion contractures, popliteal webbing/pterygia, foot deformity.
4. Neurology: a motor-predominant flaccid paraparesis with a clear motor level; sensation is often relatively preserved (a classic dissociation).
5. Other systems: perineum/anus (imperforate or stenotic anus), abdomen (palpable bladder), associated VACTERL features.

Imaging

• Plain radiographs (AP/lateral whole spine + pelvis): define the last intact vertebra, the amount of sacrum present and the iliac articulation - the basis for Renshaw typing.
• MRI of the whole neuraxis: the key study - shows the conus level, tethered cord, syrinx, lipoma and any anterior sacral meningocele. Mandatory before surgery and for any progressive deficit.
• CT / 3D reconstruction: helpful for planning spinopelvic fixation in complex bony anatomy.
• Renal tract ultrasound +/- MCUG / urodynamics: assess the neurogenic bladder and upper tracts.

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Other

• Genetic testing (MNX1) where Currarino syndrome or familial sacral agenesis is suspected.
• Maternal investigation for diabetes if not already known.

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Key distinguishing points

• CRS vs spina bifida: CRS is a deficiency of bone/cord (closed), not an open neural tube defect, and sensation is comparatively spared.
• CRS vs Currarino: Currarino has the specific triad and a stable scimitar sacrum rather than diffuse instability.
• Sirenomelia sits at the most severe end of the same caudal spectrum.

The anterior sacral meningocele is flagged in the complications table, the Currarino discussion and an unanswered viva follow-up ("What are the risks of an anterior sacral meningocele?"), so it deserves to be developed. It is the most dangerous of the Currarino presacral masses.

What it is. An anterior sacral meningocele is a herniation of the dural sac through the defect in the deficient anterior sacrum into the presacral (retroperitoneal) space - a CSF-filled diverticulum sitting in front of the sacrum, classically through the "scimitar/sickle" sacral defect of Currarino syndrome. It communicates with the thecal sac through a narrow neck.

Why it matters / its risks:

Management is neurosurgical: definitive repair is closure of the dural neck (commonly via a posterior trans-sacral or, in selected cases, anterior/laparoscopic approach) to obliterate the communication and decompress the sac, with excision of any associated solid teratomatous component. The classic teaching point that "a presacral mass needs excision" specifically includes securely closing the CSF communication of a meningocele, not simply debulking it.

Spinal and Neurological

Orthopaedic

Systemic

• Reflux nephropathy / renal failure from an untreated neurogenic bladder - the leading long-term threat to life.
• Recurrent urinary and respiratory infection, faecal incontinence.
• Presacral mass complications (meningitis, malignant change) in Currarino syndrome.

After Spinopelvic Fusion

• Early: meticulous pressure care over insensate skin; vigilant wound surveillance (high infection risk).
• Mobilisation: graded sitting/standing once the construct allows; custom seating to protect skin and support the trunk.
• Monitoring: serial imaging for fusion and implant integrity; watch for pseudarthrosis and revision needs.

Lifelong Multidisciplinary Follow-Up

• Urology: ongoing bladder management and renal tract surveillance.
• Rehabilitation: bracing, wheelchair/seating, transfers and independence training.
• Neurosurgery: monitor for tethered-cord/syrinx-related deterioration.
• Psychosocial: support for the child and family; intelligence is typically normal so cognitive expectations are unchanged.

What Determines Function

• Bony level / Renshaw type is the key predictor: lower types (I-II) and a lower neurological level favour community ambulation; high types (III-IV) more often become wheelchair/seating dependent.
• Last intact motor level governs walking potential much as in spina bifida.
• Renal preservation is the main determinant of long-term survival.

According to PubMed, Vissarionov et al. reported spinopelvic fusion in 12 children with CRS: a 100% fusion rate with restored sagittal alignment that allowed mobilisation and standing in type III patients (5 complications requiring further care), demonstrating that reconstruction can convert an unstable trunk into a stable, functional base (DOI).

Quality of Life

• Most children have normal cognition and can achieve schooling, employment and independence with appropriate support.
• Continence and skin integrity, rather than walking alone, dominate day-to-day quality of life.

Global Epidemiology

• CRS is rare (around 1 in 25,000 live births) but its incidence rises many-fold in diabetic pregnancies, making it a sentinel marker of diabetic embryopathy worldwide.
• The spectrum runs from isolated coccygeal absence to sirenomelia; partial sacral agenesis is commoner than complete.
• The Currarino subset shows a slight female predominance and autosomal dominant inheritance via MNX1.

Side-by-Side Guidance

Practice Variation

• High-resource settings: multidisciplinary clinics (orthopaedics, neurosurgery, urology, colorectal, rehabilitation), MRI neuraxis screening and spinopelvic reconstruction available.
• Limited-resource settings: diagnosis is clinical/radiographic; care prioritises continence, renal protection and seating, with referral for the rare progressive neurological case.
• Because the lesion is fixed, prevention through maternal diabetes control offers the greatest public-health impact globally.

Static versus progressive deficit Historically all deficits were assumed fixed, leading to a purely orthopaedic, "wait-and-watch" stance. Pang and Hoffman showed a treatable progressive subgroup; the modern debate is how aggressively to image and intervene for subtle deterioration versus accepting a stable baseline.

Hip reduction in non-ambulators Whether to reduce a dislocated hip in a child who will not walk is contested. Many favour a supple, pain-free hip for sitting over forced reduction, which can stiffen the hip and worsen seating.

Timing and extent of spinopelvic fusion The threshold for fusing an unstable high-type spine - and how early to do it in a growing child - is not standardised; surgeons balance sitting stability against growth, infection risk over insensate skin, and revision burden.