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Lumic Lumic: Ophthalmology Encyclopedia
Pediatric Ophthalmology & Strabismus

Unilateral coronal synostosis

Key Points at a Glance

Section titled “Key Points at a Glance”

1. What is Unilateral Coronal Synostosis?

Section titled “1. What is Unilateral Coronal Synostosis?”

Unilateral Coronal Synostosis (UCS) is a condition caused by premature fusion of the coronal suture on one side of the skull. It occurs in approximately 1 in 10,000 births.

Most cases are nonsyndromic, accounting for 12–24% of nonsyndromic craniosynostosis. However, it can also occur as part of syndromes such as Muenke syndrome or frontonasal dysplasia.

Approximately 50% of nonsyndromic UCS are caused by sporadic genetic mutations. Associations with FGFR3 and TWIST1 genes have been reported. A family history is found in 8–10% of coronal craniosynostosis cases. Environmental factors include exposure to teratogens, maternal smoking, intrauterine fetal head constraint, and oligohydramnios.

UCS affects orbital growth, eye position, and visual development in children.

2. Main Symptoms and Clinical Findings

Section titled “2. Main Symptoms and Clinical Findings”

Subjective Symptoms

Section titled “Subjective Symptoms”

Cranial asymmetry is present from birth. Parents often notice misalignment of the eyes or a difference in eye size between the two sides.

Clinical Findings (Findings Confirmed by Physician Examination)

Section titled “Clinical Findings (Findings Confirmed by Physician Examination)”

UCS presents with “facial scoliosis” and exhibits the following features.

  • Affected side: flattening (recession) of the forehead, ear protrusion and anteroinferior displacement, elevation of the supraorbital rim
  • Unaffected side: compensatory frontal bossing, narrowing of the palpebral fissure
  • Nasal root: torsion toward the affected side
  • Mandible: deviation toward the unaffected side

Harlequin deformity results from elevation of the supraorbital rim and greater wing of the sphenoid on the affected side, presenting as eyebrow elevation, widening of the palpebral fissure, and mild proptosis.

Ophthalmic Findings

Section titled “Ophthalmic Findings”
  • Strabismus: Prevalence 19–71%. Characteristic pattern is superior oblique palsy (ipsilateral to the synostosis, 87%). May progress to V-pattern strabismus. Inferior oblique overaction/superior oblique underaction is seen in 50.8%.
  • Anisometropic astigmatism: 54–60%. More severe astigmatism in the eye contralateral to the synostosis. This is thought to be due to inferior displacement of the contralateral orbital roof altering corneal curvature.
  • Amblyopia: 35–38%. Caused by high prevalence of astigmatism and strabismus.
  • Eyelid and lacrimal abnormalities: Epicanthus (26%), lagophthalmos (7%), lateral canthal dystopia (14%), nasolacrimal duct obstruction (12%).
  • Papilledema: May occur with increased intracranial pressure (4–42% in single-suture synostosis), but is relatively rare in unilateral coronal synostosis.
  • Optic atrophy: May occur with chronic intracranial hypertension, but is rare in isolated unilateral coronal synostosis.

Differentiation from deformational plagiocephaly

Section titled “Differentiation from deformational plagiocephaly”

Deformational plagiocephaly (DP) is sometimes misdiagnosed as unilateral coronal synostosis. DP is an asymmetric flattening of the skull due to birth pressure or prolonged supine positioning, with an estimated incidence of about 20–50% in infants at 6 months of age. DP can be differentiated by a parallelogram-shaped head and the absence of harlequin deformity or nasal root deviation.

3. Causes and risk factors

Section titled “3. Causes and risk factors”

The cause of UCS is premature fusion of the coronal suture. When the coronal suture fuses, skull growth on the fused side stops, leading to compensatory growth in the direction of unfused sutures.

  • Genetic factors: Mutations in FGFR3 (fibroblast growth factor receptor 3) and TWIST1 genes have been reported. Early activation of these transcription factors leads to premature fusion of cranial sutures.
  • Family history: 8–10% have a family history of craniosynostosis
  • Environmental factors: Exposure to teratogens, maternal smoking, intrauterine fetal head constraint, oligohydramnios

4. Diagnosis and Examination Methods

Section titled “4. Diagnosis and Examination Methods”
  • Physical examination: Assessment of cranial asymmetry, harlequin deformity, and facial scoliosis
  • Head CT (3D reconstruction): Confirmation of fused sutures and evaluation of extent
  • Ophthalmologic examination:
    • Refraction test (under cycloplegia): Evaluation of anisometropic astigmatism. Higher astigmatism is observed on the side opposite to the fusion
    • Ocular alignment test: Presence of superior oblique dysfunction and V-pattern strabismus
    • Visual acuity test: screening for amblyopia
    • Fundus examination: presence of papilledema
  • MRI: evaluation of lateral rectus muscle excyclotorsion and superior oblique trochlear position abnormalities

5. Standard treatment

Section titled “5. Standard treatment”

Cranioplasty

Section titled “Cranioplasty”

The main goal of treatment is to secure intracranial expansion space and ensure normal brain development.

  • Endoscopic suturectomy (ESC): A minimally invasive procedure. Usually performed within the first 3–4 months of life. Associated with shorter operative time, less blood loss, and shorter hospital stay. Postoperative helmet therapy is required. Typically not effective after 6 months of age, so timely referral is essential.
  • Fronto-orbital advancement (FOA): Reconstruction involving craniotomy. Usually performed around 1 year of age or later. More invasive but allows greater freedom in cranial reconstruction.

Ophthalmic Management

Section titled “Ophthalmic Management”

6. Pathophysiology and detailed pathogenesis

Section titled “6. Pathophysiology and detailed pathogenesis”

Mechanism of orbital deformity

Section titled “Mechanism of orbital deformity”

Premature fusion of the coronal suture elevates the superior orbital rim and greater wing of the sphenoid on the affected side, reducing orbital volume. This causes the harlequin deformity. Compensatory cranial growth occurs on the opposite side, leading to frontal bossing.

Mechanism of Strabismus Development

Section titled “Mechanism of Strabismus Development”

The following hypotheses have been proposed to explain why the superior oblique palsy pattern is characteristic of UCS.

  • Posterior displacement of the trochlea due to recession of the superior orbital rim and shortening of the orbital roof, leading to dysfunction of the superior oblique muscle
  • MRI studies have confirmed excyclorotation of the rectus muscles and abnormal position of the superior oblique trochlea within the bony orbit
  • Iatrogenic displacement of the trochlea may also occur during fronto-orbital advancement surgery

Mechanism of Anisometropic Astigmatism Development

Section titled “Mechanism of Anisometropic Astigmatism Development”

The reason astigmatism is stronger on the side opposite the fused side is thought to be that the contralateral orbital roof deviates downward, directly pressing on the eyeball and altering the corneal curvature.

8. References

Section titled “8. References”

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