Can Hemifacial Microsomia Be Undetected on MRI?
Yes, hemifacial microsomia can potentially be missed on standard MRI, particularly if the imaging protocol is not optimized for craniofacial evaluation or if the radiologist is not specifically looking for this congenital malformation.
Understanding Hemifacial Microsomia
Hemifacial microsomia (HFM) is the second most common congenital craniofacial anomaly after cleft lip and palate, characterized by underdevelopment of structures derived from the first and second branchial arches 1. The condition demonstrates extreme phenotypic variability, affecting:
- Mandible and temporomandibular joint (hypoplastic in 97% of cases) 1, 2
- Ear structures (abnormal in 95% of cases, ranging from mild deformity to complete atresia) 1, 2
- Orbit (though normal in 64% of patients) 1
- Facial nerve (weakness present in 50% of cases) 2
- Soft tissues (minimal to mild hypoplasia in 52%) 1
Why MRI May Miss Hemifacial Microsomia
Imaging Protocol Limitations
Standard brain MRI protocols frequently fail to adequately capture orbital and facial structures because they are optimized for intracranial pathology rather than craniofacial anatomy 3. The facial structures have an extensive extracranial course through the temporal bone and soft tissues that may not be fully included in standard brain imaging fields of view 4.
Osseous Detail Deficiency
MRI has inherent limitations in evaluating bony structures, which are the primary abnormalities in HFM 5. The condition primarily involves:
- Mandibular hypoplasia and temporomandibular joint malformation 1, 2
- Temporal bone abnormalities (present in 90% of cases) 2
- Orbital rim and zygomatic bone defects 6
CT imaging provides superior osseous detail and is specifically recommended for comprehensive assessment of HFM 7, 8. Three-dimensional CT allows comprehensive evaluation of the skeletal deformity without superimposition of normal and abnormal structures, which limits conventional radiography 7, 8.
Subtle or Mild Presentations
The majority of HFM patients have mild manifestations (type M1 or M2a mandibular hypoplasia in 58% of cases) 1. Subtle asymmetries may be overlooked on MRI if the radiologist is not specifically evaluating for craniofacial malformations or if comparison views of both sides are not carefully scrutinized.
Optimal Imaging Strategy for Hemifacial Microsomia
When MRI Is Appropriate
MRI of the orbits, face, and neck without and with contrast is indicated when soft tissue characterization is needed, particularly for:
- Evaluating associated soft tissue deficiency 5
- Assessing facial nerve involvement 9, 4
- Detecting marrow involvement in osseous structures 5
- Ruling out associated intracranial anomalies 5
The superior soft tissue resolution of MRI allows improved delineation of muscle hypoplasia, fat distribution, and nerve pathology 5.
When CT Is Superior
CT maxillofacial without contrast is the primary modality for evaluating the skeletal components of HFM 5. CT best depicts:
- Osseous matrix and bony remodeling versus destruction 5
- Mandibular and temporomandibular joint anatomy 7, 8
- Temporal bone structures including middle ear and ossicles 2
- Orbital volumes and osseous anatomy 5, 3
Three-dimensional CT reconstruction provides comprehensive assessment of the craniofacial deformity at low radiation dose (22% of conventional CT) using modern low-dose techniques 7.
Complementary Imaging Approach
CT and MRI are complementary modalities in evaluating craniofacial malformations 5. The optimal approach includes:
- CT maxillofacial without contrast as the primary modality for skeletal assessment 5
- MRI orbits, face, and neck without and with contrast for soft tissue characterization and nerve evaluation 5
- Temporal bone CT for detailed evaluation of auditory structures, particularly given the 86% incidence of conductive hearing loss in HFM 2
Clinical Pitfalls to Avoid
Do not rely solely on standard brain MRI to exclude craniofacial malformations—ensure dedicated orbital and facial imaging is performed if HFM is suspected 3. Standard protocols may miss orbital pathology and facial asymmetry 3.
Do not assume bilateral symmetry without careful comparison—28% of HFM cases are bilateral, though asymmetric 1. Right-sided involvement is most common (49%), followed by bilateral (28%), then left-sided (23%) 1.
Do not overlook associated anomalies—comprehensive audiologic evaluation is recommended in all HFM patients regardless of the apparent severity of craniofacial abnormalities, as hearing deficit does not correlate with the degree of visible deformity 2.
Recognize that the severity of craniofacial features correlates with temporal bone abnormality but not with the degree or type of hearing loss 2, emphasizing the need for functional assessment beyond imaging.