What is the longhorn sign?

The Longhorn Sign in Radiology

The Longhorn sign refers to the appearance of bilateral temporal horn enlargement on MRI, most prominently involving the inferolateral aspects of the ventricles, seen in certain congenital brain malformations, particularly agenesis of the corpus callosum and lissencephaly. 1

Radiological Characteristics

• The Longhorn sign is characterized by enlargement of the temporal horns of the lateral ventricles with a distinctive morphology that differs from hydrocephalic enlargement 1
• In congenital brain anomalies, the enlargement is most prominent in the inferolateral aspects of the ventricles, creating a characteristic appearance resembling longhorns 1
• This sign is frequently associated with abnormal hippocampal formations that show:
  • Small size in 62% of cases with congenital anomalies 1
  • Abnormal vertical orientation (incomplete inversion) in 82% of cases 1
  • Focal thinning of white matter lateral to the temporal horn in 50% of cases 1

Clinical Significance and Differential Diagnosis

• The Longhorn sign is most commonly associated with:

  • Agenesis of the corpus callosum (present in 100% of cases in the study) 1
  • Lissencephaly (present in 100% of cases in the study) 1
  • Not typically seen in lobar holoprosencephaly 1

• It is crucial to differentiate the Longhorn sign from temporal horn enlargement due to obstructive hydrocephalus:

  • In hydrocephalus, temporal horn enlargement is most pronounced in the superior-lateral region 1
  • Hydrocephalic patients typically have normal hippocampal size and orientation (except in severe cases) 1
  • Hydrocephalic enlargement results from increased intraventricular pressure, while the Longhorn sign represents enlargement due to incomplete development 1

Imaging Modalities and Protocols

• MRI is the preferred imaging modality for evaluating the Longhorn sign and associated brain malformations 2
• Key MRI sequences to evaluate this sign include:
  • T1-weighted images in coronal plane (best for evaluating hippocampal morphology) 1
  • T2/FLAIR sequences to assess white matter abnormalities 2
  • Susceptibility-weighted sequences may be helpful for additional tissue characterization 2

Clinical Management Implications

• Recognition of the Longhorn sign should prompt evaluation for associated developmental brain anomalies 2, 1

• Patients with agenesis of the corpus callosum or lissencephaly may require:

  • Neurodevelopmental follow-up 2
  • Evaluation for seizures and other neurological manifestations 2
  • Assessment for associated genetic syndromes 2

• The presence of the Longhorn sign should not be misinterpreted as hydrocephalus requiring surgical intervention 1

• Careful analysis of temporal lobe morphology will allow differentiation if doubt exists 1

Common Pitfalls

• Misinterpreting the Longhorn sign as obstructive hydrocephalus requiring shunting 1
• Failing to recognize associated developmental abnormalities that may have clinical implications 2
• Not evaluating for other midline brain malformations that may coexist with corpus callosum agenesis 2
• Overlooking subtle hippocampal malformations that often accompany the Longhorn sign 1

Understanding the Longhorn sign is essential for accurate diagnosis of congenital brain malformations and appropriate clinical management of affected patients.