Which feature best distinguishes anencephaly from spina bifida cystica in terms of embryological defect and clinical presentation?

Explanation

## Distinguishing Anencephaly from Spina Bifida Cystica ### Embryological Defect **Key Point:** Anencephaly results from failure of closure of the cranial neuropore (anterior), while spina bifida cystica results from failure of closure of the caudal neuropore (posterior). ### Structural Comparison | Feature | Anencephaly | Spina Bifida Cystica | |---------|-------------|---------------------| | **Defect Location** | Cranial (anterior neuropore) | Spinal (posterior neuropore) | | **Neural Tissue Present** | Absent — no cerebral hemispheres | Present — neural tissue herniates | | **Exposed Structure** | Brainstem, meninges, choroid plexus | Spinal cord (myelocele) or meninges only (meningocele) | | **Vertebral Involvement** | Skull vault defect | Posterior vertebral arch defect | | **Viability** | Incompatible with life (stillbirth or neonatal death) | Compatible with life (variable neurological deficit) | ### Clinical Pearl **High-Yield:** Anencephaly is characterized by **complete absence of the cerebral hemispheres** with exposed brain tissue (brainstem and meninges) at the vertex. This is the pathognomonic finding that distinguishes it from all other neural tube defects. The fetus typically has a characteristic "frog-like" appearance with low-set ears and micrognathia. ### Spina Bifida Cystica Variants - **Myelocele:** Neural tissue within the sac (most severe) - **Meningocele:** Meninges only, neural tissue intact (least severe) - **Myelomeningocele:** Mixed presentation **Mnemonic:** **ANENCE** = **A**bsent **N**eural **E**ncephalon (brain completely missing) [cite:Langman Embryology 15e Ch 17] ### Why Anencephaly is Incompatible with Life The complete absence of cerebral hemispheres means absence of the cerebrum, which controls all vital functions. Brainstem function alone cannot sustain life beyond a few hours to days.