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De Vries classification [14]
| Grade I | Periventricular hyperechogenicity, persisting for > 7 days |
| Grade II | Periventricular hyperechogenicity, evolving into small localized fronto-parietal cysts |
| Grade III | Periventricular hyperechogenicity, evolving into extensive periventricular cystic lesions |
| Grade IV | Hyperechogenicity extending into the deep white matter evolving into extensive cystic lesions |
