Abnormal sperm head morphology Enlarged sperm head vacuole 2023-07-03T10:22:35Z Increased size of sperm vacuoles, which are surface concavities typically located at the the tip or middle area of sperm heads. According to the WHO Laboratory Manual for the Examination and Processing of Human Semen, sperm head vacuoles are considered abnormal when exceeding 20% of the cross-sectional area of the head. Vacuoles are found in almost all samples with normal sperm morphology; this term applies if the proportion of sperm with abnormally large vacuoles is above the upper limit of normal.