A bulging fontanelle is an outward curving of an infant's soft spot (fontanelle).
Soft spot - bulging; Bulging fontanelles
The skull is made up of many bones, seven in the skull itself and 14 in the face area. They join together to form a solid, bony cavity that protects and supports the brain. The areas where the bones join together are called the sutures.
The bones are not joined together firmly at birth. This allows the head to change shape to help it pass through the birth canal. The sutures get minerals added to them over time and harden, firmly joining the skull bones together.
In an infant, the space where two sutures join forms a membrane-covered "soft spot" called a fontanelle (fontanel). The fontanelles allow for growth of the brain and skull during an infant's first year.
There are normally several fontanelles on a newborn's skull, mainly at the top, back, and sides of the head. Like the sutures, fontanelles harden over time and become closed, solid bony areas.
The fontanelle in the back of the head (posterior fontanelle) usually closes by the time an infant is 1 - 2 months old.
The fontanelle at the top of the head (anterior fontanelle) usually closes between 7 - 19 months.
The fontanelles should feel firm and very slightly curved inward to the touch. A tense or bulging fontanelle occurs when fluid builds up in the brain or the brain swells, causing increased pressure inside the skull.
When the infant is crying, lying down, or vomiting, the fontanelles may look like they are bulging, but they should return to normal when the infant is in a calm, head-up position.
Encephalitis -- swelling (inflammation) of the brain, most often due to infections
Carlo W. Physical Examination of the Infant. In: Kliegman RM, Behrman RE, Jenson HB, Stanton BF, eds. Nelson Textbook of Pediatrics.19th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 88.2.
Neil K. Kaneshiro, MD, MHA, Clinical Assistant Professor of Pediatrics, University of Washington School of Medicine. Also reviewed by A.D.A.M. Health Solutions, Ebix, Inc., Editorial Team: David Zieve, MD, MHA, Bethanne Black, Stephanie Slon, and Nissi Wang.