What is a soft spot, or fontanelle?
In infants, a fontanelle is an area where the skull bones have not yet fused completely. The spaces, covered by a tough fibrous membrane, allow the skull to be flexible during birth and to accommodate the rapid growth of the brain in the first years of life. While infants are born with several fontanelles, the posterior one on the back of the head usually closes by around 2 to 3 months of age. The anterior one on top of the head closes later, typically between 9 and 18 months.
For adults, finding a true soft spot where the bones have not fused is very rare and is typically a congenital condition. More often, what feels like a soft spot is actually an indentation, ridge, or irregularity in the bone structure that has been there all along.
Potential causes for a soft spot on the back of an adult's head
There are several reasons, both benign and serious, why an adult might feel an indentation or soft spot on their head. Many are related to normal anatomical structure or past events, while others may signal an underlying medical issue.
Normal anatomical variations
- External Occipital Protuberance (EOP): An easily palpable bump where muscles and ligaments attach to the back of the skull. Some people have a more pronounced EOP, while others have a flatter one, which can feel like an indentation in comparison.
- Sagittal Suture: The sagittal suture runs along the top of the skull from front to back, separating the two parietal bones. Some people can feel this seam where the skull bones have fused, which may feel like a long groove or dent.
- Incomplete Fontanelle Closure: In very rare cases, the posterior fontanelle may not fully close, or a different congenital condition could result in a membranous area remaining in adulthood. A doctor can confirm this with a physical exam.
Head trauma and injury
- Depressed Skull Fracture: A past head injury, even a seemingly minor one that has healed, can result in a depressed skull fracture. This leaves a permanent dent in the bone.
- Hematoma or Swelling: A recent head bump can cause a blood clot (hematoma) or swelling under the scalp, which might feel like a soft pocket. As the swelling subsides, an indentation can become noticeable.
Medical conditions
Several medical conditions can affect bone density and structure, potentially causing indentations on the skull:
- Paget's Disease of Bone: This chronic condition causes bones to grow abnormally large and misshapen. It primarily affects older adults and can lead to indentations or irregularities in the skull.
- Osteodystrophy: This is a condition that results in abnormal bone formation and can lead to a thinning of the skull bones, potentially causing deformities.
- Nutrient Deficiencies: Severe vitamin deficiencies, particularly vitamin A, can lead to soft bones and skull irregularities in some cases.
- Gorham-Stout Syndrome: A very rare disorder that causes progressive bone destruction, sometimes leading to visible deformities.
- Cancer: In extremely rare cases, a bone-destructive cancer that has spread to the skull can cause irregularities. Imaging tests like CT or MRI can help identify this.
- Autoimmune Disorders: Some autoimmune diseases, such as lupus, can cause bone involvement and lead to osteonecrosis, which can affect the skull.
Skin and scalp lesions
Sometimes, what feels like a soft spot is actually a cyst or other lesion on the scalp, rather than a depression in the skull bone itself. These may include:
- Sebaceous cysts: Fluid-filled sacs that can develop under the skin and feel soft or movable.
- Lipomas: Benign, fatty lumps that can feel soft to the touch and form anywhere on the body, including the scalp.
Comparing infant vs. adult soft spots
| Feature | Infant Fontanelle (Soft Spot) | Adult Head Indentation (Soft Spot) |
|---|---|---|
| Cause | Normal developmental stage for brain growth and birth. | Anatomical variation, past trauma, or an underlying medical condition. |
| Timing | Present at birth; posterior fontanelle closes around 2-3 months, anterior around 9-18 months. | Can be lifelong, new, or develop over time. |
| Appearance | Typically slightly depressed and pulsating, covered by a tough membrane. | Can be a subtle groove, a permanent dent from injury, or an area of thinner bone. |
| Associated Symptoms | Bulging (sign of increased pressure) or sunken (sign of dehydration) are concerning. | Pain, swelling, changes in shape/size, or neurological symptoms are concerning. |
When to see a doctor
For many, a soft spot on the back of the head is simply a normal, harmless part of their anatomy. However, medical evaluation is recommended if you notice any concerning symptoms.
Consult a healthcare provider if:
- The soft spot is new or has recently changed in size or shape.
- It is accompanied by pain, swelling, or tenderness.
- You have a history of head trauma that coincides with the appearance of the dent.
- You experience other symptoms such as headaches, vision changes, or neurological issues.
Your doctor may perform a physical examination to feel the area and determine if it is a bony or soft-tissue lesion. In some cases, they may order imaging tests, such as a CT scan or MRI, to get a detailed view of the skull and rule out more serious causes. Depending on the diagnosis, treatment may be unnecessary, or it could involve addressing an underlying medical condition.
Conclusion
While the feeling of a soft spot or indentation on the back of your head may be unsettling, it is often a normal anatomical feature or the result of a past, long-healed injury. True unclosed fontanelles in adults are a rare congenital condition. However, the key to peace of mind lies in being aware of any new or changing symptoms. If a soft spot is painful, growing, or associated with other health issues, it's prudent to consult a healthcare provider. They can provide an accurate diagnosis, rule out serious concerns, and ensure that any underlying conditions are properly managed. For more information on health conditions affecting bones, consult reliable resources such as the Cleveland Clinic.
