A fontanelle, commonly referred to as a soft spot, is a membrane-covered gap between the skull bones of an infant, allowing for brain growth and passage through the birth canal. These spaces typically close by the second year of life as the bone plates fuse together. When an adult perceives a soft spot or depression on their head, it indicates a structural change in the underlying bone or an issue in the overlying tissue. This article will explore the causes of these perceived or pathological soft spots, which range from normal structural variations to serious medical conditions.
Misinterpreting Normal Skull Structure
Sometimes, a perceived soft spot is not a defect in the bone but a normal variation in skull anatomy or an issue in the scalp tissue. The skull is not uniformly thick; its thickness can naturally vary from approximately 3 millimeters to 14 millimeters across different regions in adults. Feeling a slight indentation may simply be the discovery of a thinner area of bone, particularly in the parietal or temporal regions, which is a normal finding.
The skull bones are joined by dense, fibrous joints called sutures, which may still be palpable through the scalp even after fusion is complete in adulthood. A prominent suture line, like the sagittal suture, can sometimes be mistaken for a ridge or a depression. Localized soft tissue masses such as lipomas or cysts, which are common and benign, can also create a concave sensation, mimicking a structural bone defect.
Systemic Conditions Affecting Bone Density
A soft spot can be the physical manifestation of a systemic disease that compromises the integrity of the entire skeleton, including the skull. Paget’s disease of the bone is a condition of disorganized bone remodeling that begins with an aggressive lytic phase, characterized by intense osteoclastic resorption. This destructive process can create well-defined, large areas of bone loss in the skull, known radiographically as osteoporosis circumscripta, which presents as a palpable defect.
Metabolic diseases that affect bone mineralization, such as severe osteoporosis or osteomalacia, are another group of disorders. Osteomalacia, the adult equivalent of rickets, results in “soft bone disease” where the bone matrix is inadequately mineralized with calcium and phosphate, making the bones weak.
While osteoporosis primarily increases bone porosity, unmanaged cases can lead to diffuse thinning of the calvaria, resulting in a generalized sense of a depressed or soft area. Idiopathic focal calvarial thinning, or biparietal osteodystrophy, causes progressive, painless thinning of the skull, often symmetrically in the parietal bones, leading to a noticeable depression without a clear underlying systemic cause.
Localized Defects and Lesions
When the soft spot is confined to a single, discrete area, the cause is typically a localized structural defect or a destructive lesion. Previous trauma is a common cause, where a depressed skull fracture leaves a permanent indentation, or an old injury heals with a palpable bone defect. Similarly, a prior neurosurgical procedure, such as a craniotomy, involves the surgical removal of a bone flap, and the resulting bony defect can be felt through the scalp.
Pathological lesions that destroy or replace the normal, rigid bone structure also create localized soft spots. Metastatic cancer, most commonly originating from the breast, lung, or prostate, can spread to the skull and form osteolytic lesions that destroy the bone layers. These lesions appear as discrete defects that can be felt as a soft or tender mass. Benign tumors, such as fibrous dysplasia, replace normal bone with a weaker, fibro-osseous tissue, which can be perceived as an area of softness or fragility.
When to Consult a Healthcare Provider
Any new or changing soft spot or depression on the adult head warrants medical evaluation to rule out a serious underlying condition. Immediate consultation is necessary if the spot is accompanied by neurological changes, such as persistent headaches, vision problems, or confusion. These may indicate involvement of the underlying brain tissue or increased intracranial pressure. Other warning signs include rapid onset, a palpable mass that is growing in size, or localized pain and tenderness over the area.
A healthcare provider will typically begin the diagnostic process with a physical examination and a review of the patient’s history, including any prior head trauma or known cancer diagnoses. To determine the exact cause and extent of the structural change, imaging studies are required. A computed tomography (CT) scan provides excellent detail of the bone structure, while a magnetic resonance imaging (MRI) scan is better for evaluating the surrounding soft tissues and brain involvement. In some cases, a biopsy of the lesion may be necessary to obtain a definitive diagnosis and guide appropriate treatment.