Head circumference (OFC) is the measurement around the largest perimeter of the skull. While routine for monitoring brain development in infants, adult head size typically stabilizes by age six because the skull bones have fused. In adults, the OFC is primarily used as a low-cost screening tool in clinical settings to assess potential underlying neurological or systemic conditions.
Why Head Circumference is Measured in Adults
The purpose of measuring head circumference in adults is to investigate or monitor conditions causing structural changes to the brain or skull. A sudden or progressive change in size indicates an active disease process. This simple, non-invasive measurement approximates intracranial volume, which includes the brain, cerebrospinal fluid, and blood within the skull.
A primary reason for adult measurement is monitoring conditions causing abnormal fluid accumulation, such as acquired or normal-pressure hydrocephalus. When cerebrospinal fluid flow or absorption is impaired, the resulting pressure can slowly expand the skull, even after sutures have fused. Tracking the OFC over time helps professionals monitor the progression of these conditions.
The measurement is also useful for diagnosing certain congenital syndromes that manifest in adulthood, such as Neurofibromatosis Type 1 (NF1) or Nevoid Basal Cell Carcinoma Syndrome. These genetic conditions are often associated with a significantly larger than average head size. Furthermore, head size measurement can be a factor in assessing older adults, as a smaller head circumference has been associated with an increased risk of dementia.
Performing the Measurement Correctly
Accurate measurement requires a flexible, non-stretchable measuring tape, typically made of plastic or paper, to ensure precision. Before starting, remove any hair accessories or bulky hairstyles that could interfere with the tape’s path. The individual should be sitting or standing with their head in a neutral position, looking straight ahead.
The tape must be positioned over the most prominent points of the head to capture the maximum circumference. Start by placing the tape just above the eyebrows, specifically over the glabella, which is the smooth area between the brow ridges. Wrap the tape around the sides of the head and over the back.
The posterior landmark is the occipital protuberance, the most prominent point on the back of the head. It is crucial to ensure the tape is level on both sides, remaining above the ears and maintaining a consistent horizontal plane. The tape should be pulled snugly enough to compress the hair but must not cause indentation or discomfort to the skin.
Read the measurement where the zero mark overlaps the tape, recording the result to the nearest millimeter or tenth of a centimeter. To confirm accuracy, repeat the procedure two or three times, slightly adjusting the tape’s position to find the true maximum circumference. The largest measurement is recorded for clinical use.
Understanding the Results and Clinical Significance
Interpreting adult head circumference involves comparing the reading to established reference centile charts that account for gender and body height. Head size has normal biological variation; men generally have a larger average circumference than women, and taller individuals often have larger heads. A single measurement is not diagnostic but serves as a red flag if it falls outside the normal range.
The terms microcephaly and macrocephaly describe an abnormally small or large head circumference, respectively. These are typically defined as more than two standard deviations outside the average for a person’s age, sex, and height. Macrocephaly can be caused by an enlarged brain (megalencephaly), hydrocephalus, or conditions that thicken the skull bones, such as Paget’s disease.
Microcephaly is rarer in adults, often related to congenital conditions or severe brain tissue atrophy. While genetic factors can cause naturally large or small head sizes, a measurement outside the typical range warrants a full neurological evaluation, often including imaging studies like CT or MRI. The primary utility of the measurement is to screen for and monitor physical signs associated with serious neurological conditions.