What Causes Frontal Bossing?

Frontal bossing, also known as skull bossing, is a skeletal deformity that causes the frontal bones of the skull to become enlarged and protrude, resulting in an unusually pronounced forehead. There may also be heavier-than-normal brow ridge.

While a prominent forehead and brow may be normal inherited features (an example being Australian aboriginal people who tend to have heavy brow ridges), frontal bossing may also be a sign of a medical condition. These include rare genetic and hormonal disorders that can affect bone development in children during and after pregnancy.

This article describes the symptoms and causes of frontal bossing, including how the condition and underlying cause are diagnosed and treated.

Frontal Bossing Symptoms

Frontal bossing can differ based on the underlying cause. In some cases, the enlargement may not be recognized in a newborn and only becomes apparent as the child ages and moves into adolescence.

Symptoms of frontal bossing include:

• A very prominent forehead
• The notable depression of the nasal bridge
• A heavier-than-normal brow ridge (sometimes but not always)
• An enlarged lower jaw bone (sometimes but not always)
  

The term "frontal bossing" is sometimes used in gender-affirming surgery to describe a heavier brow ridge in males. As part of gender-affirming surgery, the brow ridge may be surgically reduced and contoured as part of the "feminization" of transgender women.

Possible Causes of Frontal Bossing

Frontal bossing may be a sign of an underlying genetic syndrome, hormonal disorder, or congenital infection (meaning one that was passed from the mother to the child during pregnancy).

Some possible causes of frontal bossing include:

• Hydrocephalus: This is a condition caused by a build up of cerebrospinal fluid (CSF) in the ventricles (cavities) of the brain.
• Acromegaly: This is a hormonal disorder in which the pituitary gland makes too much growth hormone (GH). This can cause skull, face, jaw, hand, and foot bones to grow excessively during and after pregnancy.
• Basal cell nevus syndrome: Also known as Gorlin syndrome, this is a rare genetic disorder that causes skeletal abnormalities, jaw cysts, and cancerous tumors called basal cell carcinoma. 
• Congenital syphilis: This occurs when syphilis is passed to the baby during pregnancy. Babies are often born with bone deformities that are often not noticed until after age 2. 
• Cleidocranial dysostosis: This is a rare birth defect that causes incomplete skull formation and underdeveloped or absent collar bones.
• Crouzon syndrome: This is a genetic disorder that causes premature fusing of the skull bones, also known as craniosynostosis. 
• Hurler syndrome is a disease that causes abnormal facial features, short stature, corneal clouding, and hearing problems. It is caused by an abnormal enzyme. 
• Pfeiffer syndrome: This is a rare genetic disorder that causes the bones in the skull to fuse prematurely.
• Rickets: This is a condition caused by a severe deficiency of vitamin D. It leads to bone deformities, stunted growth, and easily breakable bones.
• Rubinstein-Taybi syndrome: This is a rare birth defect that causes thick eyebrows with a prominent arch, short stature, intellectual disability, and other health problems. 
• Russell-Silver syndrome: This is a growth disorder that causes facial abnormalities, asymmetric limbs, difficulty feeding, and other health problems.
• Tridione (trimethadione): This is an anti-epilepsy drug that has been known to cause birth defects if taken during pregnancy, including frontal bossing and cleft palate.

Diagnosing Frontal Bossing

Frontal bossing can be diagnosed with a physical exam. To narrow the possible causes, the healthcare provider will start by taking a detailed medical history and family history.

Among the questions that may be asked:

• When did you first notice your child's forehead size?
• Is your child having any other symptoms?
• Did you have any illness or problems during your pregnancy?
• Is your child meeting their developmental milestones (such as sitting and rolling by 4 to 8 months or sitting unsupported by 6 to 8 months)?

From there, the healthcare provider will likely order lab tests based on the suspected cause. These might include:

• Genetic testing: If a genetic disorder is suspected, genetic blood screening may be recommended for the whole family.
• Insulin growth factor-1 (IGF) test: This is a blood test used to check levels of growth hormone (GH), which would be high if a child has acromegaly.
• Oral glucose tolerance test (OGTT): This is a blood test used to confirm acromegaly. Under normal circumstances GH levels drop after you consume GH, but not with acromegaly.
• Calcium testing: With rickets, calcium levels in the blood and urine will be decreased while parathyroid hormone and urinary phosphorus levels will be increased.
• Fluorescent treponemal antibody absorption (FTA-ABS): This is a blood test used to detect antibodies to Treponema pallidum, the bacteria that causes syphilis.
• Dual X-ray absorptiometry (DEXA) scan; Also known as a bone density scan, this imaging technique can detect bone mineral loss and other problems consistent with congenital syphilis or rickets.

Various head imaging modalities may also be used, including:

• Head ultrasound
• Head or brain computed tomography (CT)
• Magnetic resonance imaging (MRI)

It is also possible to detect frontal bossing during pregnancy during a prenatal ultrasound. This may be ordered if maternal syphilis or rickets is diagnosed or the parents have a known genetic disorder. Even so, an ultrasound may only have limited value as frontal bossing is often not seen until a child is older.

How Frontal Bossing Is Treated

There is no treatment to reverse frontal bossing itself. Treatment of the underlying condition will likely not reverse your child's condition, but it may keep it from getting worse.

Frontal bossing is not reversible and a child can't really "grow out of it." However, as a person reaches adulthood, the prominence may not be as noticeable (particularly in males who tend to have heavier brow ridges). Different hairstyles can also help minimize the appearance of an enlarged forehead.

If frontal bossing is severe or causing distress, cosmetic surgery may be explored. This is especially true if the bone malformation interferes with the field of vision.

Generally speaking, corrective surgery is not considered until skull bone growth is complete, usually around the end of puberty. Even then, there is no standard approach to surgery.

Depending on the severity of frontal bossing, the surgery may involve burring (abrading away excess bone), burring with hydroxyapatite (a mineral compound used to contour the brow), or osteotomy (the cutting, reshaping, and repositioning of bones).

Summary

Frontal bossing occurs when a baby has a protruding forehead. A child with frontal bossing may also have a heavy brow ridge. This condition usually is a sign of an underlying genetic syndrome, birth defect, or hormonal disorder.

While frontal bossing cannot be treated, the underlying condition can be treated or managed to avoid progression. In severe cases, corrective cosmetic surgery may be explored.