What Are the Sentinel Facial Features of FAS?

When diagnosing fetal alcohol syndrome (FAS), doctors look for three specific facial characteristics known as “sentinel” features ^[1]. These features are key indicators that medical professionals evaluate during an assessment to help confirm a diagnosis, as they are strongly linked to prenatal alcohol exposure ^[2][1]. The three sentinel facial features are short palpebral fissures, a smooth philtrum, and a thin upper lip ^[3].

Doctors use these specific traits because they develop during a critical window of pregnancy. While the facial structures themselves do not cause cognitive issues, they serve as visible markers of prenatal alcohol exposure, which can cause central nervous system damage and neurodevelopmental challenges ^[4]. It is crucial to understand that while these features are required for a diagnosis of full FAS, a child can still have severe, invisible neurodevelopmental challenges (under the broader Fetal Alcohol Spectrum Disorders, or FASD, umbrella) even if their face looks completely typical ^[1].

The Three Sentinel Features

During a physical examination, a doctor or dysmorphologist (a specialist in physical development and structural differences) will carefully measure and evaluate three specific areas of the face:

• Short palpebral fissures (eye slits): The palpebral fissure is the horizontal opening between the upper and lower eyelids. In children with FAS, the distance from the inner corner of the eye to the outer corner is shorter than average for their age and background ^[3][5]. Doctors measure this distance precisely, often using a small ruler, and compare it to standard growth charts ^[6].
• Smooth philtrum: The philtrum is the vertical groove between the base of the nose and the top of the upper lip. In FAS, this groove is often flattened or completely smooth, rather than deeply indented ^[4].
• Thin upper lip: The vermilion border—the reddish part of the upper lip—is unusually thin ^[3].

How Doctors Measure and Rank These Features

Evaluating these features requires precision, so doctors do not just guess based on looking at a patient. This evaluation is painless; it typically only involves looking at the doctor, having measurements taken with a small ruler, and perhaps taking standardized photographs of your child’s face from a few different angles to measure later.

Doctors use standardized clinical tools like the Lip-Philtrum Guide that doctors use to measure and rank these features on a 1-4 scale ^[2][7]. The Lip-Philtrum Guide is a visual chart that allows clinicians to compare the patient’s lip and philtrum to standardized photographs ^[7]. Doctors assign a rank to the facial features, where higher scores indicate a stronger presence of FAS characteristics. Using diagnostic systems like the 4-Digit Diagnostic Code, the overall facial phenotype is ranked on a 1-4 scale, where a rank of 4 means that all three sentinel features—short palpebral fissures, smooth philtrum, and thin upper lip—are distinctly present ^[2][8].

Because facial structures naturally vary across different races and ethnicities, doctors use specialized Lip-Philtrum Guides and measurement charts that match the patient’s specific demographic background to ensure the assessment is accurate ^[9][7]. It is also important to note that these facial features are typically most distinct during early childhood and may become less noticeable as the child grows into adolescence and adulthood. Today, some specialists even use digital 3D facial imaging or telemedicine tools to capture more reliable and objective measurements of these subtle features ^[10][11].