An Introduction to Microform Holoprosencephaly

Hearing the word “holoprosencephaly” can be overwhelming and frightening. It is a long, complex medical term often associated with severe health challenges. However, the addition of the word “microform” changes the picture significantly. Microform holoprosencephaly (mHPE) is the mildest possible version of this spectrum, and understanding why it is classified this way can help you navigate your child’s care with more clarity.

Understanding the Spectrum

To understand mHPE, it helps to know what happens in the “classic” forms of the condition. During early pregnancy, the brain normally divides into two distinct halves (the right and left hemispheres) ^[1]. In classic holoprosencephaly (categorized as alobar, semilobar, or lobar), this division is incomplete or fails to happen at all ^[1].

In microform holoprosencephaly, the situation is different. By definition, microform HPE excludes structural brain involvement ^[2]. This means that your child’s brain has successfully divided into two hemispheres ^[2]. While the condition belongs to the same family of diagnoses, the most critical part—the structure of the brain itself—is considered normal or “morphologically intact” ^[3].

Why the Diagnosis is Made

If the brain is normal, you may wonder why your child received this diagnosis. Doctors use the term “microform” when a child has the midline features (physical traits along the center of the face or body) associated with the spectrum, even though the brain is structurally unaffected ^[2]. These features occur because the same genetic “instructions” that tell the brain to divide also tell the face and other organs how to form ^[4].

Common signs of mHPE include:

• Solitary Median Maxillary Central Incisor (SMMCI): Having only one single front tooth in the middle of the upper jaw instead of two ^[2].
• Ocular Hypotelorism: Eyes that are positioned slightly closer together than average ^[5].
• Midline Clefts: A cleft in the lip or the roof of the mouth that occurs exactly in the center ^[5].
• Flat Nasal Bridge: A nose that appears flatter or less prominent at the top ^[5].

For more details on these features, see Recognizing the Physical Signs.

Long-Term Outlook

Because the brain structure in mHPE is typically normal, the long-term outlook is vastly different from the more severe forms of the condition. Children with classic HPE often face significant neurological and physical challenges; in contrast, children with microform HPE frequently have normal or near-normal neurodevelopmental outcomes ^[2].

However, because the genetic signals involved in mHPE also affect other systems, your child may need specialized monitoring in two main areas:

Genetics and Your Family

Microform HPE is often genetically heterogeneous, meaning it can be caused by changes in several different genes ^[3][4]. Sometimes these genetic traits are passed down from a parent who may only have very subtle signs and never knew they were part of the spectrum ^[7]. Genetic counseling is highly recommended to understand why this happened and what it means for your family. Read more in The Genetics of the Midline.