Central to a newborn examination is the tracking of growth parameters; head circumference is a key component in that evaluation. Identification of an abnormal head circumference, especially in the context of developmental delays, warrants evaluation by a neurologist and possibly a geneticist. Though it’s a frequently encountered exam finding, microcephaly is a poorly defined condition with a myriad of causes and lack of uniform diagnostic workup.
What is the definition of microcephaly?
Microcephaly, distinctly different from microencephaly, means “small head,” as opposed to “small brain,” which the latter more appropriately defines. That said, head size is a gross approximation of brain size, and small brain size is generally considered the reason for small head size. Recommendations from the American Academy of Pediatrics suggest head circumference should be included in well-child evaluations through at least 24 months of age. A head circumference measuring >2 standard deviations below the mean is widely considered microcephalic.
Head circumference is defined as the largest circumference measured from above the eyebrow ridge to the top of the occipital bone. Measurements should typically be performed twice for accuracy. It’s also generally good practice to measure parent’s (and where appropriate, siblings) head circumference as well.
Microcephaly is currently poorly classified; however, one convention is to distinguish primary from secondary etiologies based on time of onset. Primary (congenital) etiologies represent microcephaly present at birth, and anything thereafter, is designated secondary.
What are the causes of microcephaly?
The underlying causes vary widely; ranging from normal population variation, genetic, environmental, congenital and acquired insults that can occur prenatally, perinatally or postnatally.
When the child’s growth parameters are also small, the microcephaly may be proportionate – referred to as “relative microcephaly” – a distinction that may carry a more favorable prognosis. However, in general, microcephaly often includes developmental delay and intellectual disability as part of its sequelae.
Symptoms suggesting urgent referral and imaging:
- Poor feeding/failure to thrive
- Developmental regression
- Change in mobility
- Impaired consciousness, or excessive irritability
Physical exam findings which may suggest underlying syndromic cause of microcephaly:
- Dysmorphic features
- Behavior (excessive inappropriate laughter, stereotypic behaviors, etc.)
- Developmental delays or neurologic deficits
- Cardiac anomalies
- Skeletal anomalies
- Ophthalmologic disorders
What should the workup of microcephaly include?
Evaluation of microcephaly starts with the clinical assessment. Thorough assessment includes pre/perinatal history (fetal activity), family history (parental learning difficulties, siblings with developmental delays), exposure history (drugs, alcohol, radiation, vitamin A), parental relation (i.e., consanguinity risk), maternal health (diabetes) and physical exam.
Imaging is central to workup; MRI being the preferred study of choice. In circumstances where MRI is not readily available, head CT can be a temporizing study. However, CT imaging will eventually need to be followed up by an MRI. Identification of structural abnormalities can help inform likelihood of primary versus secondary etiologies.
Genetic evaluation is another integral part of working up microcephaly, especially when extra-CNS (dysmorphisms, cardiac or skeletal anomalies, etc.) features are present on exam. Generally, chromosomal analysis is the starting point for genetic evaluation.
When isolated microcephaly is present during the prenatal or neonatal period, an infectious evaluation, (TORCH screen) can be helpful.
When should you refer for evaluation?
Referral to a neurologist is warranted for children with microcephaly and abnormal neurological examinations, developmental delays, seizures, and other signs/symptoms of underlying genetic or metabolic etiology. Referral to genetics is appropriate for abnormal genetic studies, or concerns for genetic syndromes.
For more information on microcephaly, you can visit the following resource.
DynaMed [Internet]. Ipswich (MA): EBSCO Information Services. 1995 – . Record No. T1580143049586, Microcephaly – Approach to the Patient; [updated 2020 Mar 25, cited 2020 Sept 27]. Available from https://www.dynamed.com/topics/dmp~AN~T1580143049586. Registration and login required.
Damian Campbell, D.O.
Neurology, Cook Children’s
Cook Children’s Neurosciences team
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Contact the Jane and John Justin Neuroscience Center at Cook Children’s with your questions at 682-885-2500.
- Poduri, A., & Volpe, J. J. (2018). Chapter 5: Neuronal Proliferation. In Volpe’s Neurology of the Newborn (Sixth Edition). Elsevier.
- Curry, C. J., & Hogue, J. S. (2014). Chapter 6: Microcephaly. In Signs and Symptoms of Genetic Conditions: A Handbook (pp. 63-77). New York: Oxford University Press.
- Ropper, A. H., Samuels, M. A., Klein, J. P., & Prasad, S. (2019). Chapter 37: Developmental Diseases of the Nervous System. In Adams and Victor’s Principles of Neurology (pp. 1026-1036). New York: McGraw-Hill.