Absent CSP on Fetal Ultrasound: Meaning, ICD-10, Prognosis, SMFM Guidelines, and Genetic Causes

Absent CSP on Fetal Ultrasound:

• What is Absent CSP on Fetal Ultrasound?
• Meaning
• ICD-10
• Prognosis
• SMFM Guidelines
• Genetic Causes

What is Absent CSP on Fetal Ultrasound?

An Absent CSP (Cavum Septi Pellucidi) on fetal ultrasound refers to the failure to visualize a normal fluid-filled midline structure in the fetal brain that lies between the two frontal horns of the lateral ventricles. The cavum septi pellucidi is normally seen in most fetuses between 18 and 37 weeks of gestation and serves as an important landmark of normal midline brain development. Its absence can indicate either an abnormal brain formation or a secondary effect of other neurological anomalies.

In fetal neuroimaging, the CSP is typically visualized in the axial plane as a small anechoic (dark) space between the frontal horns. When it is not seen, further investigation is necessary to rule out serious conditions such as agenesis of the corpus callosum, holoprosencephaly, or septo-optic dysplasia. Absence of the CSP should always prompt a detailed neurosonographic examination and possibly fetal MRI for confirmation.

Meaning

The meaning of absent CSP depends on whether it represents an isolated finding or part of a broader brain anomaly. In some rare cases, it may be a benign variation, but more often, it indicates a developmental issue involving the midline structures of the brain. The CSP is closely associated with the corpus callosum and the septal nuclei, so its absence often correlates with abnormalities in these areas.

When absent CSP is detected, clinicians evaluate the integrity of the corpus callosum, optic nerves, hypothalamic structures, and ventricles. Conditions such as agenesis of the corpus callosum, schizencephaly, hydranencephaly, or severe hydrocephalus are frequently associated. The key is to determine whether the absence is isolated (normal variant) or non-isolated (pathologic), as the latter carries a higher risk of neurological or genetic complications.

ICD-10

The ICD-10 code for absent cavum septi pellucidi depends on the underlying or associated diagnosis. There is no specific ICD-10 code solely for “absent CSP,” but related codes include:

• Q04.0 – Congenital malformations of corpus callosum
• Q04.3 – Other reduction deformities of brain
• Q04.9 – Congenital malformation of brain, unspecified
• Q89.9 – Congenital malformation, unspecified (if diagnosis uncertain)

These codes are used clinically and for reporting purposes when absent CSP is found in association with a confirmed or suspected central nervous system (CNS) anomaly. Accurate documentation ensures appropriate genetic counseling, follow-up imaging, and long-term management.

Prognosis

The prognosis for a fetus with absent CSP depends on whether the finding is isolated or associated with other brain malformations. When the absence is isolated and no other structural or chromosomal abnormalities are found, the prognosis may be favorable, with many infants developing normally. However, if absent CSP occurs alongside agenesis of the corpus callosum, septo-optic dysplasia, or holoprosencephaly, the outlook is typically poor.

Associated neurological impairments can include developmental delays, vision problems, seizures, or hormonal dysregulation due to hypothalamic involvement. Postnatal MRI and neurological assessments are often recommended to determine the extent of brain involvement. Prognosis is best discussed with a multidisciplinary team including maternal-fetal medicine specialists, neurologists, and genetic counselors.

SMFM Guidelines

According to the Society for Maternal-Fetal Medicine (SMFM) guidelines, absence of the CSP on fetal ultrasound should prompt a detailed neurosonographic evaluation and consideration of fetal MRI for further assessment. The SMFM emphasizes determining whether the absence is isolated or associated with other CNS abnormalities. If associated anomalies are present, genetic testing—such as chromosomal microarray or exome sequencing—may be indicated.

SMFM also recommends that all patients with this finding receive counseling about potential outcomes, recurrence risks, and available interventions. In isolated cases, close prenatal monitoring and postnatal follow-up are usually sufficient. In complex cases, referral to a tertiary fetal medicine or neurogenetics center is advised for comprehensive evaluation and management.

Genetic Causes

The genetic causes of absent CSP can include chromosomal abnormalities, single-gene mutations, or syndromic associations affecting midline brain development. Common genetic links include chromosome 1q, 6q, and 8p deletions, as well as mutations in genes associated with holoprosencephaly and septo-optic dysplasia. In some cases, the absence of CSP is seen in combination with other findings like microcephaly, ventriculomegaly, or pituitary abnormalities.

Genetic testing is recommended when absent CSP is accompanied by other abnormalities or when the family has a history of congenital brain malformations. Advanced diagnostic tools such as chromosomal microarray (CMA), whole-exome sequencing (WES), or targeted gene panels can help identify the underlying cause. Early genetic diagnosis assists in family counseling, prognosis prediction, and planning for postnatal care and follow-up.