Neonatal Cranial Ultrasound

Neonatal cranial ultrasound images the brain through the open anterior fontanelle in preterm and term infants. The posterior fontanelle, mastoid fontanelle and temporal window are used as needed.

A 5 to 10 MHz sector or curvilinear probe is used. A 10 to 15 MHz linear probe images the superficial extra-axial spaces and the superior sagittal sinus.

It is done at the cot side in the neonatal unit without sedation and without radiation. Routine coronal views (six planes from the frontal lobes to the occipital lobes) and sagittal views (midline, and the parasagittal planes through the lateral ventricles and the basal ganglia) are acquired. Colour Doppler of the anterior cerebral artery (resistive index) and the venous sinuses is added when needed.

Ventricular size is measured (ventricular index, anterior horn width, thalamo occipital distance), as is the width of the extra-axial space. The examination takes 10 to 20 minutes.

Intraventricular haemorrhage (germinal matrix haemorrhage) occurs in 20 to 25% of infants born before 32 weeks (higher at lower gestation), and is mostly found in the first 72 hours of life. Periventricular leucomalacia, post-haemorrhagic ventricular dilatation, hypoxic ischaemic injury, infection and congenital malformations are the other main findings.

Cranial ultrasound has a sensitivity of over 90% for moderate and severe intraventricular haemorrhage and ventricular dilatation. Sensitivity is lower (about 30 to 60%) for diffuse white matter injury, cortical and posterior fossa abnormalities, which are better shown by MR.

Indications. Routine screening of all infants born before 32 weeks (and often before 34 weeks), repeated at day 1 to 3, day 7, day 14 to 28 and at term equivalent age or before discharge; screening of sick or unstable term infants. Hypoxic ischaemic encephalopathy. Neonatal seizures, apnoea, abnormal tone or abnormal neurological examination.

Suspected meningitis, ventriculitis and abscess. Congenital infection (cytomegalovirus, toxoplasmosis, Zika, rubella) and microcephaly. Macrocephaly or rapidly increasing head circumference, bulging fontanelle. Suspected congenital malformation (found on antenatal scan). Birth trauma, subdural and subgaleal collections. Cardiac surgery or extracorporeal membrane oxygenation (before and during).

Follow up of known haemorrhage, hydrocephalus and after shunting. Not after the anterior fontanelle has closed (usually 9 to 18 months).

Contraindications and precautions. None. The infant is handled minimally (cluster care) and kept warm, with monitoring of heart rate, oxygen saturation and temperature. The examination is deferred for a critically unstable infant if it does not change management. Gel is warmed and sterile gel is used for infants with open scalp wounds.

Probe cleaning follows infection control practice. Pressure on the fontanelle is light, as it can alter the intracranial pressure and the Doppler indices.

Preparation and technique. No preparation or sedation is needed. The infant lies supine with the head in the midline.

Coronal images are acquired from the frontal horns at the level of the interhemispheric fissure to the occipital lobes. Structures seen are the frontal lobes, anterior horns, foramen of Monro, third ventricle, thalami and basal ganglia, atria and choroid plexus, occipital horns, and cerebellum.

Sagittal images are acquired in the midline and parasagittally.

• Midline: corpus callosum, cavum septi pellucidi, third and fourth ventricles, cerebellar vermis, cisterna magna.
• Parasagittal: caudothalamic groove, lateral ventricle, choroid plexus and periventricular white matter.

Extra windows are added: posterior fontanelle (occipital horns, blood in the atria) and mastoid fontanelle (posterior fossa, cerebellar haemorrhage), and the temporal window.

Doppler of the anterior cerebral artery (in the midline sagittal plane) gives peak systolic and end diastolic velocity and resistive index. Measurements are made on fixed planes and compared with gestational age centiles.

Normal appearances. The brain appears with the cerebral hemispheres separated by the interhemispheric fissure, and a gyral pattern that matures with gestation (smooth in early preterm infants, with the sulci appearing from 28 weeks).

The lateral ventricles are slit like, with anechoic fluid; the choroid plexus is echogenic in the floor of the ventricle (it does not extend into the anterior horns).

The periventricular white matter is slightly echogenic, normally less than the choroid plexus. The thalami and basal ganglia are of low to moderate echogenicity, and the cavum septi pellucidi and cavum vergae are normal findings in preterm infants. The third and fourth ventricles are narrow, the cerebellar vermis is echogenic with normal fissures, and the cisterna magna is under 10 mm.

The ventricular index is under the 97th centile for gestation (for example, under 13 mm at 32 weeks; anterior horn width under 6 mm; thalamo occipital distance under 24 mm). The extra-axial space (sinocortical width) is under 3 to 4 mm and the interhemispheric distance under 6 mm. Anterior cerebral artery resistive index is 0.65 to 0.85 (higher in the first days of preterm life).

Arteries pulsate with a clear colour Doppler signal, and the superior sagittal sinus is patent.

Interpretation.

• Germinal matrix and intraventricular haemorrhage are graded by extent.
• Grade 1: haemorrhage confined to the germinal matrix (an echogenic area in the caudothalamic groove).
• Grade 2: haemorrhage within the ventricle filling under 50% of the ventricular volume, without ventricular dilatation.
• Grade 3: haemorrhage filling over 50% of the ventricle with ventricular dilatation (the ventricle is distended with echogenic clot).
• Grade 4 (periventricular haemorrhagic infarction): echogenic parenchymal haemorrhage in the periventricular white matter, usually asymmetrical, due to venous infarction.
• Blood is echogenic in the acute phase. Over 1 to 4 weeks the clot retracts and liquefies, leaving a cystic cavity (porencephalic cyst) in parenchymal lesions.
• Post-haemorrhagic ventricular dilatation: progressive enlargement of the ventricles (ventricular index over the 97th centile and 4 mm over it, anterior horn width over 6 mm, thalamo occipital distance over 26 mm), rounding of the ventricles, echogenic debris and ependymal lining thickening.
• It may come with raised resistive index, a bulging fontanelle, or rapid head growth (over 2 cm each week). It may progress to hydrocephalus needing temporary or permanent cerebrospinal fluid diversion.
• Periventricular leucomalacia: persistent periventricular echogenicity (flare) lasting over 7 days, with echogenicity equal to or greater than the choroid plexus. It evolves over 2 to 6 weeks to cystic changes (small periventricular cysts) and later loss of white matter volume with ventricular enlargement.
• Cystic lesions are specific, but non-cystic injury is better shown by MR.
• Cerebellar haemorrhage: echogenic focus in the cerebellar hemisphere, best seen through the mastoid window; hypoplasia or atrophy in the follow up of preterm infants.
• Hypoxic ischaemic injury (term infant): diffuse brain swelling with slit ventricles and loss of sulcal definition, increased echogenicity of the thalami and basal ganglia, a "bright" cortex, and loss of grey white differentiation.
• Reduced pulsatility and a low resistive index (under 0.55) in the first 72 hours indicate loss of autoregulation. Later findings are cystic encephalomalacia and atrophy.
• Infarction: focal wedge shaped increased echogenicity in an arterial territory (usually the middle cerebral artery), later cystic change.
• Meningitis/ventriculitis/abscess: echogenic ventricular debris, thickened and echogenic ependyma, ventriculomegaly with septations, widened extra-axial spaces with echogenic strands, abscess as a thick walled complex collection, and echogenic sulci; subdural empyema.
• Congenital infection: periventricular and basal ganglia calcifications (echogenic foci with shadow, cytomegalovirus: periventricular; toxoplasmosis: scattered), lenticulostriate vasculopathy (echogenic linear branching in the thalami and basal ganglia), ventriculomegaly, cysts at the frontal horns (cytomegalovirus), cortical malformations and microcephaly.
• Congenital malformations: agenesis of the corpus callosum (absent callosum and cavum, "tear drop" ventricles, radial gyral pattern), holoprosencephaly, Dandy-Walker spectrum and cerebellar vermis hypoplasia, Chiari malformation, aqueductal stenosis (dilated lateral and third ventricles with a normal fourth ventricle).
• Further malformations: vein of Galen malformation (a midline anechoic structure with turbulent flow and a low resistive index), arachnoid cysts, schizencephaly, hydranencephaly, and lissencephaly.
• Extra-axial collections: subdural haematoma (crescentic collection with echogenic content) and subgaleal haematoma.
• Subarachnoid space enlargement (benign external hydrocephalus): widened frontal subarachnoid space over 5 mm, with normal or mildly enlarged ventricles and preserved cortical vessels crossing the space.
• Vascular: venous sinus thrombosis (loss of flow in the superior sagittal sinus on colour Doppler), and raised resistive index (over 0.85 to 0.9) with raised intracranial pressure, hydrocephalus or patent ductus arteriosus with diastolic steal.
• Cystic structures: choroid plexus cysts (isolated finding, usually benign), subependymal cysts, germinolytic cysts, and ventricular septations.

Critical values and findings needing immediate action.

• Grade 3 or 4 intraventricular haemorrhage, or parenchymal haemorrhage: urgent neonatology and neurosurgery review, supportive care, cranial ultrasound at least every 3 to 7 days to monitor ventricular size.
• Rapid ventricular enlargement, bulging fontanelle, or increased head growth: neurosurgical review for temporary drainage (reservoir, ventriculosubgaleal shunt) or shunt.
• Suspected meningitis or ventriculitis: lumbar puncture if safe, blood cultures, intravenous antibiotics immediately, and neurosurgical review for obstructive hydrocephalus.
• Large infarct, extensive hypoxic ischaemic injury or severe brain swelling: neonatal intensive care, neuroprotection (therapeutic hypothermia for eligible term infants), electroencephalography, and MR at 3 to 7 days.
• Vein of Galen malformation with heart failure: urgent cardiology and neurointervention review.
• Subdural haematoma with signs of raised pressure or unexplained injury: neurosurgical review and child protection assessment.
• Venous sinus thrombosis: haematology and neurology review for anticoagulation.
• Dilated ventricles with seizures or apnoea: urgent assessment for raised pressure.

Limitations and artefacts.

• The limited acoustic window (the anterior fontanelle) restricts views of the peripheral cortex, the convexity, the posterior fossa, the far field and the extra-axial spaces over the convexities.
• The cerebellum and brainstem are better seen via the mastoid window.
• Sensitivity is low for non-cystic white matter injury, cortical injury, diffuse hypoxic ischaemic change, small posterior fossa haemorrhage, subarachnoid and small subdural haemorrhage, and myelination.
• MR at term equivalent age predicts outcome better than ultrasound alone.
• Echogenicity depends on gain settings and probe frequency. Periventricular echogenicity (flare) in the first week is normal in preterm infants. Comparison with the choroid plexus and repeat scans are needed.
• Operator dependence is considerable in grading and in measurements of ventricular size, and use of the same planes and landmarks is needed for follow up.
• Resistive index is affected by heart rate, patent ductus arteriosus, blood pressure, compression of the fontanelle, hydrocephalus, sedation, and gestation; it is not a stand alone indicator.
• Normal ultrasound does not exclude brain injury or predict normal neurodevelopmental outcome; at risk infants need developmental follow up.
• Normal variants mimic pathology: choroid plexus cysts, cavum septi pellucidi and vergae, prominent extra-axial spaces, lenticulostriate vessels, "pseudo cysts" at the frontal horns, and mild asymmetry of ventricles.
• Calcification may be missed on ultrasound; CT or MR is needed for definition when uncertain.
• The study should be done early enough (within 72 hours) to detect haemorrhage, but late (term) scans identify cystic change and atrophy; a single scan is insufficient.

Special groups.

• Extremely preterm infants (under 28 weeks): highest risk for haemorrhage and white matter injury, with scanning from day 1 and frequent follow up.
• Term infants with encephalopathy: ultrasound is a first line tool at the cot side, with MR for definitive evaluation.
• Infants on extracorporeal membrane oxygenation: high risk of haemorrhage and infarction, with daily scans.
• Infants with congenital heart disease: brain injury before and after surgery; cranial scans in critical lesions.
• Infants born to mothers with infections (cytomegalovirus, Zika, toxoplasmosis, syphilis, rubella, HIV): detailed scans.
• Infants with suspected non-accidental head injury: ultrasound is not a substitute for CT or MR and skeletal survey.
• Regions: neonatal sepsis and meningitis, birth asphyxia, kernicterus, congenital infections, and neural tube defects are common.
• Cranial ultrasound is often the only neuroimaging available, and portable machines allow cot side imaging. Early detection of hydrocephalus needs referral routes to neurosurgery, which may be limited.

Follow up tests. Serial cranial ultrasound, brain MR at term equivalent age (or at 3 to 7 days in hypoxic ischaemic encephalopathy), CT in emergencies, electroencephalography and amplitude integrated electroencephalography for seizures, lumbar puncture with cerebrospinal fluid analysis, blood cultures and inflammatory markers,

congenital infection screen (cytomegalovirus polymerase chain reaction on urine or saliva, toxoplasma serology, syphilis serology), coagulation studies and platelet count, metabolic screen, cardiac echocardiography for patent ductus arteriosus, head circumference charting, hearing and vision screening, neurodevelopmental follow up, and referral to neonatology, paediatric neurology or neurosurgery.