M.F. Boomsma,
MD
Sint Antonius Ziekenhuis, Nieuwegein, The Netherlands
Clinical
Presentation: An 8-month-old girl, ex-premature of 35 weeks part of gemelli, presented after referral by general practitioner to pediatrician because of an enlarging head. Clinically she shows normal development for her age but seems to be less able to lift her head in comparison with her twin sister. Her father has a head circumference of 2.5. SD.
Imaging Findings: Figures 1 and 2 show hypo-echoic extra-axial effusion craniocortical 6 mm and interhemispheric 8 mm. The effusion follows the cortex. The subarachnoidal space left and right are symmetrical. No membranes. Normal size and shape of lateral ventricles. As far as could be examined through the large fontanel: normal gyri and sulci pattern with normal differentiation of gray and white matter. Figure 3 shows cortical veins running through the extra axial effusion.
Figure 1A&B: Coronal and sagittal ultrasound images, obtained with curved array transducer (Philips ATL C8-5), through the great fontanel.
Figure 2.
Figure 3.
Figures 2 & 3: Coronal ultrasound images, obtained with linear array transducer (Philips ATL L12-5 38) through the large fontanel, with measurement of extra-axial effusion diameter and color Doppler evaluation of the extra-axial effusion.
Diagnosis: External hydrocephalus
Discussion: Enlarged hypoechoic SAS and increased head circumference (>95%) is idiopathic during first year of infancy. Mild developmental delay occurs in 50% of cases, consisting of delayed motoric development and language. Frontal bossing can be noted and male infants start to walk later on average. The knowledge of the head circumference is critical for the diagnosis. There is no documented genetic predisposition although there is a positive family history in 80% of cases.
Ultrasound (US) is follow up imaging modality of choice as it gives information in real time. MRI can be used to exclude chronic subdural collections and more reliably provide information of cranial anatomy, although narcosis will often be necessary to obtain high quality images. US color Doppler shows veins traversing the SAS and pulsed Doppler can show progressive cases in demonstrating increased cerebral flow. The craniocortical diameter should exceed 5 mm. The CSF space follows the cortical contour and the right and left subarachnoid spaces are symmetrical. In 66% of cases the ventricles will be mildly enlarged. CT and MR can confirm diagnosis, with CT showing >5mm widening bifrontal/anterior space. CECT demonstrates veins traversing the SAS. MR will show single layer of fluid (SAS) with traversing vessels (flow voids) and T1C+ shows enhancing veins traversing the SAS. Differential diagnosis consists of atrophy (small head circumference), acquired extraventricular obstructive hydrocephalus (EVOH, density of extra-axial collection does not resemble CSF) and inflicted non-accidental trauma (NAT should show venous stretching). Associated anomalies are anecdotal.
Danger signs consist of onset or persistence after the age of 1. Rapid enlargement of the head circumference and the SAS measuring more than 6 mm.
Benign SAS enlargement usually resolves at 18 months of age as Pacchonian granulations mature at this age and the Pacchonian granulations are then displaced into veins as Starling type resistors, thus regulating pulse pressure/venous drainage CSF when fontanels close. Before this age CSF is primarily drained via the extracellular space -the capillaries. Nevertheless the macrocephaly can persist. There will be a normal outcome with resolve of the developmental delay as the prominent SAS resolves.
The advice was given for an additional MRI for exclusion of brain anomalies as the brain could not be completely reliable evaluated by ultrasound. Two weeks after establishing the diagnosis external hydrocephalus, the patient has a head circumference of 46,5 cm (+ 1SD). Patient is referred for physiotherapy and will be seen regularly by the pediatrician.
References:
Osborne. Diagnostic imaging. Amirsys, Salt Lake City, Utah, USA. 2004 page II-1; page 12-15.
