Benign enlargement of subarachnoid space in infancy (BESS)

A 7-month-old infant was brought to the hospital with progressive enlargement of the head for past 3 months. No symptoms of irritability, loss of consciousness and vomiting. Neurological examination was normal except enlarged head circumference for the age which was above 2 standard deviation and few dilated superficial veins in the scalp.

Prominent subarachnoid spaces were noted in bilateral frontal and temporal lobes (more on the frontal lobes) with prominent sylvian fissures. Widening of interhemispheric sulcus and bridging veins along with CSF spaces were noted in the bilateral frontal regions. There was no displacement of the veins away from the calvarium. Cortical surface appears normal with no evidence of mass effect. Bilateral lateral ventricles appeared slightly prominent with no periventricular hypodensity. Anterior fontanelle was enlarged while calvarium and sutures were normal.

Benign enlargement of subarachnoid spaces in infancy (BESS) is the one of the common causes of macrocephaly in infants. Maturation of the arachnoid villi usually occurs at about 18 months of age. Due to delayed maturation of the arachnoid villi, it is not able to absorb CSF continuously, leading to the enlargement of the subarachnoid space. It is also associated with multiple metabolic conditions like glutaric aciduria type I, mucopolysaccharidosis, achondroplasia and Sotos syndrome [1, 2].

Ultrasound is the first modality for evaluation since it is a fast, safe and inexpensive tool for the scanning of the brain in infants. It helps to identify and evaluate the intracranial structures, ventricular size as well as extracerebral fluids. Three measurement tools have been proposed for evaluation of the subarachnoid spaces. They are cranio-cortical width (CC width), sino-cortical width (SC width) and interhemispheric width (IH width). Normal upper limits of measurement are CC: 4 mm, SC: 3 mm and IH: 6 mm. However, Nahid et al mentioned the more flexible range of measurement of these parameters as IH: 6 to 8.5 mm, SCW: 2 to 10 mm and CC: 4 to 10 mm. If the measurements are more than the given upper limits, subarachnoid spaces are said to be enlarged [3, 4].Ultrasound Doppler study is also important for differentiating the subdural collection and enlarged subarachnoid spaces. The bridging veins are traversing within the enlarged subarachnoid spaces and these non-displaced veins are seen in colour Doppler. But in cases of subdural collection, these cortical veins are displaced away from the calvarium along with the cortical surface of the brain on Doppler study due to internal shifting of the arachnoid membranes [5].

CT and MRI are the next modalities for the detailed evaluation of the structural anomalies of the deep brain parenchyma. Imaging findings show symmetric enlargement of subarachnoid spaces in bilateral frontoparietal lobes and interhemispheric sulcus. Mild ventriculomegaly is present and bridging veins are noted within the prominent subarachnoid spaces. These bridging veins are best seen as elongated flow voids in T2WI. Therefore, differentiation with subdural effusion can be better done with MRI [1]. Low dose CT was performed, since MRI was not available. We are presenting the case in order to think about this entity whenever an infant presents with macrocephaly.

BESS can be differentiated from brain atrophy where there is prominent subarachnoid space in frontal, parietal, temporal as well as occipital lobes symmetrically and widening of cerebral sulcus with no enlargement of the circumference of the head. The volume of the brain appears smaller in comparison to infants of the same age. Subdural collection can be differentiated with ultrasound and CT/MRI showing non-displaced bridging veins within the subarachnoid space on colour Doppler and flow voids on ultrasound and MRI respectively. Similarly, hydrocephalus can be differentiated from BESS with no significant dilatation of ventricles as well as no evidence of obstruction and periventricular hypodensity. Other metabolic causes like glutaric aciduria type I can be differentiated from BESS with the imaging findings of abnormal T2 and FLAIR high signal intensity in bilateral basal ganglia/globus pallidus, substantia nigra and dentate nuclei associated with subdural effusion and macrocephaly [6]. Canavan disease and Alexander disease can be differentiated on the basis of abnormal T2 and FLAIR high signal intensity within the white matter with macrocephaly without evidence of effusion [7].