Benign enlargement of sub-arachnoid spaces in infancy.

Benign enlargement of sub-arachnoid spaces (BESS) is one of the causes of macrocephaly in infants. It is a self-limiting condition and does not require any active medical or surgical treatment. We report a case of an infant aged 4 months who was referred for magnetic resonance imaging (MRI) of the brain as the head circumference of the infant had increased rapidly from the 50(th) percentile in the 3(rd) month to more than the 95(th) percentile in the 4(th) month of age. MRI revealed enlarged anterior sub-arachnoid spaces and mild prominence of all the ventricles. A possibility of BESS was suspected since the child was neurodevelopmentally normal. A follow-up MRI done at the age of 18 months showed a reduction in the size of the sub-arachnoid spaces with normal sized ventricles.

Introduction

Benign enlargement of sub-arachnoid spaces (BESS) has been described by various other names in literature such as extra ventricular obstructive hydrocephalus, external hydrocephalus, benign sub-arachnoid space enlargement, benign extra-axial collections, or subdural effusions of infancy. It is a self-limiting condition. Herein, we report a case of an infant with macrocephaly who was diagnosed with BESS on magnetic resonance imaging (MRI). Clinicians should be aware of this benign entity so as not to mistake it for a sinister pathology.

Case Report

A male infant aged 4 months was referred from a local clinic to the Pediatric Department of our institute for the rapid increase in his head circumference from the 50th percentile (40 cm) at 3 months of age to more than the 95th percentile (45 cm) at 4 months of age. These percentiles were measured according to the Indian Academy of Pediatrics growth charts. The anterior fontanelle was soft and of normal tension. Rest of the clinical examination revealed no significant abnormality. The child was neurodevelopmentally normal with normal milestones. The child was born at 40 weeks of gestation by normal vaginal delivery and had cried immediately after birth.

Ultrasound of the cranium was performed which revealed mild prominence of all the ventricles, suggestive of a mild communicating hydrocephalus. MRI of the brain was performed for further evaluation. MRI revealed mild prominence of all the ventricles with enlarged sub-arachnoid spaces along the anterior aspect of the brain [Figures 1 and 2]. The subarachnoid spaces along the posterior aspect of the brain were normal. The basal cisterns were also enlarged [Figure 3]. The anterior inter-hemispheric distance (widest distance between both cerebral hemispheres) and the anterior cranio-cortical width (widest vertical distance between the inner table of the skull and cortex of the brain parenchyma along the anterior aspect) were increased.

Figure 1

Axial T2-weighted image of magnetic resonance imaging of the brain reveals mild prominence of both the lateral ventricles (black arrows). The anterior inter-hemispheric distance is increased (green arrow)

Figure 2

Axial T2-weighted image of magnetic resonance imaging of the brain reveals enlarged subarachnoid spaces along the anterior aspect of the brain (black arrows). The body of bilateral lateral ventricles appears prominent (green arrows). The anterior cranio-cortical distance (black arrows) is increased

Figure 3

Axial T2-weighted image of magnetic resonance imaging of the brain reveals enlarged basal cisterns (black arrow) and fourth ventricle (white arrow)

A possibility of BESS was considered since the child was neurodevelopmentally normal and hence, follow-up imaging was advised.

The child was continuously monitored every 2 months for measurement of the head circumference and for the development of any abnormal neurodevelopmental signs. A follow-up MRI at 18 months of age revealed reduction in the size of the sub-arachnoid spaces with normal-sized ventricles. However, the child still had a head circumference of more than the 95th percentile.

Discussion

Benign enlargement of sub-arachnoid spaces is a self-limiting,[1] benign cause of macrocephaly in infants. There have been a few case reports of this entity in literature. It usually presents in infants by 3-4 months of age. Two-third of the infants are boys.[2] Some studies have shown a positive family history of macrocephaly in such infants.[3] Many possible causes of this condition have been postulated. The most probable cause is immaturity of the arachnoid villi, which is responsible for absorption of the cerebrospinal fluid (CSF).[4] Later on, this excess CSF is reabsorbed on maturation of the arachnoid villi. These infants are usually neurodevelopmentally normal and have an excellent prognosis. The only abnormal finding on clinical examination is a rapid increase in the head circumference with or without a widely open anterior fontanelle. There are no signs and symptoms of increased intra-cranial pressure[5] such as irritability, lethargy, vomiting or tense, and bulging anterior fontanelle.

Imaging studies like ultrasound or MRI are warranted in these infants to rule out other causes of macrocephaly such as hydrocephalus, subdural or extradural collections, and space-occupying lesions in the brain. Ultrasound is usually the initial imaging modality as it provides a good window to visualize the ventricles through the open anterior fontanelle. Ultrasound reveals normal or mild prominence of all the ventricles with enlarged sub-arachnoid spaces along the anterior aspect of the brain.

Magnetic resonance imaging is the best imaging modality for such cases since the exact measurement of the enlarged sub-arachnoid spaces can be made, so as to have a baseline measurement for future follow-up. MRI usually reveals normal or mild prominence of all the ventricles (mild communicating hydrocephalus) with enlarged, usually symmetric, subarachnoid spaces along the anterior aspect of the brain. The sub-arachnoid spaces along the posterior aspect of the brain are usually normal. The anterior inter-hemispheric and the anterior cranio-cortical distances are increased. However, the brain parenchyma is usually normal and shows no abnormal signal intensity or lesion within.

Enlarged subarachnoid spaces can be differentiated from a subdural hygroma on MRI by the “cortical vein” sign.[6] Cortical veins are usually seen within enlarged subarachnoid spaces (positive cortical vein sign). The presence of these cortical veins may also be seen on an ultrasound Doppler study of the cranium. Subdural hygroma causes compression of the sub-arachnoid spaces and the veins within these sub-arachnoid spaces and hence, no cortical veins are seen within a subdural hygroma.

The head circumference may still be more than the 95th percentile on regular follow-up clinical examination. However, follow-up imaging like MRI at 18 months to 2 years of age shows reduction in the size of the subarachnoid spaces with normal-sized ventricles.

There have been a few reports stating that children with BESS are at an increased risk of developing subdural hematomas,[78] either spontaneously or due to minor trauma. It is believed that the enlarged sub-arachnoid space causes stretching of the bridging veins in the subdural space, thereby making them more susceptible to bleed. Subdural hematomas in children with BESS should be differentiated from those due to nonaccidental trauma (NAT). NAT is usually associated with multiple subdural hematomas of different ages with or without brain contusions. However, subdural hematomas in children with BESS are usually isolated.

The prognosis of these children is excellent, and no significant neurodevelopmental problems are seen in the long run. Even though, BESS is a self-limiting condition, it warrants further evaluation if the enlarged subarachnoid spaces do not revert back to normal by 2 years of age or if the child develops some neurodevelopmental problems.[9]

Conclusion

Benign enlargement of subarachnoid spaces is a benign, self-limiting cause of macrocephaly in infants. Clinicians should be aware of this entity since it has a very good prognosis, and most of these infants do not have any neurodevelopmental delay. Knowledge of this condition among clinicians would help in allaying the anxiety of concerned parents and avoiding unnecessary further investigations and treatment, since this condition only requires follow-up imaging like MRI at 18-24 months of age and does not require any active medical or surgical treatment.