Management of subarachnoid fluid collection in infants based on a long-term follow-up study

Re-evaluation of medical findings in alleged shaken baby syndrome and abusive head trauma in Norwegian courts fails to support abuse diagnoses

AIM

The criteria for diagnosing abusive head trauma (AHT) are not well defined and this condition might be diagnosed on failing premises. Our aim was to review criminal AHT cases in Norwegian courts by scrutinising the underlying medical documentation.

METHODS

Cases were identified in the data registry for Norwegian courts from 2004 to 2015. Documentation was obtained from relevant health institutions. The medical co-authors first made independent evaluations of the documentation for each child, followed by a consensus evaluation.

RESULTS

A total of 17 children (11 boys) were identified, all diagnosed as AHT by court appointed experts, 15 were infants (mean age 2.6 months). A high proportion (41.2%) was born to immigrant parents and 31.3% were premature. The medical findings could be explained by alternative diagnoses in 16 of the 17 children; 8 boys (7 infants - mean age 2.9 months) had clinical and radiological characteristics compatible with external hydrocephalus complicated by chronic subdural haematoma. Six children (five infants with mean age 2.1 months) had a female preponderance and findings compatible with hypoxic ischaemic insults.

CONCLUSION

The medical condition in most children had not necessarily been caused by shaking or direct impact, as was originally concluded by the court experts.

Fetal wide subarachnoid space and its outcome in cases of macrocephaly without ventriculomegaly

Objective: To examine the occurrence and outcomes of fetuses with wide subarachnoid space (WSS) without ventriculomegaly in pregnant women with fetal macrocephaly as a sole diagnosis.Study design: A retrospective study was performed, analyzing patients with fetal macrocephaly between the years 2008 and 2018. All these patients underwent MRI, in order to detect brain anomalies. In the absence of any other brain abnormality, they were evaluated for WSS and their offspring's database was followed for at least two years after birth.Results: Ten patients were found to be carrying fetuses with macrocephaly, nine of them were diagnosed with WSS without ventriculomegaly prior to delivery. Following at least two years of follow up, all patients did not present significant neurodevelopmental abnormalities, apart from one child that had a genetic mutation of 15q21.2-22.31 deletion with other anomalies that were not diagnosed prenatally.Conclusions: We present herein for the first time in the literature a cohort of patients with a prenatal diagnosis of WSS without ventriculomegaly in fetuses with macrocephaly. Our data show that, in the presence of normal anomaly scan and normal chromosomal study, there is a low chance for significant neurodevelopmental abnormalities in fetuses with WSS without ventriculomegaly.

Benign extracerebral fluid collection complicated by subdural hematoma and fluid collection: clinical characteristics and management

INTRODUCTION

Benign extracerebral fluid collection (bECFC) can be complicated by subdural hematoma (SDH) or subdural fluid collection (SDFC). The etiology, natural history, and management strategy for SDH/SDFC in bECFC are not fully understood. We retrospectively reviewed the cases of bECFC patients complicated with SDH/SDFC and tried (1) to confirm the fact that bECFC children are vulnerable to SDH/SDFC, (2) to investigate the clinical significance of 'trauma history' witnessed by a caregiver, and (3) to determine optimal management for them.

METHOD

Among 213 bECFC patients identified from January 2000 to August 2015, 20 patients (male:female = 14:6; median age, 6.5 months; range 1-16 months) complicated by SDH/SDFC documented with brain imaging were evaluated for their clinical manifestations, radiologic features, and management outcomes. The median follow-up period was 9.5 months. They were divided into two groups (traumatic group versus non-traumatic group) according to whether objective radiologic evidence of head injury was present or not, and the two groups were analyzed for any clinical differences between them. We also evaluated the clinical significance of witnessed traumatic events by caregivers as an additional independent variable in the analysis.

RESULTS

The incidence of SDH/SDFC in bECFC patients was 9.4% (20/213) in our data. In a comparative analysis, the traumatic group is more likely to have 'acute' stage SDH, whereas the non-traumatic group is more likely to have 'chronic' stage SDH. The trauma history witnessed by caregivers did not show clinical significance in the data analysis when included as an independent variable. The prognosis of SDH/SDFC in bECFC patients was favorable without surgery in most of patients regardless of whether the patient has evidence of head trauma or not.

CONCLUSION

Benign ECFC is vulnerable to SDH/SDFC development. For the bECFC patients complicated by SDH/SDFC, the trauma history witnessed by a caregiver did not show any clinical significance. A 'wait and watch' strategy is sufficient for the management of SDH/SDFC in bECFC patients.

Epidemiology of Benign External Hydrocephalus in Norway-A Population-Based Study

BACKGROUND

Benign external hydrocephalus is defined as a rapidly increasing head circumference (occipitofrontal circumference) with characteristic radiological findings of increased subarachnoid cerebrospinal fluid spaces on neuroimaging. The incidence of benign external hydrocephalus has not been previously reported, and there is no available information on the ratio of benign external hydrocephalus in the population of hydrocephalic children.

METHODS

This study is retrospective and population-based study, geographically covering two health regions in the southern half of Norway with a total mean population of 3.34 million in the ten-year study period, constituting approximately 75% of the Norwegian population. Children with a head circumference crossing two percentiles, or greater than the 97.5th percentile, and with typical imaging findings of enlarged frontal subarachnoid spaces with or without enlarged ventricles were included. Children were excluded if they had a history of head trauma, intracranial hemorrhage, central nervous system infection, other known causes of hydrocephalus, or were born preterm defined as birth before 37 weeks of gestation.

RESULTS

A total of 176 children fitting the criteria were identified, giving an incidence of 0.4 per 1000 live births. One hundred fifty-two (86.4%) of the patients were male, and mean age at referral was 7.3 months. Increasing head circumference was the main reason for referral in 158 (89.8%) patients and the only finding in 60 (34.1%) patients. Thirty-seven (21%) children had normal ventricles on imaging; the remainder had increased ventricular size. The incidence of pediatric hydrocephalus in Norway is reported to be 0.75 per 1000 live births, thus benign external hydrocephalus accounts for approximately 50% of hydrocephalic conditions in this population.

CONCLUSIONS

The incidence of benign external hydrocephalus was found to be 0.4 per 1000 live births in this population.

A Retrospective Analysis of the Utility of Head Computed Tomography and/or Magnetic Resonance Imaging in the Management of Benign Macrocrania

OBJECTIVE

To assess whether computed tomography (CT), magnetic resonance imaging (MRI), and neurosurgical evaluations altered the diagnosis or management of children diagnosed with benign macrocrania of infancy by ultrasonography (US).

STUDY DESIGN

We queried our radiology database to identify patients diagnosed with benign macrocrania of infancy by US between 2006 and 2013. Medical records of those with follow-up CT/MRI were reviewed to determine clinical/neurologic status and whether or not CT/MRI imaging resulted in diagnosis of communicating hydrocephalus or required neurosurgical intervention.

RESULTS

Patients with benign macrocrania of infancy (n = 466) were identified (mean age at diagnosis: 6.5 months). Eighty-four patients (18.0%) received subsequent head CT/MRI; of these, 10 patients had neurologic abnormalities before 2 years of age, of which 3 had significant findings on MRI (temporal lobe white matter changes, dysmorphic ventricles, thinned corpus callosum). One patient without neurologic abnormalities had nonspecific white matter signal abnormality (stable over 6 months) but no change in management. None required neurosurgical intervention. Another 9/84 patients had incidental findings including Chiari I (3), small subdural bleeds (2), arachnoid cyst (1), small cavernous malformation (1), frontal bone dermoid (1), and a linear parietal bone fracture after a fall (1).

CONCLUSIONS

Children diagnosed with benign macrocrania of infancy on US without focal neurologic findings do not require subsequent brain CT/MRI or neurosurgical evaluation. Decreasing unnecessary imaging would decrease costs, minimize radiation and sedation exposures, and increase clinic availability of neurology and neurosurgery specialists.

Prevalence of subdural collections in children with macrocrania

BACKGROUND AND PURPOSE

The relationship between enlarged subarachnoid spaces and subdural collections is poorly understood and creates challenges for clinicians investigating the etiology of subdural collections. The purpose of this study was to determine the prevalence of subdural collections on cross sectional imaging in children with macrocephaly correlating with subarachnoid space enlargement.

MATERIALS AND METHODS

The radiology information system of a large pediatric medical center was reviewed for "macrocrania" and "macrocephaly" on reports of cranial MRI/CT examinations in children <24 months of age, over a 24-month period. Head circumference was obtained from the clinical record. Studies were reviewed blindly for subdural collection presence and subarachnoid space size. Children with prior cranial surgery, parenchymal abnormalities, hydrocephalus, or conditions predisposing to parenchymal volume loss were excluded. Chart review was performed on those with subdural collections.

RESULTS

Imaging from 177 children with enlarged head circumference was reviewed. Nine were excluded, for a final cohort of 168 subjects (108 with enlarged subarachnoid space). Subdural collections were identified in 6 (3.6%), all with enlarged subarachnoid space (6/108, 5.6%). In 4, subdural collections were small, homogeneous, and nonhemorrhagic. In 2, the collections were complex (septations or hemorrhage). Two children were reported as victims of child abuse (both with complex collections). No definitive etiology was established in the other cases.

CONCLUSIONS

The prevalence of subdural collections in imaged children with macrocrania was 3.6%, all occurring in children with enlarged subarachnoid space. Our results suggest that enlarged subarachnoid space can be associated with some subdural collections in this cohort. Despite this, we believe that unexpected subdural collections in children should receive close clinical evaluation for underlying causes, including abusive head trauma.

Benign extracerebral fluid collection in infancy as a risk factor for the development of de novo intracranial arachnoid cysts

OBJECT

Intracranial arachnoid cysts are a relatively common benign intracranial pathology, accounting for as many as 0.75%-1% of nontraumatic CNS lesions. Although it has already been demonstrated that rupture of arachnoid cysts may lead to subdural hematomas/hygromas, no study to date has investigated benign extracerebral collection in infancy as a possible predisposing factor for further development of arachnoid cysts.

METHODS

The authors performed a retrospective imaging and chart review of macrocephalic infants 12 months old or younger who were referred to neurosurgical care at OSF St. Francis Medical Center from 2003 to 2010, and who were diagnosed with benign extracerebral fluid collection in infancy on thin-slice (1-mm) head CT scans. Special attention was given to the investigation of risk factors for further development of de novo arachnoid cysts. Several epidemiological factors in the infants and mothers were analyzed, including gestational age at delivery, mode of delivery, mother's age at delivery, delivery complications, birth weight, age of macrocephaly development, degree of macrocephaly, family history of macrocephaly, prenatal and postnatal history of infection, fontanel status, presence of papilledema, previous history of head trauma, and smoking status. Imaging characteristics of the initial scans, such as location of subdural collection (frontal vs frontoparietal and frontotemporal) and presence of ventriculomegaly, were also evaluated. For those patients in whom arachnoid cysts were identified on subsequent CT scans, the size and location of the cysts were also analyzed.

RESULTS

The authors identified 44 children with benign extracerebral fluid collection in infancy. From this group, over a mean follow-up of 13 months (range 6-13 months), 18 children developed intracranial arachnoid cysts (a 40.9% incidence of de novo development of arachnoid cysts), with 27.8% presenting with bilateral cysts. In the multiple logistic regression analysis, infants who presented with an extracerebral collection restricted to the bilateral frontal region were more likely to develop intracranial arachnoid cysts (p = 0.035) than those with collections involving the frontotemporal and frontoparietal regions (odds ratio [OR] = 5.73). Additionally, children with benign extracerebral fluid collections and plagiocephaly were more likely to develop intracranial arachnoid cysts (p = 0.043) than those without plagiocephaly (OR = 4.96).

CONCLUSIONS

This is the first report in the neurosurgical literature demonstrating that benign extracerebral fluid collections in infancy may constitute a significant risk factor for development of de novo arachnoid cysts. These findings support a 2-hit hypothesis for the development of arachnoid cysts, in which the combination of an embryological defect in arachnoid development followed by a second event leading to impairment of CSF fluid absorption in early childhood could lead to abnormal CSF dynamics and the consequent expansion of fluid collections in the intraarachnoid spaces.

A pitfall in the diagnosis of child abuse: external hydrocephalus, subdural hematoma, and retinal hemorrhages

External hydrocephalus has been associated with subdural hematomas in infancy, and the hematomas have been noted to be secondary to minor trauma or have even been described as spontaneous. The author reports the case of an infant with external hydrocephalus who developed retinal as well as subdural hemorrhages after sustaining a minor head injury. Although retinal hemorrhage in infancy has been considered virtually pathognomonic of child abuse, in the setting of external hydrocephalus a more cautious interpretation may be appropriate.

Subdural hematoma in infants without accidental or nonaccidental injury: benign external hydrocephalus, a risk factor

Benign external hydrocephalus (BEH) is considered a self-limiting condition in infants. Subdural hematoma (SDH) in infants without a history of trauma indicates nonaccidental injury (NAI). The authors studied whether SDH can complicate BEH without apparent trauma. Out of 45 children younger than 3 years with nontraumatic SDH, 9 (7 boys) with mean age 6 months had BEH as risk factor. Symptoms included increasing head size (8), fussiness, and irritability (3). Three had up-gaze restriction, 1 axial hypotonia, and 6 normal examination. Neuroimaging showed prominent extra-axial spaces; SDH was bilateral (6), subacute (5). Other etiological workup for SDH was negative except NAI in 1. Two required evacuation of SDH and subdural-peritoneal shunt; others managed conservatively. Development was normal in 8 on follow-up. On follow-up imaging of 8, SDH completely resolved in 3, markedly reduced in 3, and remained stable in 2. BEH is a risk factor for SDH in infants, thus not always benign.

Benign external hydrocephalus: a review, with emphasis on management

Benign external hydrocephalus in infants, characterized by macrocephaly and typical neuroimaging findings, is considered as a self-limiting condition and is therefore rarely treated. This review concerns all aspects of this condition: etiology, neuroimaging, symptoms and clinical findings, treatment, and outcome, with emphasis on management. The review is based on a systematic search in the Pubmed and Web of Science databases. The search covered various forms of hydrocephalus, extracerebral fluid, and macrocephaly. Studies reporting small children with idiopathic external hydrocephalus were included, mostly focusing on the studies reporting a long-term outcome. A total of 147 studies are included, the majority however with a limited methodological quality. Several theories regarding pathophysiology and various symptoms, signs, and clinical findings underscore the heterogeneity of the condition. Neuroimaging is important in the differentiation between external hydrocephalus and similar conditions. A transient delay of psychomotor development is commonly seen during childhood. A long-term outcome is scarcely reported, and the results are varying. Although most children with external hydrocephalus seem to do well both initially and in the long term, a substantial number of patients show temporary or permanent psychomotor delay. To verify that this truly is a benign condition, we suggest that future research on external hydrocephalus should focus on the long-term effects of surgical treatment as opposed to conservative management.

Long-term health status in benign external hydrocephalus

PURPOSE

Benign external hydrocephalus (BEH) is characterized by excess cerebrospinal fluid in the frontal subarachnoid spaces in infants evaluated for macrocephaly. We sought to describe the natural history of this disorder, specifically its clinical presentation, disease course and long-term health status impact using the validated, disease-specific Hydrocephalus Outcome Questionnaire (HOQ).

METHODS

An inception cohort of children >5 and <18 years old, with a history of BEH was assembled by ICD-9 and a key word search of medical and radiology records. Review confirmed the diagnosis of BEH, excluded major medical comorbidities and assessed presentation, radiographic features and head size/growth velocity. The HOQ was administered by mail.

RESULTS

We identified 99 patients, 5-12 years old (55% males). Twenty were born prematurely, 12 with <33 weeks gestation. Children presented at an average age of 9 ± 4.8 months (mean ± SD). The presenting complaint was macrocephaly in 65 cases. Other presenting findings were positional head shape deformity and torticollis; 10% had a family history of macrocephaly. Developmental delay was present in 21% of patients (4% verbal, 20% gross motor, 4% fine motor delay). Four patients had small subdural hematomas, none with suspicion of a non-accidental trauma. During clinical reassessment over a mean follow-up of 13 months, the average head percentile was stable and none of the patients developed new subdural hematomas. Gross motor delay resolved in 15/20 and fine motor delay in 4/4 patients. Verbal delay resolved in 2/4 patients, but interestingly, was newly detected in 6 other children. None of the patients required cerebrospinal fluid shunting. The response rate to the HOQ was 25% (median age 7 years, 74% females). The average overall HOQ score was 0.75 ± 0.24 versus 0.68 ± 0.19 for a previously published cohort of shunted hydrocephalic children.

CONCLUSIONS

BEH patients in this series generally saw resolution of presenting motor developmental delays; however, new verbal delays were detected in a non-trivial number of patients. Quality of life measurements suggest some reduction in health status, but less so than is seen with shunted hydrocephalus.

Postoperative extra-axial cerebrospinal fluid collection--its pathophysiology and clinical management

OBJECTIVE

External hydrocephalus, introduced by Dandy and Blackfan in 1914 [Am J Dis Child 1914;8:406-482], is used to describe the clinical condition of cerebrospinal fluid (CSF) accumulation in the subarachnoid space with its space expansion and increased intracranial pressure in pediatric patients. Benign subdural effusion, subdural hygroma and pseudohydrocephalus are some terms used to describe some conditions, not clearly defined at the moment as separate pathological entities. We report 3 pediatric cases which had CSF collections after craniotomy. Neuroimaging investigations revealed CSF collection after surgery. All 3 cases had a poor clinical course with resolution after shunting. This report analyzes the mechanisms and pathophysiology of postoperative extra-axial CSF collections (PECC), indicating that CSF accumulations in the extra-axial spaces (inside or outside the arachnoid) after surgery have a common pathophysiology. Therefore, the treatment protocol can be specific.

PATIENTS

Three pediatric cases, aged from 11 months to 6 years, had large tumors located from their suprasellar region to the hypothalamus and third ventricle. The tumors were excised surgically through either an anterior interhemispheric translamina-terminalis or a transsylvian translamina-terminalis approach or both. The surgery conducted created a wide opening of the anterior interhemispheric or the sylvian fissures, dissecting the lamina terminalis and Liliequist's membrane resulting in a new artificial CSF tract from the basal cisterns to the subdural space. Clinical symptoms such as deterioration of consciousness level developed within a few weeks after surgery, and a follow-up computed tomography image confirmed subdural CSF collection. Symptoms improved after ventriculoperitoneal (V-P) shunting.

DISCUSSION

External hydrocephalus, subdural effusion and the other entities mentioned above are frequently observed in children, and little is known about their pathophysiology. To differentiate from the whole entity of CSF accumulation within the subdural and subarachnoid space, we use the term PECC to specify the main pathophysiological condition resulting from a wide dissection and opening of the arachnoid membrane and communication with the basal cisterns.

CONCLUSION

We present the occurrence of CSF collections in the extra-axial space after surgery as the specific condition of PECC. Broadly dissecting the arachnoid membrane, with a communication remaining with the ventricles, is the main factor contributing to PECC, and patients have shown that V-P shunting is an effective procedure preventing progression and effectively treating PECC.

ReviewNatural history of chronic subdural haematoma

This review will clarify the natural history of chronic subdural haematoma (SDH). Chronic SDH has dual origins, one from subdural hygromas (SDG) and the other from acute SDHs. It occurs only in patients with a suitable pre-morbid condition, i.e. sufficient potential subdural space (PSS). In unresolved SDGs, proliferation of dural border cells produces the neomembrane. Unresolved SDGs become chronic SDHs by repeated micro-haemorrhages from fragile new vessels, which were grown into the neomembrane. When PSS is sufficient, acute SDHs may become chronic SDHs. Chronic SDHs enlarge when rebleeding exceeds absorption and they become symptomatic. When the neomembrane is matured, the neocapillary is no longer fragile. If absorption exceeds rebleeding, the haematoma will disappear. Maturation of the neomembrane and stabilization of the neovasculature eventually result in spontaneous resolution. The fate of chronic SDH depends on the pre-morbid status, the dynamics of absorption-expansion and maturation of the neomembrane.

Short-term subarachnoid space drainage: a potential treatment for extraventricular hydrocephalus

INTRODUCTION

Extraventricular hydrocephalus (EVOH), defined as the enlargement of all CSF compartments in the absence of an obstructing lesion, is usually associated with an increased head circumference and a relatively benign clinical course. Occasionally, because of concern about increased intracranial pressure (ICP), treatment with diuretics is initiated. In most cases, surgical drainage or diversion is not indicated. EVOH may follow a more alarming clinical course and be associated with developmental delay and/or other symptoms of increased ICP.

CASE REPORT

We describe a 6-month-old girl with EVOH and developmental delay who was treated with temporary drainage of the subarachnoid space. Clinical response was immediate, with stabilization of the head circumference and improvement in motor performance. We propose that such a surgical procedure might be considered for more frequent use in selected cases.

Subdural hygroma as a complication of endoscopic neurosurgery--two case reports

Two cases of subdural hygroma occurred in a series of 77 neuroendoscopic procedures. An 8-year-old boy underwent neuroendoscopic cysto-cisternostomy of a left temporal arachnoid cyst. Routine postoperative magnetic resonance imaging 7 days later showed a large left-sided subdural hygroma without clinical symptoms. During the following 3 months, the subdural hygroma did not resolve spontaneously, so it was drained through a burr hole. A 3-month-old boy with aqueductal stenosis developed bilateral subdural hygromas after third ventriculostomy. Several punctures through the open anterior fontanelle relieved the hygromas but increasing head circumference required ventriculoperitoneal shunting 12 months later. Complications of neuroendoscopic procedures are increasingly reported, including various kinds of bleeding, infections, or damage of neuronal tissue. Only three previous cases of subdural hygroma or hematoma after neuroendoscopic interventions have been reported. The possible etiologies and clinical consequences of this rare complication have to be considered before selecting neuroendoscopy treatment.

Incidence and management of subdural hematoma/hygroma with variable- and fixed-pressure differential valves: a randomized, controlled study of programmable compared with conventional valves

Shunt systems with differential pressure valves are prone to the complications of overdrainage. A programmable valve permits adjustment of the opening pressure of the valve. In this paper the authors report the incidence of subdural fluid collections in a randomized trial of programmable compared with conventional valves, and they describe methodologies used in management of this complication.

A multiinstitutional, prospective, randomized trial of the Codman Hakim programmable valve and conventional fixed-pressure valves was undertaken. Two classes were defined: “new” and “replacement” valves. Randomization of the type of valve in each group was performed at each study site. Clinical and radiological studies were required at fixed intervals over a 104-week period. All complications were reported. The experimental valves were required to be reprogrammed after magnetic resonance imaging studies, but all other decisions regarding pressure setting were left to each investigator.

Three hundred seventy-seven patients were randomized; 194 were treated with a programmable valve and 183 with a fixed-pressure valve. The two groups were statistically similar in demographic composition, as were the “new” and “replacement” categories. The investigators made 540 valve pressure changes (five per patient; range one-41 changes). More than half of the reprogramming adjustments were made in the first 3 months postplacement; 70% were made within 6 months. More than half of all reprogramming adjustments were required in a group of 30 patients.

Four treatment modalities were observed: 1) 30% of the fluid collections resolved spontaneously (25% in the patients with programmable valves and 36.3% in those with conventional valves) and were largely found to be hygromas in infants and children; 2) four subdural fluid collections were unresolved and under observation; 3) the subdural hematoma was drained and the shunt removed (in 8.3% of patients with the programmable valve and 36.3% of those with the control valve); 4) the pressure of programmable valve was raised in 58% of patients (seven of 12), and this increase in opening pressure was a feature used by investigators to affect treatment.

There was no significant difference in the incidence of subdural fluid collections between the programmable and fixed-pressure valve treatment groups. The programmable feature provided a considerable advantage in treatment when subdural collections occurred.

Origin of chronic subdural haematoma and relation to traumatic subdural lesions

The origin of chronic subdural haematoma (CSDH) and the pathogenesis of subdural hygroma (SDG) are still controversial issues. These issues and relationships between these traumatic subdural lesions are discussed. The origin of CSDH is usually a SDG, although a few cases are caused by acute subdural haematomas (ASDH). Subdural hygroma is produced by separation of the dura-arachnoid interface, when there is sufficient subdural space. When the brain remains shrunken, the SDG remains unresolved. Any pathologic condition inducing cleavage of tissue within the dural border layer at the dura-arachnoid interface can induce proliferation of dural border cells with production of neomembrane. In-growth of new vessels will follow, especially along the outer membrane, then bleeding from these vessels occurs. These unresolved SDGs become CSDHs by repeated microhaemorrhage from the neomembrane. Although most victims with ASDH underwent surgery or died, some patients could be managed conservatively. Since the ASDH is usually absorbed within a few weeks, only a very few ASDHs become CSDHs, when there is a sufficient potential subdural space. Chronic subdural haematoma can arise from ASDH, but more commonly from SDG. Such transformation, or development of a new subdural lesion, is a function of the premorbid status and the dynamics of absorption and expansion.