DISCUSSION

There is no accurate report on the incidence of subdural hygroma in infants, but it has been suggested that subdural hematomas occur every 21 per 100,000 infant populations ⁸.

The mechanisms for formation of subdural hygroma are known to be idiopathic or secondary after brain damage. In idiopathic cases; cells in the granulation tissue of the arachnoid membrane does not develop adequately, which then interferes with the normal cerebrospinal fluid absorption process. Ultimately, subdural hygroma develops when the outer arachnoid membrane breaks up or when the cap cells of arachnoid membrane proliferates. But it is well known that subdural hygroma improves when the cerebrospinal fluid is absorbed normally as time goes on ^{9, 10}.  Unfortunately, if the subdural hygroma grows too large while waiting for spontaneous resolution, it may influence brain development and may also further develop in to a subdural hematoma, and needs surgical intervention ¹¹.

When the etiology is secondary, it has been demonstrated that when treatment consisted of subdural drainage or subduroperitoneal shunt, the success rate is high due to the brain expansion in the developmental age. All subdural hygromas in infants and children should be treated taking into consideration the size of the hygroma and the presence or absence of symptoms ¹².

Arachnoid cysts are common childhood developmental anomalies, but their association with spontaneous subdural hygroma is rare They are benign lesions and may be associated with complications such as subdural hematoma subdural hygroma, and intracystic hemorrhage. Minor head injury is known to result in subdural hygroma or hematoma due to rupture of arachnoid cyst; however spontaneous rupture of arachnoid cyst may occur rarely ^{6, 9}.

The idiopathic cause is most common in patients under the age 1, and especially at 3-5 months. When and how often infants or children develop subdural hygromas is not yet accurately known, but progression to subdural hematoma usually develops 3~4weeks after the appearance of subdural hygroma ¹².  In this study about 36% of cases were with unknown cause and head and birth trauma predominated the causes while 3 cases had ruptured arachnoid cyst after minor head injury.

According to Caldarelli et al ¹³, which studied 85 cases of infantile subdural hygroma, the main symptoms were increased intracranial pressure, macrocrania, mental retardation, hemiparesis, seizure, and decreased consciousness, in decreasing order of frequency. However, our study showed that delayed developmental milestones, increased intracranial pressure, seizures, large sized head, and lastly diplopia   was the most common symptoms, in descending order of frequency.

Radiological characteristics of subdural hygroma in CT and MRI can be distinguished from external hydrocephalus. As subdural hygroma shows expansion of the subdural space and not the subarachnoid space due to accumulation of CSF in the subdural space. So, the enhanced vessels by contrast media are compressed to adhere to the brain ^{14, 15}. When a subdural hygroma appears on the inside of a cerebral cortical vessel, it should be differentiated from external hydrocephalus. If the patient is under age of 1 month, it should also be differentiated from other transient diseases, such as benign perisylvian fluid collection and benign familial subdural fluid collection. Of course the subarachnoid membrane can be seen without enhancement on the MRI, and therefore it can be certain that the location of the hygroma is not in the subarachnoid space, but in the subdural space ¹⁶. Mri brain was done for all cases in this series and carefully studied by radiologist and us to differentiate subdural hygroma from other possibilities.  

Many authors tried subdural aspiration first in infantile patients with subdural hygroma, and then subdural drainage. This is a still used surgical procedure, but subdural aspiration may be of benefit for some cases ¹². In this study, subdural aspiration (tapping from anterior fontanelle) was performed in 3 cases by transfontanell aspiration. The other 5 cases were done by burr ho le drainage in older children. Among the 8 cases, 6 cases improved and 2 cases needed subduroperitoneal shunts.

Subdural drainage through subdural catheter as an external drain may be tried for subdural hygromas, but it is most effective when a subdural hematoma develops, and a success rate of 60~70% had been reported ^{13, 17, 18, 19, 20, 21, 22}. In this study subdural drain was successfully used in 2 patients. One case developed extradural hematoma after a subdural external drain causing deterioration of conscious level after initial clinical improvement in a 5 years old boy having a left sided ruptured temporal arachnoid cyst and managed by evacuation and insertion of subduroperitoneal shunt in same site at same session.

In spite of the fact that the success rate is higher after subduroperitoneal shunt compared to subdural drainage, it is not the more preferred mode of treatment for subdural hygroma. This is because of the complication of the abdomen due to the abdominal operation, though infection rate is similar to that of subdural drainage while obstruction is frequent, which does not occur with subdural drainage, and finally the shunt catheter needs to be removed after recovery ²². In addition, subduroperitoneal shunt cannot be performed in the presence of subdural hemorrhage because of the high risk of obstruction ²².

Nevertheless, the reason why the use subduroperitoneal shunt is being widely reported is that it may be performed when subdural drainage is ineffective, and its success rate is as high as 70~90%. With regard to the choice of unilateral or bilateral subduroperitoneal shunt in bilateral subdural hygroma, Aoki ²³ reported that the course of subdural hygromas was improved after unilateral subduroperitoneal shunt only ²⁴. In this series 9 cases (41%) were treated by unilateral subduroperitoneal shunts with no incidence of complications. Success rate more than 90%   in comparison to previous studies.

 CONCLUSION

Subdural hygroma is not uncommon disease in our community. Early diagnosis and proper management is essential for better outcome. Subduro-peritoneal shunt is an effective management of subdural hygromas and can be applied if conservation and repeated tapping fail to control symptoms.

Limitation 

Small number of patients and short term follow up.