The meninges is a membrane that envelopes the brain and the spinal cord. It is made up of 3 layers namely, the dura mater, the arachnoid space and the pia mater. The main function of the meninges is to protect the brain and the spinal cord enclosed within. It protects the brain and the spinal cord from any neurological trauma or injury. The subdural space (also known as subdural cavity or the epiarachnoid space) is the space that is potentially formed as the arachnoid mater separates from the dura mater. This occurs as a result of trauma or other pathological conditions such as subdural hematoma or subdural empyema. The subdural space is generally called as the potential or the artificial space.
An Overview of Subdural Empyema
Subdural empyema is a condition which is characterised by collection of purulent material in the subdural space between the arachnoid mater and the dura mater. It is a type of abscess caused by intracranial focal collection of pus like material. It has been noted that about 95% of the cases affects within the cranium predominantly in the frontal lobe. A minor percentage of the cases involve spinal nexus. The former type or the intracranial type is associated with inflammation of the brain and surrounding meninges, and also compression of the brain.
Subdural empyema is a life threatening infection and studies have shown that about 15-22% of all focal intracranial infection is caused by subdural empyema. The first case of subdural empyema was reported about 100 years ago. Before the discovery of penicillin, the mortality rate of subdural empyema was very high.
Symptoms of Subdural Empyema
The symptoms of subdural empyema are secondary to increased intracranial pressure, meningitis and infection. Occurrence of cerebral infarction is quite distressing. The most commonly encountered symptoms of subdural empyema include:
- Fever, nausea, vomiting, malaise, persistent headache, stiff neck, lethargy, confusion, drowsiness, papilledema, seizures, one sided weakness, abnormal sensation with numbness, blurry vision, difficulty speaking and coma.
If subdural empyema is not treated immediately, it can lead to permanent damage. It can lead to neurological complications and even death in certain cases.
Epidemiology of Subdural Empyema
In United States, about 15 – 22% of focal intracranial infections are caused by subdural empyema. Sinusitis, otitis media and mastoiditis are the most common predisposing factors. With development in antibiotic, the mortality rate has reduced significantly. In developed countries the mortality rate is about 6 – 35% with morbidity rate (neurological deficit) of about 55%. The decrease in mortality rate is also because of the advancement in investigative procedures. Subdural empyema is more common among males than in females (accounting up to 80% of the cases). The exact reason for higher incidence in men is yet unknown. The most accepted theory being that there is a difference in the anatomical development of the paranasal sinuses in male and female. It can occur at any age; however it is most commonly seen between the ages of 10 – 40 years.
Causes and Risk Factors of Subdural Empyema
Bacterial infection is the most common cause of subdural empyema followed by fungal and protozoal infection. In children, subdural empyema can be caused as a complication of meningitis. Other routes through which the infection can invade includes:
- Penetrating head injury or traumatic head injury
- Post-operative bacterial infection from the skin
- Infiltration from surrounding area such as paranasal sinus, middle ear, osteomyelitis of the skull or mastoiditis
- Dissemination from distant sites such as heart valves and lungs.
- Other infectious diseases such as tuberculosis and malaria.
Pathophysiology of Subdural Empyema
As mentioned earlier, subdural empyema is caused by focal infection between the arachnoid space and the dura mater. The infection spreads rapidly until limited by structures such as falx cerebri, base of the brain, foramen magnum, tentorium cerebelli etc. The infection usually spreads unilaterally. Over a period of time the intracranial pressure increases due to spread on infection. This also leads to intraparenchymal penetration, disruption of blood flow and CSF or cerebrospinal fluid flow which leads to cerebral oedema and hydrocephalus. There may be septic venous thrombosis of contiguous veins and thrombosis of the cortical veins or cavernous sinuses which may lead to cerebral infarction. In children, subdural empyema may be secondary to meningitis.
The infection usually enters through the frontal sinus or the ethmoid sinuses. It can also enter through the sphenoid sinus, mastoid cells and the middle ear. The condition is predisposed by sinusitis, which is followed by spread of infection into the venous sinuses through thrombophlebitis. Infection can also spread due to erosion of the posterior wall of the mastoid or frontal sinus, or directly from an intracerebral abscess. Very rarely, the source of infection is a distant foci from complication of trauma, surgery or septicaemia.
Complications of Subdural Empyema
Subdural empyema is characterized by acute afebrile condition with rapid progressive neurological manifestation. If left untreated, subdural empyema can be fatal. It can lead to coma and permanent neurological deterioration.
Diagnosis of Subdural Empyema
Diagnosis and management of subdural empyema is done by an experienced neurologist. The symptoms of subdural empyema are closely evaluated. Based on the symptoms, investigative examinations are recommended for further evaluation and planning. A contract enhanced MRI of the brain is helpful in the diagnosis of the condition. Gadolinium enhanced MRI provides better clarity. A contrast enhanced CT scan may also be considered. A comprehensive blood work is also obtained for evaluation of the condition. Blood samples and other surgical specimens are usually cultured and studied anaerobically and aerobically. Gram staining and sensitivity tests are considered too. Lumbar puncture is not very helpful in this condition and it is often avoided. Lumbar puncture can lead to transtentorial herniation. In children and infants, subdural tap is done to relieve pressure.
Treatment of Subdural Empyema
Treatment for subdural empyema involves surgical drainage followed by antibiotic coverage based on the causative pathogen. Once subdural empyema is diagnosed, immediate surgical evacuation is advised. The most common surgical procedure that is considered for management of the symptoms of subdural empyema is called craniotomy. It is a surgical procedure which involves adequate brain exposure and exploration which in turn helps in better evacuation of the purulent material. Other surgical options include stereotactic burr hole placement. In rare cases with increased risk of mortality, surgical intervention may be contraindicated. Antibiotic coverage post-surgery is generally given for a period of 3 to 6 weeks. In some cases anticonvulsants and measures to reduce intracranial pressure is required.
Other Treatment Options For Subdural Empyema Include:
- Consultation with neurologist, otolaryngologist, and thoracic surgeons
- Physical therapy, gait and balance training, speech therapy, occupational therapy etc.
- Management of residual cognitive deficit by psychologist
- Ophthalmological consultation
- Home care aides and social work.
Subdural empyema is a condition caused by infection and collection of focal purulent material in the space between the dura mater and arachnoid mater. It is most commonly predisposed by sinusitis, otitis media and mastoiditis. In children, subdural empyema may be predisposed by meningitis. The symptoms of subdural empyema are secondary to increased intracranial pressure and include increased temperature, nausea, vomiting, generalised weakness, headache etc. In adverse cases, it can lead to severe neurological issues, coma and even death. Hence, seeking immediately medical help is advised to reduce the damage to minimum.