A 53-year-old man starts to experience bifrontal headaches. The headaches are relieved by aspirin, but they recur periodically over the next two weeks. The man and his wife begin to wonder what is going on because he has no previous history of persistent headaches. As a matter of fact, his health has been very good. He has never smoked, drinks only in moderation, and is not taking any medications. Nevertheless, the man's periodic headaches persist for another week, at which point he suddenly starts to have focal seizures with involuntary movement of the right side of his face and arm. His wife rushes him to the ER where, while awaiting treatment, he suffers a generalized seizure that is controlled with intravenous diazepam, phenytoin, and phenobarbital. The man's vital signs are T = 37ºC, P = 110, R = 18, and BP = 140/80 mm Hg. On physical examination, he is sleepy and has a decreased attention span. He can move all of his extremities, although he moves his right arm less than his left. His left optic disk is slightly blurred, but there are no other remarkable physical findings. The ER physicians ask the man's wife about his medical history. She says that, aside from the headaches, his only recent complaint was a painfully sore tooth that resulted in an extraction and bridge work about five weeks ago.

Question 5.1: What is the differential?

The blurring of the left optic disk implies increased intracranial pressure, which can be caused by a wide range of problems. The combination of persistent headaches followed by focal seizures is suggestive of a parameningeal infection, such as a subdural empyema, brain abscess, epidural abscess, or thrombophlebitis of the major dural venous sinuses and cortical veins. The history of a recent dental procedure might also support one of these diagnoses. Other possible causes of the patient's condition might include viral encephalitis, parasitic infections (e.g., toxoplasmosis and cysticercosis), acute hemorrhagic leukoencephalitis, a cerebral infarction, neoplasm, or a chronic subdural hematoma. More information is needed to make a definitive diagnosis in this case.

Question 5.2: What tests should you do?

A lumbar puncture is contraindicated because of the likelihood of increased intracranial pressure. Therefore, the diagnosis must be based on the physical examination, imaging studies, and laboratory tests. The ER physicians working on this case ordered a CBC with differential, a urinalysis, serum chemistries, blood cultures, an ECG, chest x-rays, and a CT and MRI scan of the head.

Test Results:

All of the lab tests and the chest x-rays were normal. The blood cultures were negative. The CT and MRI scans showed a 1.5-cm localized ring-enhancing lesion in the left parietal hemisphere.

Question 5.3: What is your diagnosis?

The MRI suggests a brain abscess, a focal suppurative lesion within the parenchyma of the brain, with ring enhancement and adjacent edema, that is almost always caused by a bacterium. Moreover, the patient's symptoms are generally consistent with this diagnosis. The most common symptom of a brain abscess (70% of cases) is a dull, aching, poorly localized headache. Focal neurologic symptoms depend primarily on the location of the lesion and develop in 30-50% of cases. Fever is seen about 50% of cases, whereas papilledema appears in only 25% of cases. WBC counts are usually normal, and blood cultures are positive in only 10% of cases.

Question 5.4: How should you treat this case?

The diagnosis of a brain abscess generally justifies immediate intervention. Treatment options include: antibiotic therapy alone for multiple, small, and/or deep-seated abscesses or when the general condition of the patient does not allow surgery; abscess aspiration (usually by stereotaxic surgery); or open craniotomy and evacuation of the abscess cavity or excision of the abscess with the capsule. In the present case, the patient underwent a surgical procedure in which the lesion was biopsied and then completely removed. Pathologic examination of the tissue suggested that the lesion was several weeks old. Long-term anticonvulsant therapy was started following surgery, and the patient was treated with antibiotics for two weeks. He had no additional seizures or neurologic deficits. One year later, the anticonvulsant drugs were discontinued and a follow-up CT scan was negative.

Question 5.5: What was the source of the infection?

Brain abscesses can develop in three ways: (1) direct spread from a contiguous cranial infection (e.g., paranasal sinusitis, otitis media, mastoiditis, or dental infection), (2) following head trauma or a neurosurgical procedure, or (3) hematogenous spread from a remote site of infection. In this case, the dental infection that led to an extraction was probably the source of the brain infection.

Question 5.6: What is the likely causative agent?

Streptococci, Prevotella spp., and Porphyromonas spp. are the most common causes of brain abscesses related to dental infections. In this case, the laboratory isolated Prevotella melaninogenica and Streptococcus anginosus when they cultured necrotic material from the brain lesion. The most frequent causes of brain abscesses are determined primarily by the source of infection. For example, those related to paranasal sinusitis are most often caused by microaerophilic and anaerobic streptococci (mixed infections are common), Haemophilus spp., Bacteroides spp. (except B. fragilis), and Fusobacterium spp. (For information on agents associated with other sources of infection, see the general CNS Handout for BMS 6302.)

It is important to realize that an increasingly significant percentage of brain abscesses are caused not by classic pyogenic bacteria (like those named above), but rather by Nocardia braziliensis and mycobacterial spp., and by fungi (mostly Aspergillus spp. and Cryptococcus neoformans) and Toxoplasma gondii (a protozoan). This distribution pattern reflects the importance of brain abscesses in hosts who are immunocompromised as a result of HIV infection, organ transplantation, etc.