Chapter 25 Infections of the central nervous and locomotor systems
As the cerebrospinal fluid is devoid of effective antimicrobial defences, generalized infection rapidly sets in when pyogenic organisms enter the subarachnoid space and the cerebrospinal fluid. This may be caused by:
Bacterial meningitis is more severe than the viral type and remains a serious cause of morbidity and mortality despite antibiotic therapy. Prompt diagnosis is of the essence in preventing disabling sequelae of infection and death.
Symptoms include severe headache, fever, vomiting, photophobia and convulsions leading to drowsiness and unconsciousness. Signs are mainly those of meningeal irritation, i.e. neck and spinal stiffness, and Kernig’s sign (pain and resistance on extending the knee when the thigh is flexed). These cardinal signs and symptoms may be absent in neonatal meningitis and meningitis in the elderly and the immunocompromised. Sequelae include encephalopathy (altered cerebral function), cranial nerve palsies, cerebral abscess, obstructive hydrocephalus and subdural effusion of sterile or infected fluid.
N. meningitidis (the meningococcus) is the main agent of meningitis in the UK and USA and most infections are caused by group B strains. The disease is common in children and young adults. Penicillin is the drug of choice: cefotaxime and chloramphenicol are alternatives. Haemophilus meningitis is mostly seen in children between 1 month and 4 years old and is treated with chloramphenicol or cefotaxime. Pneumococcal infection, common in older patients and those without a functioning spleen, is treated with penicillin. Tuberculous infection is managed by ‘triple therapy’, as described in Chapter 23.
Meningitis may spread quickly in close household contacts. Avoiding overcrowding in living and working conditions is helpful. Chemoprophylaxis with antibiotics (e.g. rifampicin) in meningococcal infection can eliminate the carrier state, which may develop in some.
Examination of the cerebrospinal fluid, usually obtained by a lumbar puncture, is essential. Changes that occur in the cerebrospinal fluid, depending on whether the aetiology is acute pyogenic, tuberculous or viral, dictate appropriate and timely therapy (Table 25.1). Cerebrospinal fluid should also be centrifuged and the deposit Gram-stained and cultured to isolate and identify the causative agent. Blood cultures are also useful in the diagnosis of bacterial meningitis.
The major routes of viral entry into the body are the respiratory and gastrointestinal tracts. From these portals, they spread to the central nervous system by direct migration via the olfactory nerves or indirectly via blood. Cells involved in viral spread include capillary endothelial cells, epithelial cells of the choroid plexus and infected leukocytes.
Infection of the brain substance (as opposed to the meninges) is called encephalitis. This is a somewhat artificial division as patients often show signs and symptoms of meningitis and encephalitis at the same time.
Encephalitis occurs after childhood illnesses such as measles, chickenpox and rubella, and rarely after immunization with vaccines such as pertussis. Affected patients often die or have debilitating sequelae.
The portal of infection is the mouth, and the virus multiplies in the lymphoid tissue of the pharynx and the intestine. It then enters the blood stream and causes a viraemia, with resulting spread into the central nervous system, causing neurological disease. The disease is an influenza-like illness, with meningitis and encephalitis. In some, damage to the anterior horn cells of the spinal cord leads to respiratory failure (requiring artificial ventilation) or permanent lower motor neuron weakness and paralytic poliomyelitis.
Although epidemics of poliomyelitis were common in the past, it is now rare in the West, owing to effective polio vaccine. However, the disease is still prevalent in developing countries, where universal vaccination programmes are difficult to implement, despite the goal of the World Health Organization to eradicate the disease by the year 2000. The polio vaccines are of two types: the killed (Salk) vaccine and the live attenuated (Sabin) vaccine (Chapter 37).
Many bacteria may cause brain abscesses. These include streptococci (Streptococcus milleri and Streptococcus pneumoniae) enterococci (Enterococcus faecalis), staphylococci, anaerobic cocci and coliforms. The infections are mostly polymicrobial in nature (i.e. mixed infections).
The infective agent may reach the brain in the blood or by direct extension. In the latter case, a brain abscess may result as a direct extension of sinus infection caused by oral bacteria or, rarely, as a complication of acute or chronic dental infection. Infection may also follow traumatic injury to the maxillofacial region.