The meninges and cerebrospinal fluid
Raised or decreased intracranial pressure
The circulation of cerebrospinal fluid may be obstructed so that it accumulates in the skull. This condition, called hydrocephalus, may result from congenital stenosis, or narrowing, of the aqueduct of Sylvius, tumours, meningitis, or blood accumulating within the ventricles. Accumulation of cerebrospinal fluid causes massive enlargement of the skull, degeneration of the brain, and increased intracranial pressure. Symptoms of hydrocephalus may include motor disturbances, cranial-nerve palsies, visible swelling of the head, headache, dementia, and vomiting. Surgical insertion of a tube called a shunt is necessary to drain cerebrospinal fluid from the skull.
An increase in intracranial pressure can result from any mass lesion in the head (a blood clot, tumour, or abscess, for example) or external compression of the cerebrospinal fluid pathway (such as the Arnold-Chiari malformation). The brain cannot shrink, and compensation is possible only through a reduction in the amount of fluid present in the ventricles or of blood in the vessels. Benign intracranial hypertension is a condition in which there is increased intracranial pressure caused by various metabolic and endocrine diseases; lesions and fluid obstruction are absent.
Cerebral edema is the presence of excess fluid within either the cells or the extracellular tissues of the brain. This disorder also causes brain swelling and a rise in intracranial pressure. Head injuries, encephalitis, abscesses, lack of oxygen, tumours, strokes, and toxic agents are the most common causes of cerebral edema.
Cerebrospinal fluid may leak through skull fractures or after a lumbar puncture (spinal tap). As a result of reduced fluid pressure, the brain pulls upon the meninges, causing what is called postlumbar puncture headache.
Blood clots
Blood clots lying outside or just below the dura mater (called extradural or subdural hematomas) are other complications of trauma. Extradural hematomas may be complications of fractures of the temporal bone that rupture the middle meningeal artery. Arterial blood, shed under pressure, separates the dura from the underside of the skull bone, forming a rapidly expanding mass that raises intracranial pressure, compresses the brain, and may be fatal unless evacuated surgically. Chronic subdural hematomas expand very slowly and may only be discovered because of seizures, dementia, or other neurological signs.
Meningitis
Meningitis is an inflammation of the meningeal coverings of the nervous system, with possible involvement of the brain. Viruses such as mumps, Coxsackie, and ECHO viruses, tuberculosis, fungi, spirochetes, bacteria, protozoa, and some chemical agents may cause the disease. Organisms most often reach the meninges via the blood, but direct spread may occur with skull fractures, middle-ear or nasal-sinus infections, or congenital defects of the meninges. Symptoms depend upon the infectious organism and the resistance and age of the patient, but they usually include lethargy and drowsiness, fever, headache, stiffness of the neck, vomiting, and (in smaller children) seizures. Patients with nonbacterial, or aseptic, meningitis also have fever, headache, and other meningeal signs, but they are not so obviously ill. Residual consequences of meningitis include cranial nerve palsies (especially loss of hearing), hydrocephalus, and brain damage.
Since treatment with the correct antibiotic is essential to treat meningitis, the most important diagnostic aid is lumbar puncture, the examination of the cerebrospinal fluid. The amount of protein, the number and type of cells, and the glucose level of the fluid confirm the type of meningitis.
Tumours
Tumours affecting the meninges are usually malignant and commonly spread from cerebral tumours such as medulloblastoma, from distant tumours such as carcinoma of the lung or breast, and from lymphomas and leukemia.
Benign tumours arising from the meninges are called meningiomas. These tumours occur over the convexity of the brain and on the floor of the cranium, where they compress and damage the brain or cranial nerves and may cause seizures. Meningiomas may be removed successfully.
The peripheral system
Neuropathies
Neuronal neuropathies
Neuronal neuropathies affect the axon or cell body of ventral-horn neurons or dorsal-root ganglion neurons. Damage to the ventral-horn neurons causes reduced muscle tone and power and reduction or loss of reflexes with no change in sensation. Damage to the dorsal-root ganglion neurons also causes reduced reflexes, in this case because the afferent, or sensory, limb of the reflex arc is interrupted. Loss of joint-position sense, discriminative light touch, vibration, pain, temperature, and scratch sensations may be lost or impaired. Pure sensory neuronal neuropathies may be hereditary. Other causes include toxic drugs or other agents, diabetes, herpes zoster, vitamin deficiency, and cancer. Mixed (sensorimotor) neuropathies have similar causes; in most acquired conditions they represent a late stage of a sensory disease.
Poliomyelitis
Poliomyelitis is an acute viral infection. Initially it may cause only a brief, febrile illness, but groups of ventral-horn cells of the spinal cord may be destroyed. This may later cause severe pain with further fever, occasional delirium and meningism (due to accompanying encephalitis), and a rapid onset of muscle atrophy, fasciculations, and weakness that may be localized or diffuse, mild or profound, and that may be fatal if the respiratory or bulbar muscles are involved. Only supportive treatment is available for poliomyelitis, but some recovery occurs in the majority of patients who survive the acute stage of the illness. Since the advent of immunization programs in the 1950s, this disease has been rare in the Western world.
Hereditary motor neuropathies
Hereditary motor neuropathies (also known as spinal muscular atrophies and as Werdnig-Hoffman or Kugelberg-Welander diseases) are a diverse group of genetic disorders in which signs of ventral-horn disease occur in babies or young people. The usual symptoms of muscle atrophy and weakness progress more slowly if the disease begins at a later age (5 to 15 years); at later ages the disease may also pass, leaving only chronic mild weakness and secondary skeletal deformities such as scoliosis (rotation of the spine). Less commonly, muscle weakness occurs in specific patterns—for example, involving only the facial, shoulder, or calf muscles—and the progress of the disease is much slower. Babies with these disorders may exhibit respiratory insufficiency, poor ability to suck, and severe limpness and weakness of all muscles except those of the face and eyes; the muscles of the shoulder and pelvic girdles are primarily affected. Motor development is delayed or absent. Diagnosis of hereditary motor neuropathy is confirmed by electromyography and muscle biopsy. Enzyme deficiencies are the cause of some cases of hereditary motor neuropathy, but in most cases the etiological basis of the disease is unknown.