The history and examination remain most valuable ‘tests’ in neurology, but computed tomography (CT), magnetic resonance imaging (MRI), and other non-invasive tests have revolutionized the management of patients.
look at the patient:
Examine upper limbs:
Posture of outstretched arms
Examine lower limbs:
Power (hip flexion, ankle dorsiflexion), tone
Ask the patient
Examples of helpful routine investigations.
Plain X-rays should not be done unnecessarily. Examples of diagnostically important changes are:
FRACTURES AND LESIONS OF THE VAULT OR BASE OF SKULL (e.g. metastases)
ENLARGEMENT OR DESTRUCTION OF THE SELLA TURCICA (e.g. intrasellar tumour, raised intracranial pressure)
INTRACRANIAL CALCIFICATION (e.g. tuberculoma, oligodendroglioma, wall of an aneurysm, cysticercosis) PINEAL CALCIFICATION (to show midline shift)
These show fractures and degenerative, destructive and congenital bone lesions.
This technique uses a collimated X-ray beam moving synchronously with detectors across a slice of brain between 2 mm and l3 mm thick. The transmitted Xirradiation from an element, or pixel of that slice « 1 mm”) is processed by computer and a numerical value (the Hounsfield number) is assigned to its density (air = -1000 units; water = 0; bone = +1000 units). The difference in X-ray attenuation between bone, brain and CSF makes it possible to distinguish normal and infarcted tissue, tumour, extravasated blood and oedema. Examples of normal CT scans.
The image can be enhanced with intravenous contrast media to show areas of increased blood supply and oedema more clearly. Additional information about the subarachnoid space and the cerebral ventricles is obtained by scanning after the intrathecal injection of water-soluble contrast media (e.g. metrizamide) or air. In general, lesions greater than 1 ern in diameter can be visualized on CT scans.
The method is safe (apart from occasional systemic reactions to contrast); the irradiation involved is small.
CT scanning is used for the diagnosis of:
• Cerebral tumours
• Intracerebral haemorrhage and infarction
• Subdural and extradural haematoma
• Subarachnoid haemorrhage
• Lateral shift of midline structures and displacement of the ventricular system
• Cerebral atrophy
• Pituitary lesions
• Spinal lesions (with CT myelography)
The CT scan can also be used to show that a brain is anatomically normal with a high degree of accuracy. Limitations
LESIONS UNDER 1 CM IN DIAMETER may be missed. LESIONS WITH ATTENUATION CLOSE TO THAT OF BONE may be missed if they are near the skull.
LESIONS WITH ATTENUATION SIMILAR TO THAT OF BRAIN may be difficult to diagnose (e.g. ‘isodense’ subdural haematoma).
THE RESULTS ARE POOR WHEN THE PATIENT CANNOT COOPERATE-a general anaesthetic may occasionally be necessary.
Magnetic resonance imaging
This technique makes use of the properties of protons aligned in a strong magnetic field. The protons are bombarded with radio frequency waves at right angles to generate images. The equipment is expensive and still restricted to specialized units. MRI scanning can distinguish between white matter and grey matter (Fig. 18.2) in the brain. Brain tumours, syringomyelia, the lesions of multiple sclerosis (MS) and lesions in the posterior fossa and at the foramen magnum are demonstrated well. In the spinal cord the technique can visualize tumours, cord compression and vascular malformations and is replacing myelography. Cerebral angiography and digital
This demonstrates the cerebral arterial and venous systems. Contrast is injected intra-arterially or intravenously. Carotid and vertebral arteriography is used for the demonstration of aneurysms, arteriovenous malformations and venous occlusion. Films of the aortic arch and the carotid and vertebral arteries demonstrate occlusion, stenoses and atheromatous plaques. Spinal angiography is used to investigate arteriovenous malformations of the cord.
Conventional arteriography is invasive and requires a general anaesthetic; it should rarely be performed outside a specialist centre. It carries a mortality of around 1% and a 1% risk of stroke.
Digital subtraction angiography (DSA), using a computerized subtraction technique is superseding traditional angiography. Contrast is injected intravenously or intraarterially.
No anaesthetic is necessary.
A water-soluble radiopaque dye is injected into the lumbar (or rarely cervical) subarachnoid space and viewed by conventional X-rays or CT. This is used in the diagnosis of tumours of the spinal cord and other causes of cord compression.
Radiculography is an examination confined to the lumbar region to show the anatomy of nerve roots. Isotope brain and bone scanning A radioisotope, usually [99ffiTc]pertechnate is injected intravenously to detect:
• Vascular tumours
• Arteriovenous malformations
• Cerebral infarcts
• Subdural haematoma
Isotope brain scanning is safe, non-invasive and cheap but has largely been overtaken by CT scanning because of the high incidence of false-negative isotope scans. Isotope bone scanning is useful for detecting vertebral lesions (e.g. metastases).
The electroencephalogram (EEG) is recorded from scalp electrodes on 16 channels simultaneously for 10-30 min The main value of the EEG is in the diagnosis of epilepsy and diffuse brain diseases.
Epilepsy Spikes, or spike and wave abnormalities occur, but it should be emphasized that patients with epilepsy may have a normal EEG between fits.
Diffuse brain disorders
Slow-wave EEG abnormalities are seen in encephalitis, dementia and metabolic states (e.g. hypoglycaemia and hepatic coma).
The EEG is isoelectric (i.e. flat). An EEG is no longer necessary to confirm the diagnosis of brain death in the UK.
Fig. 18.2 Multiple areas of high signal on T2 weighted images in the peri-ventricular white matter in a patient with multiple sclerosis.
Electromyography and nerve conduction studies.
A concentric needle electrode is inserted into voluntarymuscle. The amplified recording is viewed on an oscilloscope and heard through a speaker. The following can0 be demonstrated:
• Normal interference pattern
• Denervation and reinnervation
• Myopathic, myotonic or myasthenic changes
Peripheral nerve conduction
Four measurements are of principal value in the diagnosis of neuropathies and nerve entrapment:
1 Mean conduction velocity (motor and sensory)
2 Distal motor latency
3 Sensory action potentials
4 Muscle action potentials
Visual-evoked potentials record the time taken for the response to a retinal stimulus to travel to the occipital cortex. Their value is chiefly in documenting previous retrobulbar neuritis, which causes a permanent delay in the latency despite clinical recovery of vision.
Similar techniques exist for the measurement of auditory and somatosensory potentials (from an arm or leg). Lumbar puncture Examination of the CSF The indications for lumbar puncture are: DIAGNOSIS OF MENINGITIS AND ENCEPHALITIS DIAGNOSIS OF MS AND NEUROSYPHILIS INTRATHECAL INJECTION OF CONTRAST MEDIA AND DRUGS
DIAGNOSIS OF SUSPECTED SUBARACHNOID HAEMORRHAGE
MEASUREMENT OF CSF PRESSURE (e.g. in benign intracranial hypertension
REMOVAL OF CSF THERAPEUTICALLY (e.g. in benign intracranial hypertension)
DIAGNOSIS OF MISCELLANEOUS CONDITIONS (e.g. certain polyneuropathies, sarcoidosis, intrathecal neoplastic involvement)
Biopsy is useful in the diagnosis of inflammatory and dystrophic disorders of muscle.
Biopsy, usually of the sural nerve, is carried out to aid diagnosis in certain polyneuropathies, e.g. due to vasculitides.
Brain biopsy (e.g. of a non-dominant frontal lobe) is undertaken to diagnose inflammatory and degenerative brain diseases. CT -guided stereotactic biopsy of intracranial mass lesions is being used increasingly. It is less traumatic and more accurate than conventional biopsy through a skull burr hole or craniotomy.
Psychometric assessment Formal psychometric testing is used to assess intellectual function. Preservation of the verbal IQ (a measure of past attainments) in the presence of deterioration of the performance IQ (a measure of present abilities) is a useful indicator of dementia. Low subtest scores (e.g. for block design, speech or constructional skills) indicate impaired function of specific regions of the brain. The main limitation of these techniques is that depression and lack of concentration can also impair scores.
Certain specialized tests are employed in the diagnosis of individual (and often rare) neurological diseases. Examples are:
SERUM ENZYMES LIBERATED FROM MuscLE-greatly raised in many primary muscle diseases.
Creatine phosphokinase is the enzyme usually assayed in most laboratories.
SERUM COPPER AND CAERULOPLASMIN in Wilson’s disease .
ANTIBODIES TO ACETYLCHOLINE RECEPTOR PROTEIN in myasthenia gravis.
Lumbar puncture should not be performed in the presence of raised intracranial pressure or when an intracranial mass lesion is a possibility.
The patient is placed on the edge of the bed in the left lateral position with the knees and chin as close together as possible
The third and fourth lumbar spines are marked. The fourth lumbar spine usually lies on a line joining the iliac crests Using sterile precautions, 2% lignocaine is injected into the dermis by raising a bleb in either the third or fourth lumbar interspace
The special lumbar puncture needle is pushed through the skin in the midline. It is pressed steadily forwards and slightly towards the head When the needle is felt to penetrate the dura mater, the stylet is withdrawn and a few drops of (SF are allowed to escape The (SF pressure can now be measured by connecting a manometer to the needle. The patient’S head must be on the same level as the sacrum. Normal (SF pressure is 60-150 mmH20. It rises and falls with respiration and the heart beat Specimens of (SF are collected in three sterilized test tubes and sent to the laboratory. An additional sample in which the sugar level can be measured, together with a simultaneous blood sample for blood sugar measurement, should be taken when relevant (e.g. in meningitis). Patients are usually asked to lie flat after the procedure to avoid a headache that may develop but this is probably of little value.
Analgesics may be required
Contraindications for lumbar puncture
Suspicion of a mass lesion in the brain or spinal cord. Caudal herniation of the cerebellar tonsils (‘coning’) may occur if an intracranial massis present and the pressure below is reduced by removal of (SF. This is extremely dangerous.
Any cause of raised intracranial pressure
Local infection near the site of puncture
Congenital lesions in the lumbosacral region (e.g. meningomyelocele). Platelet count below 40 x 10’/litre and other clotting abnormalities, including anticoagulant drugs Unconscious patients and those with papilloedema must have a (T scan before lumbar puncture These contraindications are relative, i.e. there are circumstances when lumbar puncture is carried out in spite of them.
The composition of the normal.