Multiple sclerosis: making the diagnosis

So let us start from where we left off. Just how do we (doctors) go about making (confirming) the diagnosis of MS.

MRI scan: Well one of the test most commonly requested (infact done in nearly every patient) is a MRI scan of the brain and at times of the cervical spinal cord. What are we looking for you may ask? Well multiple sclerosis on the MRI is characterized by plaques (lesions) which are disseminated in space and in time. What does that mean? In a typical patient of MS, the MRI scan shall show evidence of disease activity which is scattered around in different parts of the brain. Meaning there are MS lesions seen in different parts of the brain white matter (typically MS is a white matter disease though recent research indicates involvement of the grey matter too). So for example a typical MRI scan shall show plaques scattered  in the white matter of the frontal lobe, parietal, temporal lobe, cerebellum and so forth. Moreover the MRI scan shall indicate that these plaques are of different age (which indicates that the disease has been present for sometime now). Remember what I said –relapsing and remitting MS. Sometimes to help secure the diagnosis, your doctor shall also order a MRI scan of the spine most commonly cervical spine. The intention is the same and that is to see evidence of dissemination of the disease process in the brain and spinal cord.

Spinal tap: a lumbar puncture is usually carried out. Does every patient need a spinal tap to help secure the diagnosis of MS? No. Remember the diagnosis of MS can be made clinically in some patients. In patients where the characteristic history is not forthcoming and in whom the MRI scan does not prove helpful (does not evidence of dissemination of disease process in space and time), a spinal tap may be warranted.  The spinal fluid of MS patients is analyzed for certain proteins which suggest evidence of disease process. These include myelin basic protein (MBP), oligoclonal bands (OCBs) and IgG index.

Other tests: these tests may be requested in special circumstances (usually when the diagnosis remains elusive inspite of MRI scans and spinal tap).

1) Visual evoked potential (VEP), brainstem auditory evoked potential (BAEP) and somatosensory evoked potentials (SSEP):  these tests usually involve testing the integrity of different pathways in the brain. VEP tests the visual pathway from the eye to the occipital (visual) cortex, BAEP–tests the brainstem auditory pathways while SSEP check for the integrity of the white matter tracts carrying somatosensory information (vibration, joint sense and position sense) from the periphery (arm or leg) to the somatosensory cortex.  MS lesions involving any of these pathways cause a delay in the rate of conduction of nerve impulse and provide ancillary evidence of involvement of white matter tracts of the brain by a demyelinating disease process.

I hope these two posts help you all in understanding how the diagnosis of MS is made.

 

Nitin Sethi, MD

Normal pressure hydrocephalus (NPH)

Normal pressure hydrocephalus (NPH) is another potentially reversible cause of “dementia” or rather memory problems in the elderly. NPH is characterized by the triad of gait disturbance, urinary incontinence and memory problems. The dementia in NPH is of the subcortical type meaning that it is mostly characterized by psychomotor retardation (patients are slow to respond), unlike cortical dementias like Alzheimers disease they do not have language deficits (aphasia), inability to do learned things (apraxia) or agnosia.

The etiology of NPH is still not fully elucidated but it is thought to involve some obstruction to the normal flow of the cerebrospinal fluid (CSF). This obstruction may be due to previous history of meningitis or CNS trauma. When you image these patients with either a CT scan or a MRI scan, one finds the ventricles are dilated (large enlarged ventricles) but there is not much corresponding cortical (brain0 atrophy. This is against what is found in cortical dementias like Alzheimers disease where apart from the ventricles been dilated and enlarged, the brain also is shrunken (atrophy).

So in which patients should the diagnosis of NPH be entertained? Mostly these are patients who are elderly and who have had a subacute onset of memory problems accompanied by difficulty in walking (patients who have NPH have a characteristic gait (they walk slowly and stiffly, we call their gait as magnetic gait). They may or may not have urinary incontinence (the entire triad may not be present in all the patients). Neurologists entertain the diagnosis of NPH if they see a patient with the above symptoms and if the imaging is characteristic.

To confirm the diagnosis though requires further testing. Normally what we do is a therapeutic as well as diagnostic spinal tap. What does that mean you may ask?

Well we bring the patient into the hospital and do a spinal tap. Before the tap is done the patient is examined to determine the memory deficits. A timed walking test is carried out ( we make the patient walk a fixed distance and measure the time taken to do so ). Then a good amount of spinal fluid is removed about 20-30 cc. The pressure of the spinal fluid is measured at that time and in a typical patient of NPH it should be normal (hence the name normal pressure hydrocephalus). Once the spinal fluid has been removed the patient is again tested. Has the memory improved. We make the patient walk the same distance as before. If now the walk is much faster and steadier, then we document a positive reponse to large volume CSF removal. Our diagnosis of NPH is now strengthened and there has also been a therapeutic response to the procedure ( remember I told you the test is both diagnostic and therapeutic).

Before I usually subject the patient to a surgical option for more definitive treatment, I usually like to repeat the above test at least two more times. If there is a consistent positive response to large volume CSF removal, then I feel confident in going ahead and asking the neurosurgeon to place a shunt.  What is a shunt? Well a shunt is a device which as the name suggests shunts the spinal fluid from the brain into the peritoneal cavitiy (the gut). It has a valve which can be set to open at a particular pressure. So whenever the CSF pressure rises above that pressure, the valve shall open and the extra spinal fluid shall be shunted from the brain into the gut.

Simple device but does have its own risk of complications. Shunts can get infected, they can get dislodged from the brain and start migrating, they may get obstructed and have to be replaced etc. Hence before I advise putting any sort of hardware into the brain, I try to be as sure as possible that my patient indeed does have NPH and not a cortical dementia like Alzheimers or Parkinsons disease (as these do not respond to shunt placement).

It is Friday the 20th, I am off home on my vacation but shall be keeping the blog active. Please do contact me if you have any questions or want me to discuss something particular.

Personal Regards,

Nitin Sethi, MD