Relying on Tests to Diagnose MS, Part 1
Why are doctors so heavily reliant on MRI and lab tests to diagnosis MS?
During a recent online chat about MS, one person asked the question: “why are doctors so heavily reliant on MRI and lab tests to diagnosis MS?” This is a good question. In order to answer it, let me briefly review the criteria for diagnosing MS. A diagnosis of MS can be made in a patient who has two episodes of neurological dysfunction that occur in different places in the central nervous system at different times, and there must be objective evidence of an abnormality on the neurological exam. In early or equivocal cases, the MRI and lumbar puncture can provide enough additional evidence to make a diagnosis of MS. In many individuals, making the diagnosis of MS can be quite straightforward. In patients who present with characteristic ocular findings and a typical MRI, for example, the diagnosis of MS is not difficult to make.
However, there are a large number of patients in whom the diagnosis of MS is not as clear based on either their clinical presentation or the MRI findings. Clinically, MS can present with a wide variety of symptoms. According to a survey of 300 patients with MS in England, there were a total of 19 symptoms that at least one patient experienced that led to a diagnosis of MS. It is certainly the case that some symptoms were much more common than others. Sensory complaints, weakness, incoordination (termed ataxia), and visual complaints were the initial symptoms in almost 90% of patients. However, while these symptoms are common in MS, they are by no means specific to MS. Symptoms such as weakness, tingling, and visual disturbance can occur for numerous reasons, including peripheral nerve disease, muscle disease, diseases of the eye, rheumatological disease, psychiatric conditions and many other diseases of the central nervous system.
It is for this reason that doctors are so reliant on the MRI and spinal tap results to make a diagnosis of MS, especially if the history and physical exam are in any way equivocal. A lumbar puncture will show an abnormality, termed oligoclonal bands, in about 90% of patients with MS. So while 10% of MS patients may have a normal lumbar puncture, this is a powerful test for ruling out MS. Additionally, in almost all patients with MS, the MRI will reveal characteristic lesions in the white matter of the brain and spinal cord. (For further details on this, please see my previous bog posts on the use of the MRI in MS.)
The clinical exam combined with the lumbar puncture results and MRI form a powerful combination for ruling out MS. A study in 2008 examined 143 subjects at an MS clinic in Detroit all of whom had a normal neurological exam, a normal or nonspecific brain MRI, and a normal spinal tap. 109 of these patients were seen for a second evaluation an average of 4 years later, and none of these patients had developed MS. Patients and doctors can rest assured that with a normal exam, MRI and lumbar puncture, the likelihood that patient has MS is extremely low.