N E U ROLOG Y
The key to a good d ifferential diagnosis is to start with a broad , all-inclusive
differential based on the major points of the case and narrow it down logically
to a smaller, "working" differential based on the specifics of the case. If the patient's
signs/symptoms become atypica l , or new information becomes ava ilable,
you can go back to the broad differential; in this way, you will not miss
u nusual presentations of disease . Consider a 50-year-old man with hypertension,
diabetes, and hyperlipidemia presenti ng with stroke: the expanded differential
should i nclude all causes of stroke, but the working d ifferential features
atheroth rombotic disease, hemorrhage, a nd embolism as l i kely etiologies. If
the CT scan shows a mass lesion, then you'd better return to the broad differential
to include tumor and brain abscess. If the patient develops a fever and is
found to have a sed i mentation rate of 1 00, then you should return to the expanded
differential and retrieve vasculitis and infection to add to your working
differential.
Without a complete initial differential diagnosis, or a return to .the initial differential
when a typical case becomes atypical, you will miss diagnoses.
General Considerations
H ow to M a ke a Broad D ifferential Diagnosis
Despite the stereotype of neurology as a mysterious and arcane "black box , "
there are several effective methods to help any physician make a complete differential
diagnosis for neurological disease.
Differential by Etiology
The MEDIC I N E DOC mnemonic is a useful sta rting place to develop a
complete neurologic differential diagnosis. To review:
Neurology • 203
b
Metabolic disease (e . g . , metabolic encephalopa thy, leukodystrophy,
Wilson's disease)
E ndocrine d isease (e.g . , diabetic neuropathy, myxedema como, hypoglycemic
seizures)
Drugs/medicines (e.g . , iatrogenic, occidental , self-administered)
Infections (e.g . , meningitis, herpes encephalitis, HIV dementia, neurosyphilis)
Congen ital abnormalities (e.g . , spino bifida, Chiari malformations, muscular
dystrophy)
Immunologic disease (e. g . , vasculitis, myasthenia gravis)
Neoplasms (e.g , primary tumors, metastatic disease)
Exotic ( "stra nge" diseases of u ncertai n etiology, e . g . , m ultiple sclerosis,
Guillain-Barre)
Degenerative processes (e. g . , Alzheimer's, Parkinson's)
Occupational exposures (e.g . , environmental or occupational toxins, trauma)
Cardiovascular (e. g . , infarction, hemorrhage, embolism, aneurysm, arteriovenous
malformation)
Diffe re n ti a l by Anatomy
A detailed understanding of neuroanotomy, although useful in precisely localizing
lesions, pinpointing d iagnoses, and i mpress i n g/bori ng colleagues
during rounds, is not required to make basic differentials. The simplest method is
to sta rt at the m uscle and work bock anatom ica lly to the cerebral cortex.
Examples of on anatomic differential for "weakness" are in parentheses:
1 . Muscle ( polymyositis)
2 . Neuromuscular j unction ( myasthenia gravis)
3 . Peripheral nerve (Guillain-Barre)
4. Nerve plexus (brachial amyotrophy)
5 . Nerve root (disc herniation)
6. Meninges/subarachnoid space (arachnoiditis)
7. Spinal cord (spinal cord tumor)
8. Brainstem ( pontine infarction)
9. Subcortical structures - basal ganglia, thalamus (lacunar infarction - internal
capsule)
1 0. Cortical structu res - cerebr u m , cerebellum ( m iddle cerebral a rtery
i nfarction)
H ow to M a ke a " Worki n g " D iffe rential Diagn osis
Essential in the development of a useful working differential diagnosis is taking
clinical characteristics from the case in point and using them to narrow down the
broad differential . Herein lies the a rt of d iagnosis: how do you know when a
history of alcohol abuse is helpful in making the diagnosis of alcohol withdrawal
204 . Neurology
seizure, or simply a " red herring" distracting you from the correct diagnosis of
meningitis? If things don't fit, go bock to the brood differential Make sure nothing
has been overlooked .
Differential by Time Course
In many diseases, and particularly in neurologic diseases, the time course of
symptomatology is critical to intelligently narrowing down the differential. Different
etiologies are suggested by d ifferent time courses . I ntermittent sym ptoms with
complete resolution between episodes invoke a different set of diagnoses than
chronic, gradually progressive symptoms. A history of headache with nausea
and vom iting might be due to migraine in a patient with the first time course, but
the second course is more consistent with a brain tumor. It also is possible for a
history of episod ic symptoms to be gradually progressive (such as crescendo
transient ischemic attacks). Take a careful history of the timing of the symptoms.
A good history of the time course includes not only whether the symptoms
are i ntermittent or conti nuous, but also the following i nformation: whether the
symptoms were maximal at onset, rapidly progressive, or progressing in