This latin maxim translates to “Plurality should not be posited without necessity.” William of Ockham (1285-1349) was a controversial 14th-century philosopher often credited with the supposition, “Things should not be multiplied unnecessarily,” although many experts take exception and argue that this is not exactly how he originally stated what is now called the law of parsimony or “Ockham’s Razor” (sometimes spelled Occam’s Razor). Regardless of origin or intent, the general rule that the simplest explanation tends to be the best (or most likely) serves us well in clinical neurology. For instance, if a person is weak on the entire left side of their body (face, arm, and leg), it is most likely related to a single “lesion.” You know from basic neuroanatomy that this problem would likely be located in the right cerebral hemisphere. Even Ockham himself (with limited knowledge of neuroanatomy) would agree that this pattern could not realistically be attributed to an abnormality of every nerve or root on the left side of the body (without any on the right being affected). Similarly, it is your basic understanding of neuroanatomy that will allow you to postulate which area of the nervous system is most likely affected given the patient’s presenting signs and symptoms. Remember to KEEP IT SIMPLE! Assume from the start that the simplest explanation is the most likely (i.e., The law of parsimony or Ockham’s Razor).

If after proper use of this four-step process the patient’s diagnosis remains elusive, then you may assume the problem is more complex than first hypothesized (e.g., possibly involving several different levels of the NeurAxis). However, you should still rely on the patient’s signs and symptoms to direct you to the next most likely level(s) of the NeurAxis for appropriate testing.

Students of neuroanatomy will frequently hear this throughout their studies. The question may sound intimidating, but it doesn’t have to be. Start by assuming that there is a simple explanation for the constellation of findings (remember the law of parsimony?). In clinical neurology, arriving at the correct diagnosis usually depends on first simply “bracketing” the problem to two or three potential levels based on the presenting information. These initial possibilities can then be narrowed down further with more focused questioning, a good neurologic examination, and then laboratory testing if indicated. The column of axis levels (see chart) consists of ten functionally separate areas. A distinct pattern of symptoms and abnormal signs is expected when damage or dysfunction occurs at any particular level. Keeping the chart in mind during the evaluation will help you conceptualize the nervous system as a set of interrelated levels instead of an overwhelming list of unrelated diseases. Because some overlap exists between the levels, your initial impression may include several potential areas of involvement and will quickly evolve as additional information becomes available. Your focused history and examination will extract the necessary additional information, leading you to an accurate final impression. The signs and symptoms listed across the top of the chart are a very good place to start (but, of course, are not exhaustive). They consist mainly of the more discriminating and/or common ones encountered and should provide a sense of direction (guide) when collecting and compiling clinical data. The remainder of this chapter covers salient issues related first to each of the NeurAxis levels, and then to the individual signs and symptoms.

The NeurAxis levels are listed as follows: