Cracking the optometry code

Have you ever wondered what a diopter is? After my most recent trip to the optometrist, I decided to find out what an eyeglass prescription means.

Wikipedia, as usual, is a great place to start. At a high level, the “spherical” correction number indicates how much isotropic correction is needed — magnification that applies equally in all directions. The “cylindrical” correction applies to astigmatisms, which require magnification preferentially in one direction to fine-tune the spherical correction. “Axis” specifies the orientation of that cylindrical correction.

The units in which the spherical and cylindrical corrections are specified are called diopters. A diopter has a physical meaning; it is the reciprocal of the focal length in meters. So a spherical correction of -0.50 corresponds to a lens with a focal length of 2 m.

I collected my prescriptions since 1998, when I first got glasses. Now I can track my visual degradation graphically (except for an elusive 2006 prescription, which I cannot find — argh!):

Apparently my distance vision isn’t terrible (something like 20/50), but my astigmatism is what causes me blurring trouble, and it keeps getting worse. Next up: a regression analysis in which I forecast the date on which I’ll be legally blind!

Wikipedia also includes an interesting discussion of presbyopia, the gradual decline in ocular lens flexibility. This is what permits your eye to focus over a wide range of distances. Children generally can accommodate a range of more than 10 diopters, while those older than 50 can only accommodate 2. Check out this plot:

Wikipedia claims that kids can focus on something only 10 cm (~4 inches) from their eyes. Can you? Try it! (I can’t, darn!) This calculator purports to determine how much reading-glass (near-vision) correction you would need, as a function of your age; unfortunately, it only works if you’re at least 37.

As an interesting etymological note, the root “presby-” in presbyopia means “old” or “elder” (i.e., presbyopia = elder-eye), and is the same root in “priest” (“presbyter”) and “presbyterian.”

Implantable radio science

I had no idea what a broad range of topics the field of “radio science” covers. I recently attended the National Radio Science Meeting in Boulder, CO, to talk with other researchers about the latest advances in radio astronomy data analysis (e.g., hunting for pulsars). Other topics in the multiple parallel sessions included lightning detection, antenna design, remote sensing of rain, “biophotonics”, “metamaterials”, space plasmas, and “telemetry for monitoring and biosensing”. Intrigued by some of the talk titles, I attended one of the latter sessions.

One goal of this field is to develop and test low-power, efficient radio communications for implantable medical devices (IMD). One envisioned application is for people in very rural areas who don’t have regular access to a doctor. Internal sensors could monitor blood pressure and various nutrient levels, then report them to an external base station they could visually check. As one presenter imagined, “Low potassium? Push a button and find out what you should eat for the next week!”

The devices are still under development, and in the initial work they’re focusing on the ability to monitor blood pressure. They aren’t yet up to human trials. Researchers from Texas A&M and Mississippi State University described how they’d started with rats. They showed pictures of the rat surgeries needed to implant the tiny antennas and then described the experiments, which aimed to evaluate whether the simulated response from the antennas was the same as what was observed when it propagated through rat muscle, fat, and skin. Unfortunately, the presenter noted, they’d been forced to euthanize all of the rats after the tests, because they hadn’t coated the antennas with a “biocompatible material” and therefore by animal testing rules they could not let the animals live. (It seems odd to me that this oversight would not have been caught during the protocol review process!) At any rate, the results showed a not very good match between the simulated response and what they actually got, which they attributed to differences between human skin (in the simulation) and rat skin (in reality).

As a side note, I kept wondering if these tests really qualified for the “in vivo” term the presenters applied, since the rats went to sleep for the surgery and (presumably) never woke up. The point at which they were euthanized was never specified. I started wondering whether live fat/muscle/skin tissue has different dielectric properties than dead tissue, which I assume it must, since circulating blood probably affects any signal propagation. This particular experiment seemed perfectly designed to test both cases. But I wasn’t quite up to asking this question after the talk.

The next presenter (from the same group) continued on to describe their subsequent experiments with larger animals (pigs). Pig skin apparently is a much better match to human skin (insert obligatory “white meat” joke here), and they got an excellent match with their simulation. In this case, they used a proper coating and the pigs were permitted to live. The presenter also commented on how very expensive these particular bred-for-experimentation pigs are (about $10,000 each), although I had to wonder whether one must purchase an entire pig to do a radio antenna transmission test, or whether one can give it back afterwards to be used for other experiments, or possibly time-share with other researchers. But again I wasn’t actually able to ask a question, being more sort of transfixed in a rather distasteful fascination and slightly nauseated by all of the graphic surgery images!

These talks didn’t spend much time on other important IMD constraints, like where power for the wireless transmitter comes from and how to dissipate the excess heat generated without cooking the animal (or human) internally. They noted that the devices had a 25 day lifetime if in continuous use, or 1.7 months if only transmitting periodically, so I’m guessing that limit was based on some nonrenewable power source being exhausted.

Overall, the envisioned future of such devices is certainly promising—and I was kind of disappointed to see how premature such investigations apparently are (if this represents the state of the art). I would also have liked to hear more about the kind of technology used for the sensors that collect the data to be sent by the antennas!

Behind the pharmacy counter

I’m used to giving tours to friends and visitors at work, but it’s not often that *I* get to take a tour of a friend’s workplace. Last Friday, I was treated to a behind-the-scenes view of a Kaiser pharmacy (thanks, Jim!).

I spotted Jim as soon as I entered the waiting room. He was not only a head taller than everyone else working behind the counter, but he also was the only one not wearing a white lab coat. I figured his brown jacket was a clever disguise to allow him to mix unnoticed with patients in the lobby, but later learned instead that it is his way of protesting the pharmacy thermostat’s 60 F setting. It’s too cold without a jacket, and too cumbersome and restrictive to wear a lab coat on top of his jacket. But the drugs are comfortable!

Most of the other people milling around were “technicians”, who fill the prescriptions via an efficiently organized assembly line. A pharmacist has to check and confirm the result of each order, and is also required whenever the really good drugs are requested, which are in a locked cabinet. They are also the ones who answer patient questions about the drug, how often to take it, what its side effects may be, etc. A technician needs only a high school diploma, while a pharmacist must press on through college and grad school and pick up a PharmD before he or she can be hired. Even without the brown jacket, Jim’s air of authority and advanced degree-ness would have marked him as the pharmacist in charge!

I was fascinated to learn that modern pharmacies also include a station where the pharmacist (and sometimes even technicians) can mix their own drugs. This comes up when a patient needs something in a non-standard strength, or to convert a solid drug into a liquid form for a child to consume. The station contains a rack of different materials for mixing, including coal tar (yes, actual tar; it’s good for skin conditions), testosterone, and magic mouthwash (a “mucositis agent”). Now I’m wondering if Pharmacy School includes labs in which you’re tested on your ability to accurately mix new concoctions (or maybe that’s taken care of in a chemistry prerequisite). This hands-on, custom-made aspect of the job (apparently only rarely called for here) intrigues me. But no doubt in a day-to-day setting one would much prefer to be able to dole out a fixed number of pre-packed pills when possible!

It’s too bad that I don’t have Kaiser health insurance now, so I’m unlikely to be back at this particular pharmacy with any regularity… and just when I’d figured out where they stash the doughnuts, too!