Exophthalmometer

An Exophthalmometer is an instrument used for measuring the degree of forward displacement of the eye in exophthalmos. The device allows measurement of the forward distance of the lateral orbital rim to the front of the cornea.[1] Exophthalmometers can also identify enophthalmos (retraction of the eye into the orbit), a sign of blow-out fracture or certain neoplasms.

An Exophthalmometer

Methods

There are several types of exophthalmometers: Hertel and Luedde exophthalmometers measure the distance of the corneal apex from the level of the lateral orbital rim, while Naugle exophthalmometers measure the relative difference between each eye.[2]

  • Hertel exophthalmometers take a measurement from the lateral orbital rim to the corneal apex. If a patient presents with an orbital fracture or after lateral orbitotomy, the use of a Hertel exophthalmometer may be complicated because the lateral orbital rim serves as a reference point for this instrument. Consideration should be given to the use of the Naugle exophthalmometer in these cases.
  • Naugle exophthalmometers use fixation points slightly above and below the superior and inferior orbital rims (cheek bones and forehead). Naugle exophthalmometers measure the difference in proptosis between the two eyes, in contrast to the absolute measure obtained with the Hertel method.
  • Luedde exophthalmometers fix on the lateral orbital wall and use a transparent ruler to measure the amount of protrusion.

Normal values

The normal range is 12–21 mm. Upper normal limit for people of African origin is a little higher, about 23–24 mm.[3]

A difference greater than 2 mm between the eyes is significant.

In children and teenagers mean exophthalmometric measurements increase with age: Less than 4 years old (13.2 mm), 5–8 years old (14.4 mm), 9–12 years old (15.2 mm) and 13–17 years old (16.2 mm).[4]

Axial Length of the eye affects exophthalmometer reading. Pseudoproptosis may be seen in severe myopia.[5]

gollark: Git stands for GIT Is Tremendous.
gollark: The stages of git clone are: Receive a "pack" file of all the objects in the repo database Create an index file for the received pack Check out the head revision (for a non-bare repo, obviously)"Resolving deltas" is the message shown for the second stage, indexing the pack file ("git index-pack").Pack files do not have the actual object IDs in them, only the object content. So to determine what the object IDs are, git has to do a decompress+SHA1 of each object in the pack to produce the object ID, which is then written into the index file.An object in a pack file may be stored as a delta i.e. a sequence of changes to make to some other object. In this case, git needs to retrieve the base object, apply the commands and SHA1 the result. The base object itself might have to be derived by applying a sequence of delta commands. (Even though in the case of a clone, the base object will have been encountered already, there is a limit to how many manufactured objects are cached in memory).In summary, the "resolving deltas" stage involves decompressing and checksumming the entire repo database, which not surprisingly takes quite a long time. Presumably decompressing and calculating SHA1s actually takes more time than applying the delta commands.In the case of a subsequent fetch, the received pack file may contain references (as delta object bases) to other objects that the receiving git is expected to already have. In this case, the receiving git actually rewrites the received pack file to include any such referenced objects, so that any stored pack file is self-sufficient. This might be where the message "resolving deltas" originated.
gollark: UPDATE: this is wrong.
gollark: > Git uses delta encoding to store some of the objects in packfiles. However, you don't want to have to play back every single change ever on a given file in order to get the current version, so Git also has occasional snapshots of the file contents stored as well. "Resolving deltas" is the step that deals with making sure all of that stays consistent.
gollark: A lot?

References
  1. "Exophthalmometer in TheFreeDictionary". Farlex. Retrieved 13 April 2013.
  2. Onofrey, Bruce E.; Leonid Skorin, Jr.; Nicky R. Holdeman. Ocular therapeutics handbook : a clinical manual (Third ed.). Wolters Kluwer/Lippincott Williams & Wilkins. pp. 71–72. ISBN 1605479527.
  3. de Juan E, Jr; Hurley, DP; Sapira, JD (Sep 1980). "Racial differences in normal values of proptosis". Archives of Internal Medicine. 140 (9): 1230–1. doi:10.1001/archinte.140.9.1230. PMID 7406621.
  4. Dijkstal, JM; Bothun, ED; Harrison, AR; Lee, MS (Jan 2012). "Normal exophthalmometry measurements in a United States pediatric population". Ophthalmic plastic and reconstructive surgery. 28 (1): 54–6. doi:10.1097/iop.0b013e3182392f05. PMID 22262290.
  5. Chen M, Zhou XT, Xue AQ, Wang QM, Sheng W, Yuan YM, Qu J (Jun 2007). "Myopic proptosis and the associated changes in axial components of the eye". Zhonghua Yan Ke Za Zhi. 43 (6): 525–9. PMID 17897530.
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