Corneal Thickness, Eye Pressure, and Retinal Health: Key Insights

Corneal thickness and intraocular pressure, retinoscopy and refractive error, interpretation of retinal lesions, and storage of eye medications?

1.
I measured my corneal thickness at a teaching hospital using a pen-like device after applying topical anesthetic.
The thickness in both eyes was 618/609 micrometers.
Four months later, using the same equipment at the same hospital, the measurements were 657/678 micrometers.
The doctor mentioned that this could be due to measurement error.
However, I believe this discrepancy is quite significant.
Is it possible that other conditions could cause changes in thickness? If it is indeed measurement error, which value—higher or lower—would be closer to the true value?
2.
Corneal thickness can cause errors in intraocular pressure (IOP) measurements using a non-contact tonometer.
Does this also apply to handheld tonometers? I consulted two doctors; one said it could cause errors, while the other claimed that handheld IOP values are not affected by corneal thickness.
What is your perspective on this? Is the conversion formula for thickness and IOP (every 100 micrometers of corneal thickness corresponds to a 4 mmHg increase in IOP) considered a reliable reference?
3.
The eye drop medication Combigan should be stored at 25 degrees Celsius.
I have stored it in the refrigerator; will this affect its efficacy? Or is it acceptable to keep it at room temperature during the summer?
4.
I have been wearing glasses with a prescription of -8.00 diopters for a long time.
Recently, I had my prescription measured at an optical shop using a retinoscope, which indicated a prescription of approximately -5.00 to -6.00 diopters, not exceeding -7.00.
However, during a hospital examination of the fundus, the short-acting dilating drops still showed a prescription of -8.00, which did not decrease.
My actual vision seems to require -8.00 diopters to achieve 20/20 vision.
Does this indicate ciliary muscle spasm or loss of lens elasticity? How can this be diagnosed? What treatment options are available? Is vision training for near and distance sight effective?
5.
After dilation at a clinic, I was informed that there was a black spot at the edge of the fundus in my right eye.
The doctor used high-magnification fundus photography to show me, stating it was a retinal tear that required laser treatment.
I sought second opinions from three different doctors at another teaching hospital and a clinic, none of whom found a retinal tear but noted that the retina appeared thinner.
One doctor suggested the black spot might be a pigment deposit, while another stated that a retinal tear would not appear as a single black spot.
Have you ever encountered black spots on the retina, and how would you interpret such a finding? I have many questions and hope you can address my concerns.

Xiao Ming, 20~29 year old female. Ask Date: 2015/06/02

Dr. Wang Zhishun reply Ophthalmology

Hello,
1.
The comparison of various methods for measuring corneal thickness has always been a popular research topic.
The use of anesthetic drops can potentially introduce errors, and there are many other factors that can also affect the measurements.
Currently, the conversions are based on statistical values, but I believe that long-term follow-up data for each individual is more important.
2.
It is better to store Combigan in the refrigerator.
3.
A long-acting cycloplegic agent can be used one week later in conjunction with subjective and objective examinations to determine the prescription.
Vision training for near and distance sight primarily aims to relax the eyes.
4.
After examining the fundus, if there are any uncertainties, I will use a contact fundus lens for further confirmation.
Thank you for your questions, and feel free to discuss any issues further!

Reply Date: 2015/06/02

More Info

Understanding corneal thickness, eye pressure, and retinal health is crucial for maintaining optimal eye health, especially for individuals with a history of eye conditions. Let's address your questions systematically.

1. Corneal Thickness Measurements: The variation in corneal thickness measurements (618/609 to 657/678) could indeed be attributed to measurement error, but it’s also important to consider other factors. Corneal thickness can fluctuate due to hydration levels, time of day, and even the technique used during measurement. If the measurements were taken under different conditions or by different technicians, this could explain the discrepancy. Generally, if one measurement is significantly higher than the other, it may be prudent to consider the higher value as more reflective of your true corneal thickness, but this should be interpreted cautiously. A consistent measurement over time is more reliable than a single reading.

2. Impact of Corneal Thickness on Eye Pressure Measurements: Both non-contact (air puff) and contact (hand-held) tonometry can be influenced by corneal thickness. Thicker corneas can lead to falsely elevated eye pressure readings, while thinner corneas may result in lower readings. The formula you mentioned (for every 100 microns of corneal thickness, eye pressure can vary by about 4 mmHg) is a general guideline, but individual variations exist. Therefore, it’s important to interpret eye pressure readings in conjunction with corneal thickness measurements for a more accurate assessment of intraocular pressure (IOP).

3. Storage of Eye Drops: Regarding the storage of Combigan (a combination eye drop for lowering eye pressure), it is crucial to follow the manufacturer's guidelines. Storing it in the refrigerator may not be advisable unless specified, as cold temperatures can affect the drug's efficacy. Room temperature (around 25 degrees Celsius) is typically recommended. If you have inadvertently stored it in the fridge, it’s best to consult your pharmacist or the prescribing physician for guidance.

4. Refractive Error and Ciliary Muscle Function: Your experience with varying prescriptions (from 800 to 600-700 diopters) suggests that there may be an issue with the ciliary muscle's ability to accommodate, or it could indicate changes in the lens's elasticity. This is common with age or prolonged use of corrective lenses. To diagnose this accurately, a comprehensive eye exam including cycloplegic refraction (using drops to temporarily paralyze the ciliary muscle) would be necessary. Treatment options may include vision therapy, new prescriptions, or in some cases, surgical options like refractive surgery, depending on the underlying cause.

5. Retinal Findings: The presence of a black dot on the retina can be concerning. It could represent various conditions, including pigment deposits, retinal tears, or other anomalies. The fact that multiple doctors did not confirm a retinal tear but noted thinning suggests that the initial assessment may have been overly cautious. Black dots can sometimes be benign, such as retinal pigment epithelium changes. However, if there is any doubt, further imaging studies like optical coherence tomography (OCT) can provide more detailed information about the retinal structure.

In summary, maintaining regular check-ups with your eye care provider is essential, especially given your history of high myopia and retinal concerns. Each of these factors—corneal thickness, eye pressure, and retinal health—interrelates and requires careful monitoring to ensure your eyes remain healthy. If you have ongoing concerns, don’t hesitate to seek a second opinion or further testing to clarify any uncertainties.

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