Original Medicare forms the foundation for most seniors' healthcare coverage in retirement, but it's not perfect. In fact, it's got some pretty big holes in it. You can patch these with supplemental insurance policies if you don't want to pay for them out of pocket, but you only have a short window to do so. If you want coverage for the following four things, you'd better seek it out before the annual open enrollment period ends on Dec. 7.

1. Hearing aids

Hearing aids are one of Original Medicare's most surprising omissions, given that nearly 70% of Medicare beneficiaries aged 71 and older suffer from some degree of hearing loss, according to one study. But the program doesn't provide any coverage for these devices. The average senior with hearing aids spends about $914 per year out of their own pocket on hearing care, according to the Kaiser Family Foundation. Fortunately, other insurance policies can take some of these bills off your plate.

Smiling patient talking to healthcare worker wearing mask.

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Medicare Advantage and Medicare supplement plans are good options. Both plans are available through private insurers, but they do different things. Medicare Advantage plans are an alternative to Original Medicare. They cover all the same things as Original Medicare but with some additional services that may include hearing aid coverage. Medicare supplement plans are stand-alone plans that complement Original Medicare by giving you coverage for extra services.

Medicare Advantage and Medicare supplement plans have their own deductibles and premiums, so you'll have to compare your options to see which is more affordable. Compare this to how much you'd pay out of pocket for your hearing aids if you didn't purchase additional insurance.

2. Dental care

Medicare beneficiaries spend nearly as much out of pocket on dental care as they do on hearing care, with the average senior paying about $874 for dental services each year. This high cost is due in part to Original Medicare not providing coverage for preventive dental care, like cleanings, or services like fillings or root canals.

Opting for a Medicare Advantage or Medicare supplement plan is one way to get help with these bills. But know that these plans vary in terms of what they cover and how much you pay out of pocket. So be sure to compare a few options before deciding.

You might also look into a separate dental insurance plan or even a dental discount plan. The latter isn't technically insurance, but it gives you discounts off the regular price of dental care at participating dentists. However, it still has a monthly fee, and you'll still owe something when you visit the dentist.

3. Vision care

A whopping 92.4% of Medicare beneficiaries 65 and older wear glasses, according to a 2018 study, but once again, Original Medicare provides no help with these bills. Average out-of-pocket vision costs for seniors are only about $230 per year, but you might be able to pay even less if you have insurance that covers glasses and contacts.

Your options are pretty much the same as they are for dental care. You could try a Medicare supplement or Medicare Advantage plan. Or you could look into a separate vision care policy. It doesn't hurt to price out all these options to see which is the most affordable.

4. Long-term care

Long-term care costs anywhere from about $1,690 per month for adult day healthcare services up to $9,034 per month for a private room at a nursing home, according to Genworth. We'd all like to stay healthy enough to avoid this, but many are not so lucky.

Finding coverage for long-term care isn't easy because even Medicare Advantage and Medicare supplement plans usually don't include this. There are specific long-term care insurance policies, but these are often costly. A $165,000 policy could cost as much as $7,225 per year, according to the American Association of Long-Term Care Insurance. Younger adults and those in good health could pay less, but most will probably still pay thousands per year for this coverage.

You could also look into a life insurance policy with a long-term care rider. This way, you can tap some of your life insurance funds early if you need long-term care. However, doing so will reduce the death benefit available to your beneficiaries.

Don't wait

The Medicare open enrollment period ends Dec. 7. If you don't purchase additional insurance by then, you'll have to wait until next year to make changes to your policy. You don't want to get caught off guard by high medical bills you could have avoided, so set aside some time to compare your options within the next month.