Mr. Heim Stillear, at 70-years-old, seems older and less intelligent to his family, since he was hospitalized for a stroke about two months ago. They come to the appointment with him and help him into the exam chair. When I ask Heim what is going on with him, the long pause before his strained answer begins makes his family uncomfortable.

His daughter interjects, “We are having a very difficult time understanding him.”

I learn that the right side of his body is still not completely under his control. He is on blood thinners to prevent another blood clot from floating though his system, plugging up another part of his brain. In rehabilitation, physical therapists teach him to walk and speech therapists coach him to improve his swallowing and his speaking.

When I address him again, and he responds, his voice is loud enough, but his words are slurred and unclear. I notice that his lips at the side of his mouth droop. His smile is out of kilter. I look further: one side of his tongue has little writhing movements (fasciculations) and when he sticks his tongue out, it always moves off toward one side.

Some people ask, aren’t voice and speech the same thing? While they are related, they are not the same. However, in practice, speech problems and voice problems can be easily confused. Let’s distinguish them.

An easy mental image we can use to separate speech from voice is to draw a line, which we will call the “speech line,” across the neck above the Adam’s apple. This speech line roughly separates speech production above from voice production below. The two systems interact and to some degree overlap.

Yet in general, below the speech line, sound is created, above the speech line, sound is modified into language.

Below the speech line, the larynx produces audible vibrations — sound — in the normal human system. Above the speech line sound is modified. More specifically, vowels are the modification of the resonance cavities (mostly the pharynx and tongue modify the shape of the resonating cavities) and consonants are the interruptions or restrictions of the airflow. The interrupters include the palate, the tongue and the lips. These alterations in the airflow, when combined, coalesce to form words, then phrases, then sentences and we begin to communicate. We achieve speech.

Since Mr. Stillear’s problem is with difficulty producing words clearly, the issue is most likely in the upper half of this system. A physician would call this dysarthria. His problem is not with sound production because his volume is good, and any single sound is quite clear on its own. When I look at his vocal cords, they are indeed vibrating well.

However, I focus my examination above the speech line: on his face he has difficulty moving his lips; in his mouth he has difficulty controlling the movement of his tongue. These motion impairments garble his speech. In adults, most new problems with speech are neurologic problems. His stroke has impaired the neural input to his tongue and his lips so his speech lacks clarity. It is even possible that his intelligence is entirely unaffected by his stroke. The slurring merely gives his family the perception of diminished intelligence.

Mr. Stillear makes sound, and makes it loudly enough. He just cannot transmit clear content because of his tongue and lip weakness. He does not vary his pitch or volume very much and he comes across as having not much emotion. He can be heard, but not understood. He has a problem that the physician will find above the speech line. Problems above the speech line are not voice problems.

James P. Thomas is an laryngologist and author of Why is there a frog in my throat? A Guide to Hoarseness. He blogs at voicedoctor.net and is also on YouTube, Twitter, and Facebook.